IN1173 Flue System Planning Guide Multifuel and Pellet Stoves

Transcription

Euroheat
Natural Energy Company
Flue Systems and Installation Planning Guide
for
Wood, Multifuel & Pellet Stoves
IN1173 Edition D October 2010
Contents
Why is a Flue Necessary?........................................................................................................4
What is Flue Draught?.............................................................................................................4
What Causes Flue Draught?.....................................................................................................4
Using an Existing Chimney......................................................................................................6
Yet More Thermal Influences..................................................................................................8
Looking at the Wind................................................................................................................9
Methods of Controlling the Flue...........................................................................................12
The Barometric Damper........................................................................................................12
Draught Stabilizer Installation..............................................................................................12
The Chimney Cowl.................................................................................................................13
Cowl Installation....................................................................................................................16
Installation into an Existing Chimney...................................................................................17
Installation into an Existing Chimney...................................................................................18
Register Plate........................................................................................................................19
Ventilation.............................................................................................................................20
Equivalent Area (Free Air Requirement)...............................................................................21
Terminal Positions Need to Meet Regulation Requirements ..............................................22
Flue Outlet Options...............................................................................................................24
Bends in Flue Pipe................................................................................................................25
Sweeping Access...................................................................................................................25
Pre-fabricated Flues..............................................................................................................26
Installation into a Flue Block System....................................................................................27
Examples of Flue Installations..............................................................................................28
Spring and Autumn Syndrome..............................................................................................34
Flue Size Comparison / Volume Increase.............................................................................35
flue is an anagram of fuel
;
both need to be correct if
your stove is to work satisfactorily.
© EUROHEAT DISTRIBUTORS (H.B.S) LTD. October 2010
2
Technical Guide IN1173 Edition D
Cannot
be
assumed to work
The document has been written to highlight the importance of the flue in any stove installation and to give you
an insight into why such an apparently simple item is far more complex than imagined when working.
Although we have illustrated flue installations to show some of the many options available, this document is not
intended to be an installation guide, and it should be noted that all flue installations and modifications to existing
chimneys and flues are governed by so many regulations and legal requirements that no person should attempt
any such work without being appropriately qualified.
No installation should be undertaken unless the installer is suitably qualified or Local
Authority Building Control Department permission has been granted.
The installation, replacement of or alteration to the position of a solid fuel combustion appliance is subject to
Building Regulations. Before proceeding with such works the householder is required by law to give building notice
or deposit full plans with the Local Authority Building Control Department and obtain permission to proceed.
However, for England and Wales, only, the coming into force on 1st April 2002 of SI 2002 No 440 exempts the
householder from this legal requirement for the installation of solid fuel fired appliance whose rated heat output
is 50kW or less in a building having no more than 3 storeys (excluding any basement) if a "Competent Engineer"
is employed who is registered under the Registration Scheme for Companies and Engineers involved in the
Installation and Maintenance of Domestic Solid Fuel Fired Equipment operated by HETAS Ltd. These registered
Competent Engineers may also carry out associated building work necessary to ensure that the installed appliance
complies with Building Regulations without involving the Local Authority Building Control Department.
Further information on the operation of Building Regulations and the exemptions under SI 2002 No 440 can be
obtained from the Explanatory Booklet entitled “Building Regulations” which is available from the Office of the
Deputy Prime Minister, PO Box 236, Wetherby, West
Yorkshire, LS23 7NB.
In Scotland SI 2002 No 440 and its permitted exemptions do not apply. However, HETAS Ltd in its guide also lists
under the scheme contractors in Scotland employing competent engineers which have met the HETAS scheme’s
requirements for Registration.
3
Why is a Flue Necessary?
The flue removes the unpleasant gasses and vapours produced by the combustion process away from the stove to
be diluted by the atmosphere. Unless you regard a room filled with smoke to have a rustic appeal, the advantages
of sending the products of combustion up and away from the house should be obvious, but more importantly, in
performing this task a well designed flue should be performing two other, very important, tasks.
While the flue is exhausting the waste products of combustion it allows fresh supplies of air to be introduced into
the stove, but because the air should be supplied constantly and proportionally to the fire size, the flue should
also exhaust consistently, and proportionally to the fire size. As will be explained in the following pages a flue is
constantly being subjected to conditions that can make this sometimes extremely difficult to achieve.
Some of the products of combustion are either hazardous or potentially hazardous to the fabric of the flue and
property. Consideration must be given to these risks when fitting a stove because a sub standard flue system will
endanger your property and the people in it.
What is Flue Draught?
Flue draught is the flow and rate at which air or the products of combustion travel up the flue. Because the flue
plays such an important part in the efficiency and controllability of all naturally aspirated fuel burning appliances,
its performance is quantified with the term “flue draught”. (Do not be confused by the flue draught measuring
devices of American origin where the word draught is annoyingly spelled "draft") Flue draught can be measured
by speed or weight of gasses but it is normally measured as the difference in pressure between the inside of the
flue and the air outside the flue from which all other quantities can be measured.
What Causes Flue Draught?
Flue draught is caused by two very different effects which the flue is subjected to. Firstly flow is induced by
the difference in temperature between the gasses within the flue and that of the air outside the flue, and
secondly, the effects of air flow around the property and the flue termination. Temperature difference induced
flow follows relatively simple rules and although it is often difficult to predict the performance of individual flues
accurately because too many "unknowns" are involved, but once the performance has been established the flue
will perform consistently to these rules. The flue draught induced by wind is a veritable minefield of constantly
changing, conflicting and misunderstood effects that have driven even the sane and rational to buy an electric
radiator and take up needlework.
Flue Draught without Wind.
After being involved in the combustion process the gasses making up the products
of combustion are heated and have expanded to become less dense than the
surrounding air, and being less dense they weigh less than the surrounding air
and are motivated upwards. Why they should rise is not complicated and can
be illustrated by releasing water (a dense substance) over a bucket containing
air (a less dense substance), where the water will fall to the bottom of the
bucket attracted by gravity, forcing the air
upwards, because it is less attracted by
gravity. This principle can also be illustrated
by releasing a "ping pong" ball under water.
Whilst air and water have very differing
densities, cold and hot gasses behave in
the same way, and it is the weight of the
cold, surrounding air which forces the hot air up the flue. The greater the
differences in temperatures, the greater differences in densities and the
faster the gasses will be driven up. A more colourful example of this is a
hot air balloon, where the rate of the balloon's ascent or decent is governed
by the difference in temperature between the air in the balloon and the
surrounding air.
4
From this we can establish several important facts:1. A hot flue does not “draw” air into a stove, it is the differences of
densities that cause the heavier air to motivate the lighter gasses
upwards. Remember that gasses do not go upwards unless cold air is
available to push it up.
2. The greater the temperature difference between the gasses within
the flue and the surrounding air, the greater the difference in densities
and the greater the motivation. Remember that the hotter the fire,
the faster the flue gasses can potentially travel.
3. In a perfect world it could be said that the taller the flue, the greater
the weight of the equivalent volume of denser air, and the greater
the motivation. Reality dictates that all flues lose heat and the taller
the flue the greater the heat loss which will lower the temperature of
the gasses and cancel out the benefit of height. Remember that not
all tall chimneys will work better than a short one.
From Theory to Practice.
Having established that we need to have the flue system warmer than the surrounding air, to ensure the products
of combustion are removed, we have to consider the amount of heat being sent to the flue. All surplus heat is
wasted heat, but insufficient heat will not only give a poor flue draught, it will allow the products of combustion
to damage the flue. The chemical changes during combustion result in water being formed, which for as long as
it remains a vapour causes no problems, indeed water vapour weighs less than air and so increases the flue's
efficiency. However, if the vapour cools sufficiently to condense it will mix with the other gaseous products of
combustion to form acids and tars which will be deposited on the flue wall. The acids will eventually eat their
way through the fabric of the flue and the tars will cause the flue to block, pose a fire risk, cause unpleasant
smells and possibly run down the flue to the stove and hearth.
The damage caused by insufficient flue temperatures can often be seen on the outer walls of older properties,
where the flue is no more than a brick or stone duct. Many were even
purposely built with several changes of direction to prevent rain falling
directly and unsightly into the fireplace. Despite the enormous amounts
of heat sent to the flue from an open fire, the walls of the flue were
often torturous passages over cold and wet masonry which chilled the
flue gasses, and the resultant fluids ate their way through masonry. Of
special interest is that the worst damage is usually seen where a range
cooker was installed. These cookers were far more efficient than an open
fire and their flue gas temperatures were far lower. Further signs of the
damage can often be seen on interior walls as a brown staining, not of
smoke, but of the tars and acids having eaten their way through the
masonry and plaster. These tars are inflammable and in the event of a
chimney fire are difficult to extinguish, making extensive and expensive
remedial building work necessary. As stoves are being designed to give
ever improving efficiency, the temperatures of the gasses leaving the
stove become progressively lower and the flue's design and construction
become increasingly important if the risk of tar build up and destructive
effects of acids are to be avoided.
To reduce the risk of condensation within the flue we need to keep the motivation requirements to a minimum,
by making the route of the flue gasses as simple and smooth as possible, and to maintain as much of the original
gas temperature as possible. How this is achieved will differ if a completely new flue is being constructed or an
existing chimney is being utilized and adapted to meet up to date standards.
5
Using an Existing Chimney
An existing chimney may have only its masonry duct forming the flue way, which is completely unsuitable for
a stove for several reasons. The masonry will be capable of absorbing moisture from the atmosphere which,
unless the stove was used continuously, would never dry out. This moisture would absorb so much heat from the
flue gasses that they would begin to condense which would deposit even more moisture into the masonry. The
walls of such a flue represent such a huge, unnecessary, surface that even if the masonry was dry the heat losses
would cool any flue gasses to an unacceptable temperature. The walls would have a large surface area even if
they were smooth but with the deterioration of the mortar that would have occurred the total surface area will
have increased with voids, cracks and loose or missing masonry. The poor standard of wall surface will cause
turbulence to the flow of gasses further slowing and allowing more heat to be extracted. If you have ever tried
to light a fire below an old chimney and wondered why the room filled with smoke for several hours, the above
should have given you the explanation.
Fitting an acid resistant stainless steel liner
to the chimney is a simple solution to the
problems of thermal and friction losses in
the flue. Although almost all drawings of
flues illustrate a straight vertical path from
the appliance to the terminal, this has more
to do with the illustrator's idleness than
reality because many chimneys follow a
torturous route of bends and twists to align
the flues from several rooms to an orderly
group of terminals at the top of a single chimney structure. For flues such as these a flexible stainless steel liner
designed specifically for wood and coal burning appliances can be fitted, this not only gives a smoother wall but
also radiuses the typical abrupt changes in direction that are so often found. For additional insulation vermiculite
or similar insulating materials can be poured between the liner and the chimney duct.
If the chimney flue way is in very poor
repair it is worth considering having
it lined with an insulating cement.
To do this an inflatable "sausage" is
positioned within the flue way and
the cement is poured into the space
between it and the masonry, leaving
a smooth insulated finish when the
"sausage " is deflated.
Whatever lining system is used it is important to use one of the diameter specified by the stove manufacturer.
Increasing the diameter as a precaution against it being restricted by tar deposits is sometimes advocated, which
is counter productive to both reducing the surface area, and reducing the volume to speed the flow to achieve
less tar deposits. Having a "reserve" of flue in which to allow tar to build up is a ridiculous proposal because any
The correct size diameter will
have the minimum surface area
through which to lose heat
and because the gasses will be
travelling faster they will have
less time in which to lose heat.
An over sized diameter will lose
heat unnecessarily through its
increased surface area and the
slower moving gasses will have
more time in which to lose even
more heat.
deposits of tar are a fire hazard. Always have the flue swept by a qualified sweep regularly and before significant
deposits build up. The sweep will be able to advise you on the condition of your flue and if tar deposits are
excessive, attention should be paid to the fuel being used and the way the stove is being operated.
6
If the chimney has a clay or salt glazed lining system it should be impervious to the effects of acids but the
system was designed for open fires rather than a stove. The diameter will be bigger than many stoves require
and the thermal losses through the clay to the masonry of the chimney will be large, so the problems of heat
loss, tar deposits and condensation may manifest themselves as diluted tar running into the stove or appearing
as dirty puddles on the hearth.
If the existing flue is one of a number serving rooms in the house it will probably be found that the route taken
from the room it is serving to the terminal will have nothing to do with any aspect of thermal performance, it
will be taken to facilitate a neat row of chimney pots at the top of the building. The routes taken were often
purposely made even more torturous to allow any rain falling down to hit almost horizontal sections and be
absorbed by the masonry rather than fall into the fireplace. While it is possible to coax a flexible stainless steel
liner through many twists and turns, all changes of direction reduce the efficiency of the flue.
The term "effective height" of a flue is the height of an equivalent flue that is vertical with no changes of direction.
Each change of direction requires a vertical distance to provide the motivation to overcome the resistance to the
flow of gasses these changes of direction impose. Various formulae and "rules of thumb" giving the straight
vertical length necessary to overcome each change of direction have been published. Care should be taken
when using any of these because they give absolute angles with direction changes, something that is impossible
to achieve when using a flexible liner, and because many are based on gas burning appliances they have not
allowed for differing surfaces, gas speeds, deposits in the flue and seldom adequately allow for heat losses in the
flue because these will be unknown until the flue is operating, by which time the performance will be a reality
rather than a theoretical estimation.
If an existing flue has operated satisfactorily with an open fire it would be reasonable to assume that when lined
with the correct sized and insulated liner it will work with a multifuel stove. If you are not sure as to the flue's
performance look firstly at it its height, any flue less than 5 metres will be a potential problem. Check that the
flue cross sectional area specified by the stove manufacturer can be accommodated and that if there are any
changes of direction that these can be negotiated with a flexible liner. Finally look to ensure it is possible to
insulate the liner adequately because this may be necessary rather than optional if the flue's route is complex.
Back filling the void between the flue liner and existing flue way with vermiculite or other suitable insulating
granules is both an effective insulation and a relatively simple operation.
7
Yet More Thermal Influences
Even before any wind blows a flue is affected by other factors, because a flue does not operate in isolation, it is
a component of a system which includes the stove and house. Whilst we normally think of flues only when the
stove is operating, the same rules of heat induced flow apply although the stove is not operating. If the house
is hotter than the outside air, the flue will gently draw air from the room if the stove's air controls are open.
However, on those odd days when it suddenly becomes warm and the outside temperature is higher than the
temperature in the house, the flue will operate in reverse and draw down outside air, together with sooty smells,
into the house, and for this reason the air control on the stove should be closed whenever the stove is not being
used. We call the phenomenon of downward flue flow the "Spring and Autumn Syndrome" because it is these
times of year it occurs and people have difficulty lighting their stove. The reversed air flow may provide sufficient
air for the stove to ignite and burn poorly, but the products of combustion will spill into the house, and because
so little of the heat from the stove is entering the flue the fire will either extinguish or fill the house with smoke.
Solutions to the Spring Autumn Syndrome are detailed on page 31.
A more complex example of thermally induced reversed flow in flues occurs in houses with multiple flues serving
different floor levels or low extensions with flue terminals well below the terminals of existing flues. The problem
is made worse when natural, or advantageous, ventilation is
reduced with improvements to door and window sealing, and
the tendency for the taller flue to draw air down the shorter flue
is increased, making the lighting of an appliance at the short
flue difficult. If the taller flue is being heated with an operating
appliance, air will almost certainly be drawn down the shorter
flue, and if the outside temperature is colder than the inside
temperature, the room will be subjected to a flue smelling cold
draught. This will make lighting an appliance at the shorter flue
almost impossible and it will inevitably be a smelly and smoky
task. Installing adequate ventilation to the room with the short
flue will help, but the disparity between flue heights will always
be a potential danger if they are being used at the same time.
A typical example would be where both an open fire burning with a tall flue and generating a high flue draft and
a stove with the short flue and generating a low and ideal flue draught are operating at the same time. Under
these conditions it is possible for the shorter flue to reverse its flow and the stove to emit poisonous products of
combustion and not all of these gasses have an easily detected unpleasant smell. So far we have assumed only
one tall flue, but the reality is that many houses have multiple flues that will be balanced against a single short
flue and the problem becomes and almost certainly a safety hazard. Installing a fan to assist the shorter flue is
not a solution because the fan will fail during a power cut, or may fail due to mechanical failures at any time. If
the shorter flue is the only one required in the property "capping off" the taller flues may be the only possible
option if the shorter flue cannot be extended to the outlet levels of the other flues.
8
Looking at the Wind
Typical drawings to be found showing the effects of wind on flues illustrate a symmetrical house, "A" with high
pressure areas on the windward side and low pressure areas on the leeward side. These drawings are similar to
those used in building manuals to show the loadings to walls and roofs caused by blocked and stalled air in high
winds. There is scant, if any, text with these drawings to explain why the wind would affect the flue draught
in the neat little house illustratrated. Explanations seen to be reserved for a drawing, "B" showing an additional
flue incorrectly terminating in either a high or low pressure area, and the permitted flue termination positions
allowed by the British Standards are often given to support these sorts of drawings and whatever explanations
are given. Some illustrations go further by
showing what happens when a door or
B
A
window is opened, again neatly directly on
the windward or leeward side of the house.
The effect will of course be dramatic but doors
and windows are seldom opened on a cold
windy day and the drawings explain nothing
about why a flue should work as an extraction
system when the wind blows and the doors and
windows are tightly closed to conserve heat.
Even those with little interest in science and have never heard of Bernoulli's Principle will know that a fast moving
train or car tends to draw you towards it as it passes. You need know nothing about Ludwig Prandtl to know
aeroplanes fly, but even if you paid no attention to science at school you might like to know that not standing
close to a station platform edge, aeroplanes flight, and wind causing havoc to flue draught are for the same
reason.
As a child you may have blown over a piece of paper to cause it to lift, if not, now is your opportunity to hold
the edge of a piece of paper horizontally just below your bottom lip and blow, this will cause the whole piece
of paper to lift horizontally because the air above the paper is travelling faster, with a lower pressure, than the
air below the paper. Looking at an aeroplane wing will reveal that its top surface is much more rounded than its
underside which means that air passing over the top of the wing has to travel further and therefore faster than
the air passing the underside of the wing. Because the air is travelling faster its pressure reduces and so the upper
wing surface is pulled upwards. If you have ever stood on a cold, windy, station platform you might not agree
that the air is relatively static but compared with the air being forced to push around a fast moving train it is. The
air at the platform is at a higher pressure than the air surrounding the fast moving train and so you are pulled
towards the train as it passes. The same happens when wind blows over a house, the air, having to travel over
and around the house, has to travel further and faster to keep up with the general flow and its pressure drops,
so air passing over the flue terminal will be at a lower pressure than the air within the house, causing the flue to
exhaust more easily whenever the wind blows.
If you are beginning to grasp the theory of pressure dropping with air speed you might like to contemplate what
happens when a ball is thrown with and without a spin. Or, if you have a ping pong ball and a vacuum cleaner
that will blow through the hose, try balancing the ball on the vertical stream of air, it is easier than imagined and
it balances for the same reason that wind induces flue draught.
9
Knowing the cause of increased flue draught when the wind blows
should make it easier to control, but we have only found the prime
source and we need to look at the other influential factors. Going
back to our little simplistic house being subjected to wind we can
draw the air flow travelling over the roof and it becomes obvious
how much further the air has to travel to pass over the roof. We
can illustrate the area of low pressure with shading where the air
reaches its highest speeds but however simplistic we make the
drawing the profile of the house is very different from an aeroplane
wing and the smooth flowing lines showing the air as travelling in
convenient lamina strata are unrealistic because air will only hold
together if the changes of direction are smooth and proportioned to
its speed. A house roof and chimney are not shaped to give smooth
air flow and the abrupt changes of direction they cause the air flow
to take results in the air flow fragmenting to form pockets that spin
and travel in differing directions as the wind speed increases.
Because the directions of these pockets of air are often opposite
to that of the main flow they briefly form zones of high pressure
which may cause the flue system to stall or even reverse flow
momentarily. If you can bring yourself to stand and look at the
terminal when a wind is accompanied by snow, the confused air
flow will be seen easily. Early chimney builders knew nothing
about the science of air flow but they knew that putting a chimney
terminal high above the roof lessened the tendency of the flue
to have a draught fluctuating between positive and negative
with each wind gust. The humble chimney pot took the terminal
away from even the turbulence caused by the straight sides and
squared edges of the chimney stack. The stove manufacturers
who understand flue draught have always tried to reduce the
cross sectional area of the flue to the minimum possible, not only
to reduce the thermal losses through the flue walls but because
the area exposed to wind induced draught will also be minimised. Some more modern houses have often been
built with little or no consideration to flue performance and often have the terminal only just higher than the
roof ridge, causing the terminal to be in the worst area of changing pressures. In very recently built houses you
might notice the terminations below the roof ridge, this is not because of a reversal of scientific knowledge but
rather a chimney with no flue way, put up as a non-functional roof ornamentation.
So far we have looked at the wind induced flue draught associated with our little symmetrical, two dimensional,
house, but it is enough that we understand the very basics of what is happening when the wind blows over a
house. Trying to analyse what happens when a multi chimney, asymmetrical house is subjected to wind will
produce differing results with each small change of wind direction and would make aeronautics look like child's
play. Again, watching the terminal during a snow storm will demonstrate what is happening when the wind
travels over the roof and terminal and this will be especially interesting because you can relate it to the way the
wind is affecting the stove's performance.
What part Ludwig Prandtl plays in some aeroplanes having the ability
to fly upside down will be discussed in a later document.
10
Whilst we are not going to look at wind passing over the house in any more detail several other issues that affect
the wind induced draught need to be looked at. Firstly if all this air is travelling up the flue it needs to be replaced
with air entering the house and how we do this in a planned manner will be looked at later but all doors and
opened windows will affect the flue's performance. An opening to the windward side will increase the draught
and opening to the leeward side will reduce the draught and if you have been paying attention you will know
that wind rushing past open windows midway between windward and leeward will reduce the flue draught.
Having looked at the house itself it is important that its location and surroundings are looked at to see how they
will affect the wind before it reaches the house. Much of this will be an educated guess because again it will
change with each change of wind direction and the permutations are endless, but it is possible to identify some
potential problems by looking at the contours of the ground which will affect the wind.
If before looking for potential problems with the property that may be caused by obstructions nearby, a far wider
area should be looked at, taking time to look at other chimneys. If every old house nearby has what seems to be
a disproportionately tall chimney it will be because of difficult wind patterns in the area and not because every
builder liked working at high altitudes.
Obstructions near to the property may cause problems and trees in particular should be noted because without
leaves they may have little or no effect on the passage of wind but when the stove is operated during the cold
spells in spring when the trees are in leaf they may cause havoc to the flue draught; not having caused a problem
all through the winter they are often overlooked as the cause of any flue problems. For similar reasons trees that
have been planted for many years may not have caused a problem, but one year they may reach a critical height
and width to completely disrupt the wind flow.
There is rarely a simple and totally satisfactory solution to overcoming the problems with the effects of wind
turbulence.
11
Methods of Controlling the Flue
The Barometric Damper.
If there is to be any hope of controlling the flue it must be installed to put the terminal in the best possible
position, it must have sufficient height and it must maintain temperature. So returning to the situation where
the flue needs to operate on a mild, still day there will be too much negative pressure on a cold day, when the
density differences between the atmosphere and the flue gasses are greater. If we add a strong wind to our
cold day the result would be far too much flue draught. The device for sorting out this little problem is called a
“barometric damper” or “flue stabilizer”, which can be fitted to the flue. It consists of a hinged flap with the hinge
point above the centre point so the flap always tends to adopt the closed position. As the air pressure within the
flue falls, the air on the outer side of the flap pushes the flap open, spilling air into the flue way this immediately
reduces the excessive air flow through the stove and because the air allowed into the flue is relatively cold slows
the flue's thermally induced flow. An adjustable counterbalance weight allows the flap to be held closed until the
necessary pressure difference has been reached.
With the draught pressure
below its set point the
stabilizer remains closed.
When the draught pressure
exceeds its maximum the
flap opens to spill in cool air.
The adjustable counter balance
allows the damper to be set
correctly for a wide range of flues.
Draught Stabilizer Installation
1. A stabilizer must have the same cross sectional area as the flue.
2. The stabilizer must be fitted to the manufacturers instructions
and must not be modified.
3. The stabilizer should ideally be fitted no closer than 600mm to the
flue outlet of the appliance. In certain instances where there is not
enough clearance above the stove to give 600mm it is permissable
to fit the stabilizer closer to the flue outlet, however it should never
be fitted less than 300mm from the flue outlet.
600mm
300mm
4. The flue stabilizer should be fitted in the same room as the stove
installation.
5. Where a flue draught stabilizer is used the total free air area
should be 300mm² for each kW/hr of rated output of the stove
up to 5kW. Above 5kW of rated output the free air area should be
850mm² per kW above 5kW
12
The Chimney Cowl
We now move to the top of the flue and look at the terminal itself. While you were outside watching the snow
swirling and travelling in any direction other than those drawn in “wind passing terminal” drawings you may have
noticed that the simple chimney pot, which has been used for hundreds of years, is finally being replaced by more
scientific terminals. With so many diverse shapes and sizes you would be forgiven for wondering which science
they were based on, and indeed many of us wonder. In an age where technology with computer modelling and
all the necessary test equipment have narrowed the discrepancies between the designs of solutions to problems
it is interesting that cowls would seem to be the exception. But perhaps even more interesting is that each cowl
design will have a number of people who regard it to be a perfect solution and a similar number of people who
regard it as worse than useless. None of them will prevent the low pressure zone over the house when the wind
blows but they can smooth or redirect the flow of air at the terminal, changing the effect that wind has on the
flue.
In its simplest form the chimney cowl is nothing more than a rain cap. These were unnecessary in a brick chimney
because most of the rain falling down was absorbed by the bricks and the absorbed water evaporated out
whenever the fire was lit. With appliances like stoves so little heat is wasted that we have to line and insulate
the flue which gives rain a direct passage to the stove. The following drawings show that fitting a rain cap on a
chimney pot will almost reverse the effect of vertically rising or falling wind.
Almost all winds moving in an upward direction will be caught by the cowl to form an area of high pressure above
the flue, but wind moving in a downward direction will also be able to blow into the flue as its angle decreases.
When this angle is reached the effect will be a sudden change and all sudden changes are difficult to control.
Changing the diameter of the cowl and its height above the flue in an attempt to stop this will make the flue worse
in upward wind directions or let the rain in. Before dismissing the simple rain cap cowl it must be remembered
that we have given no consideration to effects the house itself will have on the wind direction and those together
with other pressure factors might make a cowl like this of certain proportions the perfect terminal.
It is this unknown element which makes the choice of cowl so difficult. A cowl which gives the solution to one
problem flue may exacerbate the problems of a flue with a seemingly identical problem in a similar house in
a different location, but cowls can be put into very loose categories as to their purpose. Whilst almost all cowls
claim to be “anti down draught” others purport to do more.
The left hand snorkel is mounted on the chimney pot on a bearing allowing it to be rotated by wind blowing at
its vane and causing the opening to face down wind consistently. Whilst solving some problems, by maximizing
the effect of wind, the resultant difference in flue draught between wind and windless conditions is too great for
any stove and draught control system to cope with. The cowl on the right is often seen as an enormous terracotta
affair on low or badly sited old chimneys and works by ensuring that all air passing is redirected, resulting in all
air flow causing air to be drawn out of the flue. This cowl manages to cope with almost every wind direction
and a modern, light weight version is available, but as with the snorkel it tends to amplify the differences in the
flue’s performance.
13
There are many variations using the basic principles of these cowls with varying degrees of success but all are
capable of increasing the flue draught beyond the limitation of our control for stoves because they were originally
designed for open fires.
Another approach to a cowl which should give less variation in wind generated flue draught is to cover the mouth
of the flue with a box vented with slots, louvers or course mesh in an attempt to slow the wind speed passing
the mouth of the flue. These may slow the air speed but because the box will need to be large enough to spread
the area of the flue diameter it creates its own negative pressure on the downwind side while the upwind side,
if the slots are doing anything at all will be limiting the higher pressure entering. The net result will be a slower
air flow but an increase in the negative pressure over the flue mouth caused by the box obstructing the air flow
and creating a negative pressure pocket on the downwind side. The effects of winds blowing in anything but
a horizontal direction will depend on many things but the angle of the air stream may be such that it is not
prevented from acting directly onto the flue mouth. Increasing the diameter of the box to prevent this will only
increase the negative pressures generated at the downwind side.
Another solution to the problem of varying wind speed is to direct air passing over the flue downwards, and so
create a high pressure zone above the flue which is proportional to the negative pressure and thereby cancelling
each other out.
If the flue terminal was some hundred feet in the air, with nothing but flat land for a radius of a mile, the air
would be passing the terminal in only one plane and this would work well. However flue terminals do not exist
in isolation, they exist in close proximity to obstructions which divert the wind to act in many planes which will
vary with wind direction. Whatever shape above the flue caused a high pressure zone when the wind passed it
horizontally will not produce an identical effect with an air stream at anything other than horizontal flow. It can
be improved by adding disks to divert the air into a horizontal flow and these will improve the ability to cope with
air flow away from the horizontal, but the efficacy of the discs will be related to their diameter and at some point
away from horizontal the air will simply slip past the discs. At the point at which the discs fail they will only serve
to increase the effective diameter of the flue resulting in a greater negative pressure if the air stream is upwards.
What happens when the air stream is downwards will depend on the proximity of any obstruction.
14
A more complex approach to the problem is the range of cowls that would seem to have been designed on a
kitchen table using an assortment of mixing bowls. These cowls are designed to divert the air over the larger
bowl creating a high pressure zone at the middle. This bowl has an open top through which a smaller bowl
protrudes. The high pressure restricts the flue gasses passing up through the cowl, sending them downwards to
exit under the rim of the larger bowl. The shape and combination of the two bowls restricts the air flow out of
the cowl when the air stream is in an upwards direction creating a positive pressure to counteract the negative
pressure that would have been created above the flue mouth. The effect of a downwards flowing air stream will
be diverted away from the flue mouth and again the combination of openings at the top and bottom of the larger
bowl will prevent flue down draught affecting the flue. Unfortunately, the differing sizes and shapes of the dishes
means that their influence will not remain consistent with each for all wind speeds and all directions.
No cowl will solve all problems and no cowl will perform identically in every installation because no flue operates
consistently. A flue which is operating at a low temperature is very different to one operating at its maximum
temperature and flow rate. Cowls are tested as an isolated piece of equipment and some cowl manufacturers
recognize that so many other factors will affect its performance that they offer a guarantee to accept its return if
it fails to perform as anticipated in any particular installation. Although this gives the opportunity to try several
cowls, simply fitting cowls randomly is not recommended because the novelty of scaffolding and the adrenaline
rush of roof walking are fickle emotions and may evaporate before you find a suitable model. The correct choice
of cowl is often a case of two wrongs nearly making a right and before seeking the advice of cowl manufacturers
you should identify what the problem is and if possible rectify it before resorting to the expense of having a
complex cowl fitted.
N WE S
15
Cowl Installation
A cowl must be fitted to a chimney pot to raise the cowl away from the chimney brickwork. The pot must be in
perfect condition and be securely embedded into the chimney masonry. Do not fit a cowl to any other than a
simple pot, those with louvers or any ornementation are unsuitable.
X
X
X
Manufacturers' Instructions
The manufacturers' fitting instructions must be followed implicitely, no improvised fixing methods or alternative
positioning should be used. Some cowls have extra security fittings available as an optional extra if you live in
an exeptionally windy area. Ensure the cowl is approved for solid fuel combustion.
X
X
X
X
Cowl and Liner
The liner must extend to the mouth of the chimney pot. If the liner terminates at the base of the pot, the cowl may
not perform as it is designed to do, due to increase in diameter, and heat losses caused by the cold pot will cause
turbulance. The liner must have a
support plate fitted to secure the liner
and the cowl must be fitted exactly
to the manufacturers' instructions.
If room exists an insulating material
such as glassfibre wool can be fitted
between the liner and pot.
X
16
Installation into an Existing Chimney
X
Cowl
to
prevent
ingress of rain, birds
and to assist with flue
stabilization.
Flue liner reaching to top of the
termination and insulated.
Flue liner support collar which
is required for both flexible and
single wall liners.
Insulation
Weatherproof chimney
capping and pot.
Sound chimney brick work.
Stainless steel liner.
Flue height 5m or more.
Flexible to single wall
adapter if flexible liner
is fitted.
Register plate preventing the
escape of heat, positioned
as low as practicable to aid
convection.
Access for cleaning.
Level and stable supporting
hearth.
Sufficient clearance behind stove for
maintenance and to allow the air to
circulate to and from the stove .
The diagram above shows an unsuitable installation.
A masonary chimney which has not been lined.
Unstable brick work within the chimney allowing soot to be
trapped so increasing the chance of a chimney fire.
Unstable chimney pot.
The stove installed with no clearance behind it.
The flue pipe installed at an angle.
No cleaning access to either the flue pipe or chimney.
The total height of the flue less than 4m.
No cowl fitted to prevent ingress of rain, birds and/or to assist
with flue stabilization.
17
Installation into an Existing Chimney
Maximum 150mm
horizontal run
Cleaning
access
Access for cleaning, minimum
horizontal path, 150mm or less.
With no flue liner fitted, no access for cleaning
and positioned on an insufficient hearth makes
this installation dangerous and illegal.
Register plate in
correct position
Access door
for cleaning
Register plate too
high, allowing hot
air to accumulate
within the fireplace
T piece
debris trap
With no flue liner fitted, incorrect 90 degree
bend badly fitted, no protection against flue
blockage, no access for cleaning and the
stove badly positioned on an insufficient
hearth makes this installation dangerous
and illegal.
Stove positioned to allow
heated air from the stove to
circulate easily for cleaning
18
Register Plate
If an existing fireplace is to be used to house the stove all but an opening to allow the new flue pipe of the original
flueway throat will need to be sealed off. This plate is called a register or closure plate and needs to be given
more than a little thought. The plate must be manufactured using 1.5mm thick rust resistant metal to conform
to building regulations. If the plate is to be painted, do not use ordinary household paint, it must be a heat proof
paint. If the space between the old flue and new liner is to be back filled with vermiculite or similar insulating
material the a steel plate can be one complete sheet but if the space is not to be insulated a closable aperture
should be made in the plate to allow
the removal of debris that will
inevitably collect as the chimney
drys and combustable deposits fall
from the brickwork.
The plate should be secured by
fixing a continuous ledge of angle
iron around the opening, fill with
mortar any gaps left by uneven
brickwork and screw the register
plate to the angle iron. This not only
gives a very secure fixing it also
allows its removal and replacement
to be done simply and quickly if
access is ever required.
The position of the register plate is a conflict between being irritated by seeing the plate and the need to have
air flowing around the stove, circulating to the room easily. Positioning the plate high into the throat of the flue
way will render it invisible but it will trap hot air from the stove that should be heating the room rather than
the masonry of the fire place. Do not be missled into believing that the masonry becomes a heat store because
a high proportion of this heat will be lost if the chimney is against an outside wall. A well fitted plate painted
in a suitable colour, with heat proof paint, will eventually become all but invisible, but the loss of heat will be a
constant expense and irritation when the room gains temperature too slowly and looses it rapidly.
X
If you are tempted to
fit concealed lighting
to illuminate the space
behind the stove, your
electrician
must be
made aware of the high
temperatures that the
fireplace and especially
the register plate will
operate at, and that
ordinary fittings, cable
and even light bulbs
will be unsuitable.
These
considerations
will apply even if the
lights are
intended
to be illuminated only
when the stove is not
operating.
19
Ventilation
Having achieved a reliable flue draught to exhaust the products of combustion our attention now turns to the
supply of fresh air, and it will come as no surprise to learn that getting air into the house is almost as complicated
as it was pushing it up the flue. If the stove has the facility to draw its air for combustion directly from outside
of the property no additional ventilation but it is law that a stove has sufficient ventilation into the room it is
installed if it is supplied with air from that room. The legal requirements for ventilation give the size of ventilation
that must be fitted to achieve sufficient air supply for specific stove sizes. The term "equivalent area" has taken
over the old term of "free air" because so many ventilation grills were available that purported to allow air flow
with no draughts. Given that draught is air flow I have no idea what science they were based upon but many were
little more than a complicated obstruction behind the opening grill to restrict the air flow and hence the need
for all ventilation grills to be marked with their effective air flow. With the law requiring us to fit a permanent,
meaning one that cannot be closed, ventilator of a specific size, we now have to find a suitable position to install
it. The legal requirement is for the area of ventilation but makes no mention of this area being achieved with
a single or multiple ventilators nor where they should be positioned, apart from allowing air from outside the
property directly to the room in question,
Our little house has not had the wall removed for ventilation but to
simplify my drawing of the air coming in at the left side and blowing
up the chimney. If the ventilator is in a wall facing the wind it will
be subjected to a high pressure as the air struggles to find its way
around the obstruction. This will obviously cause a great deal of air
to enter the ventilator, slightly pressurising the house and ensuring
the stove has a more than adequate air supply. If the ventilator is
positioned at the opposite end of the room to the stove the room
will be subjected to a draught, whatever patented draught reducing
device may be fitted and this air will obviously be cold or there
would be no reason to have lit the stove. For how long differing
people will tolerate a cold draught before becoming irritated is the
proverbial piece of string, but the string will eventually snap for
everyone and the ventilator will be at least partially blocked with
an item of furniture or piece of card. This might not be a problem if the wind is strong and facing directly at the
ventilator, but all winds eventually blow themselves out leaving the property with an insufficient air supply
because it is unlikely that the obstruction will be removed. It is therefore important to position the ventilation
point as close to the stove as possible to minimise the cold air passing through and cooling the room and its
occupants.
What happens when the wind blows in
the completely opposite direction will be
the complete opposite to the previous
example. The ventilator will now be facing
the low pressure side of the house and
any wind blowing will tend to cause the
ventilator to evacuate rater than supply
air to the house. This is obviously an
unsatisfactory situation and by knowing
that if air is rushing past the ventilator the
ventilator will again evacuate rather than
supply air we have our ventilator supplying
air only when the wind is blowing from
one of the four possible directions. (A
more obscure example of a ventilator working to evacuate rather than supply air is where the vent has been
fitted to a wall very close to a road and is affected by every large vehicle passing.) As with all the effects of wind
we have looked at, the reality is far more complex than wind coming from neatly defined directions to hit our
little symetrical house, and ensuring reliable ventilation whatever the wind is doing is something that cannot be
achieved easily and is often overlooked as the cause of a badly performing flue.
20
The most obvious solution is to purchase a stove which has the facility for being supplied with air directly from
outside the property. This removes the requirement for a permenant ventilation to the room in which the stove is
situated and because the inlet to the stove will be very much smaller than a room ventilator the wind will have
a proportionaly smaller effect.
Whilst it is unlikely that it will be possible to fit ventilators to all different facing walls of the room in which the
stove is installed if the total effective area of the ventilators is sufficient to meet with the legal requirements
fitting a ventilator to more than one wall will mean that each can be smaller and less affected by the wind and its
direction. Many double glazed windows now have permanent "Trickle" vents but unless you can establish what
the efective area of these will be when the curtains are closed I doubt that many building instpectors will allow
them to be taken to be part of the total ventilation area.
If the house has a suspended floor it might be possible to fit ducts under the floor to access air from all sides of
the property. Again the ventilators would be small and the wind direction would be of little importance but it
would be an expensive option.
Whatever ventilaton system is used the ventilators must be examined regularly to ensure they are free of
leaves and anything else that might blow about in the garden.
Equivalent Area (Free Air Requirement)
Equivalent area and free area of ventilators:
Equivalent area has been introduced into the Approval Document instead of free area for the sizing of background
ventilators (including trickle ventilators). Equivalent area is a better measure of the airflow performance of a
ventilator. Free area is simply the physical size of the aperture of the ventilator but may not accurately reflect
the airflow performance which the ventilator will achieve. The more complicated and/or contorted the air flow
passages in a ventilator, the less air will flow through it. So, two different ventilators with the same free area will
not necessarily have the same airflow performance. A new European Standard, BS EN 13141-1:2004 (Clause 4),
includes a method of measuring the equivalent area of background ventilator openings. As an approximation, the
free area of a trickle ventilator is typically 25% greater than its equivalent area.
As equivalent areas cannot be verified with a rule, it will be difficult to demonstrate to building control bodies
that trickle ventilators and similar products have the correct equivalent area unless it is clearly marked on the
product. For this reason, it is preferable to use ventilators which have the equivalent area (in mm2 at 1Pa
pressure difference) or equivalent area per metre (where the equivalent area of the product varies according to
length) marked on the product in an easily visible location. Where it is not practical for the manufacturer to mark
the ventilator because it can be used in conjunction with a range of other components, some form of temporary
marking for the installed system should be acceptable to the building control body.
Air requirement equivalent area. Building regulations Document J, advises that an air supply,permanently open
vents, should be installed for appliances:
If design air permeability >5.0m³/(h.m²) then 550mm²/kW of appliance rated output above 5kW
or
_
If design air permeability <5.0m³/(h.m²)
then 550mm²/kW of appliance rated output
Equivalent air is as measured according to the method in BS EN13141-1:2004
It is unlikely that a dwelling constructed prior to 2008 will have an air permeability of <5.0m³/(h.m²) at 50pa
unless extensive measures have been taken to improve air-tightness.
Example
A stove with a rated or nominal heat output of 8kW.
The heat output above 5kW in this instance is 3kW which at 550mm² per kW gives a requirement for equivalent
_
air at 1650mm² when permeability >5.0m³/(h.m²). If permeability <5.0m³/(h.m²)
then equivalent air would
need to be 4400mm².
21
Terminal Positions Need to Meet Regulation Requirements
If modifications have been done to the house or when a new flue is proposed which is not utilizing an existing
chimney note should be taken of the following requirements to be found in Document J of the Building
Regulations, 2nd edition, dated October 2004.
D
A
B
C
Point where flue passes through
Clearances to flue outlet
weather surface (Note 1,2)
A at or within 600mm of the at least 600mm above the
ridge
ridge
at least 2300mm
B elsewhere on a roof
horizontally from the
(pitched or flat)
nearest point on the
weather surface and:
a) at least 1000mm
above the highest point
of intersection of the
chimney and the weather
surface: or
b) at least as high as the
ridge.
below (on a pitched roof) or
C within 2300mm horizontally at least 100mm above the
to an openable window or top of the opening.
other opening. (Note 3)
within 2300mm of an adjoining at least 600mm above the
D or adjacent building, whether adjacent building.
or not beyond the boundary.
(Note 3)
Notes:
1) The weather surface is the building external surface, such as its
roof, tiles or external walls
2) A flat roof has a pitch less than 10°
3) the clearances given for A or B, as appropriate, will also apply.
22
Datum for
horizontal
measurements
150mm
max
Datum for
vertical
measurements
The datum for vertical
measurements is the point
of discharge of the flue, or
150mm above the insulation,
whichever is lower
Further Regulations Applicable to a House with an "easily ignited roof
covering"
1800mm
A
600mm
B
B
At least
2300mm
At least
1800mm
Outlets should
be above the
shaded areas
Area
Location of flue outlet
A
at least 1800 mm vertically above the weather surface
and at least 600 mm above the ridge
B
at least 1800 mm vertically above the weather surface and at
least 2300 mm horizontally from the weather surface
Sundry Items of Legislation That Govern Installations.
Debris Collection Space
Where a chimney cannot be cleaned through the appliance, a debris collecting space which is accessible
for emptying, and suitable sized opening(s) for cleaning should be provided at appropriate locations in the
chimney.
Masonry and Flue Block Chimney
Masonry chimneys should be built in accordance with Document J paragraphs 1.27 and 1.28. Flue block chimneys
would be built in accordance with Document J paragraphs 1.29 and 1.30. The thickness of the walls around the
flues, excluding the thickness of any flue liners shall be in accordance with Document J diagram 2.4.
Separation of Combustible Material from Fireplaces and Flues
Combustible material should not be located where it could be ignited by the heat dissipating through the walls
of fireplaces or flues. A way of meeting the requirement would be to follow the guidance in Document J diagram
2.5 so that combustible material is at least:
a)
200 mm from the inside surface of a flue or fireplace recess; or
b)
40 mm from the outer surface of a masonry chimney or fireplace recess unless it is a floorboard, skirting
board, dado or picture rail, mantel-shelf or architrave. Metal fixings in contact with combustible materials should
be at least 50 mm from the inside surface of a flue.
23
Flue Outlet Options
Top Flue Outlet
This is the most common flue option, with the flue pipe rising
vertically from the flue collar on the top of the appliance. We
would strongly recommend that there is a cleaning access
plate fitted to this first section of flue pipe.
Rear Flue Outlet
The flue collar is fitted to the rear of the appliance and a
"T" piece is used to attach the flue pipe to it. The maximum
distance horizontally allowed from the rear of the appliance
is 150mm (6 inches).
We recommend this flue option with stoves with an enamel
finish as any moisture from the ingress of rain water or
condensates from the products of combustion will not come
into contact with the stove directly.
Bottom Flue Outlet
80°C
01:03:2010
The bottom flue option can be found on pellet stoves
and boilers where a flue fan is incorporated within the
stove.
They are generally supplied with an adapter pipe as
illustrated in the picture opposite.
24
Bends in Flue Pipe
45° bend
Top Exit
A flue pipe shall have no more than four bends, each providing a maximum
change of direction of 45º, there should be not more than two of these bends
before an access point for sweeping and two between a sweeping point and the
flue terminal.
Offset
45° bend with
sweeping access
plate
Back Exit
For a back outlet application using a “T” piece, this should be treated as two 45º
bends. If a “T” piece is to be used, the horizontal flue run from the back outlet of the
stove shall only be used to connect the stove to a “T” piece and shall not be more
than 150mm in length.
On top exit stoves, ideally the flue should rise vertically 1 meter before the first
bend. It is permissible to have a bend no greater than 45° from the top flue outlet,
or off the top of a “T” piece.
Sweeping Access
Although many of the stoves Euroheat supply can give access to the flue
for it to be swept through the appliance, we would strongly recommend
that whereever possible a suitable sized opening for cleaning the flue
should be provided. This in general would be on the first length of flue
pipe from the stove.
In models where there is no access for cleaning the flue through the stove
it is mandatory that there is a suitable sized opening for cleaning the flue
provided.
As we strongly recommend that the flue should be lined this one cleaning
access opening will be sufficient to accomplish the sweeping of the
continuous flue liner. However in cases where the stove has been installed
into an unlined flue further sweeping access may be required within the
length of the flue.
25
Pre-fabricated Flues
Made in interlocking sections with a stainless steel outer casing surrounding high performance insulation and a
flue liner made of stainless steel. Some systems have a ceramic or refractory concrete flue liner which offers even
better resistance to corrosion.
The metal lined systems should give a normal life of 10 to 15 years or more when correctly installed, operated and
maintained. However, prolonged periods of slow burning particularly using solid fuels, combined with inadequate
cleaning of the flueways can cause corrosion damage which may reduce the expected life of the liner. If there is
a risk that these conditions can occur the non-metallic lined systems are a better choice and under normal use
should give a life in excess of 20 years.
APPROVALS
These systems must have a British Standard Kitemark to BS 4543:
1990 (1996) Part 2 (Note: Part 3 deals only with suitability for oil
firing systems) or to BS EN 1856-1: 2003. Alternatively a BSRIA
certificate or similar test reporting e.g. by TÜV that indicates
the product can satisfy Building Regulation requirements with
respect to the burning of solid fuel. The Approval Status listed
for the products was correct at the time of printing but it is
recommended that the manufacturer be consulted on the current
approval status prior to specification or purchase of the product.
NOTE: BS 4543 was withdrawn in March 2005. It was replaced
by BS EN 1856-1:2003. Products meeting the requirement of
the 2002 Edition of Approved Document J and the ‘Approved
Document J: 2002 Edition: Guidance and Supplementary
Information on the UK Implementation of European Standards
for Chimneys and Flues should have an equivalent designation
according to BS EN 1856-1 of T400 N1 D Vm L40040 Gxx where
L40040 is the minimum material specification in the National
Annex to BS EN 1856-1 and xx is the necessary separation from
combustible materials, when the product is tested in the fully
enclosed arrangement specified in BS EN 1859:2000 including
firestops. Alternatively products may have the designation T400
N1 D V3 L40040 Gxx having been independently tested for their
corrosion resistance according to Annex A3 of BS EN 1856-1:2003.
Such products will carry a CE mark.
IMPORTANT
The manufacturers, instructions for the installation of the flue
systems must be complied with at all times.
26
Installation into a Flue Block System
Factory Made Precast Block Built Chimneys
Precast concrete blocks incorporating a flue way for building into brick or block walls, or freestanding over limited
heights. These lightweight units are easy to handle and therefore can offer a reduction in the cost of installing a
chimney in a new house. The sections are designed to fit together and require only a small amount of mortar to
provide an airtight joint.
The minimum dressing to the finished chimney is a cement wash although the chimney can be brick clad above
the roof level.
When correctly installed, operated and maintained these systems should last the life of the dwelling.
APPROVALS The chimney block system must satisfy Building
Regulations. This can be achieved by meeting the requirements
of the 2002 Edition of Approved Document J that specifies
“flueblocks" whose performance is at least equal to that
corresponding to the designation T450 N2 S D3, as described
in BS EN 1443:1999 (now 2003 with the changed designation
T450 N2 D 3 Gxx where xx is the necessary separation from
combustible materials).
Reference is made to the example of clay flueblocks at least
meeting the requirements for Class FB1 N2 as described in
BS EN 1806:2000. Concrete flueblocks at least meeting the
requirements for Class C2 as described in BS EN 1858:2003
(equating to T400 N2 D3 Gxx) are also considered suitable by
reference to the ‘Approved Document J: 2002 Edition: Guidance
and Supplementary Information on the UK Implementation of
European Standards for Chimneys and Flues’.
In the absence of there being a UK Notified Body covering
testing to these standards, manufacturers may seek to
have their chimney block systems tested by a recognized
independent body that provides a certificate confirming the
performance levels on the basis of testing to the relevant
procedures specified in the appropriate standard. Recognized
testing bodies might include the BBA, BRE Certification, BSRIA
and CERAM. Other solutions for demonstrating compliance with
the Building Regulations may be acceptable as defined in the
Approved Document J.
It is recommended that the validity of certification for the
liners or evidence of compliance with Building Regulations be
checked prior to purchase.
IMPORTANT
The manufacturers' instructions for the installation of the flue
systems must be complied with at all times.
27
Examples of Flue Installations
Example 1
Using an Existing Chimney or Claypot Lined Flue.
Rear exit using a Tee piece and the claypot lined flue lined with an approved stainless steel liner the same
diameter as the flue outlet of the stove.
Mulitfuel cowl
Chimney pot
Flexible liner
Clay liner (when fitted)
Horizontal register plate
Flue with debris trap
and cleaning access
Provision of
non-combustible
hearth
28
Example 1b:
Using an Existing Chimney or Claypot Lined Flue.
External flue cleaning access, the liner being the same diameter as the flue outlet on the stove.
Clay liner (if fitted)
Flexible liner
Liner support bracket
Flue cleaning access
(external whenever
possible)
Register plate
Provision of
non-combustible
support hearth
29
Example 2:
Precast Chimney Block System Internal to the Property
Precast chimney blocks
Warm air
ducts to room
Warm air ducts
to room
Chimney
adaptor
Chimney support lintel
with warm air ducting
facilities
Chimney breast
support lintel
Cleaning
access
Provision of
non-combustible
support hearth
30
Example 3:
Precast Chimney Block System External to the Property
Multifuel cowl
Brick cladding
Precast chimney blocks
Vitreous enamel
flue with
cleaning
access
Flue cleaning access
(external whenever
possible)
Provision of
non-combustible
support hearth
31
Example 4:
Prefabricated Twin Walled Chimney System Internal to the Property
The stove Situated in a Fireplace Recess.
Twinned walled metal
flue to suitable height
with cowl
Through roof flashing
Ceiling support plate
Fire stop plate
Flue support
bracket
Cleaning access
Chimney breast
support lintel
provision of
non-combustible
hearth
(with supports
where required)
Optional fireplace
surround
32
Example 5:
Prefabricated Twin Walled Chimney System Internal and External to the Property
The Stove is Free Standing in the Rroom.
Twin walled metal
with cowl
Twin walled metal
with cowl
Through roof flashing
Ceiling support plate
Wall bands to support
flue system
A
B
Fire stop plate
135º Tee with end cap
for cleaning access.
Cleaning access
A. Twin wall insulated flue system internal to property.
B. Twin wall insulated flue system routed externally.
33
Spring and Autumn Syndrome
During the very changeable weather conditions of Spring and Autumn the outside temperature can rise suddenly
and become warmer than the temperature within the house. This causes the air within the flue to reverse its
normal flow pattern and air travels down the flue. The most obvious outcome of this will initially be a smell from
the flue and whilst this is not harmful it may be unpleasant if the flue has not been swept as often as it should
have been.
Because of the warmer outside temperature the house will feel colder than it actually is, and the desire to light
the stove and at least match the outside temperature will reveal another problem, the stove will not light. If
sufficient air is coming down the flue the stove will appear to begin its lighting cycle but smoke will emanate
from what are normally air inlets and into the room. The stove may continue to operate in this fashion for a
considerable time but because the flue is operating in reverse there is no possibility of any hot air produced by
the stove travelling up the flue, to warm it, and reverse the flow.
If the house feels colder than the outside temperature do not light the stove without clarifying the that the air is
travelling up, rather than down, the flue. As mentioned previously a smell of soot is an indication that the flue is
operating in reverse but by opening the stove’s door and placing a hand within the stove it should be possible
to confirm the air flow.
Leaving the stove door open a few millimetres on lighting for a few minutes may allow enough air through the
flue to warm its fabric sufficiently to at least stall the air flow which will make lighting possible, never leave the
stove unattended when the door is open. Another way of getting some heat into the flue is by crumpling a few
sheets of newspaper and lighting those, again just pushing the door nearly closed allowing some air in around
the door, this will cause a rush of heat into the flue and once the paper has all burned away you can then continue
with the normal lighting procedure for the stove. If this fails the practice of directing warm air from a hair dryer
into the stove is a solution chosen by some, who report it to be effective. However, do not attempt this procedure
unless the stove is scrupulously clean and free of all ash, dust and any other debris; the air flow from a hair dryer
is surprisingly powerful.
If lighting the stove under these conditions proves to be more than an infrequent irritation you might like to
consider purchasing an electrical flue heater band which is permanently attached to the flue pipe and when
required heats the flue pipe noiselessly and without dust.
This is only available for the stoves with a 125mm (5 inch) flue outlet.
Top flue
Rear flue
Run Back Timer
MS10000
This
switch
when
activated will allow
power to the heater
band for up to 4 minutes
heating the heater
band to its optimum
and then switching
it off automatically
prolonging the life of
the heater element.
The heater band clamps around the flue pipe close to the stove (see diagram) and plugs directly into a standard
electrical socket. Prior to lighting the stove the heater band should be switched on and allowed to heat the flue
pipe for a period of time, this will depend greatly upon the flue environment. Heating the flue pipe will introduce
heat into the flue so helping to initiate a flue draught. Once the stove is lit the heater band must be turned off as
prolonged use may cause damage to the heating element. To control the switching of the heater band the use
of a run back timer ( MS 10000 ) is recommended.
34
Flue Size Comparison / Volume Increase
The change of flue size by an inch or two might not seem significant but the resultant change in flue volume and
surface area can be dramatic.
Pipe Diameter / Cross Sectional Area
5” or 125 mm = 19.6 in²
6” or 150 mm = 28.3 in²
7” or 175 mm = 38.5 in²
8” or 205 mm = 50.3 in²
9” or 230 mm = 63.6 in²
or
or
or
or
or
12271 mm²
17671 mm²
24053 mm²
33006 mm²
41548 mm²
Pipe Diameter / Area Increase
5” or 125 mm to 6” or 150mm is an increase of 20%
6” or 150 mm to 7” or 175 mm is an increase of 17%
Pipe Diameter / Volume Increase
Pipe Diameter / Circumference
5” or 125 mm = 15.7in
6” or 150 mm = 18.8in
7” or 175 mm = 22in
8” or 205 mm = 25in
9” or 230 mm = 28in
35
or
or
or
or
or
393mm
471mm
550 mm
644 mm
722 mm
Need more info?
with over 10,000 pages of technical information, spare parts,
product shots, news and 1001 other things, - you will not find
a more comprehensive solution to your queries, whatever
time of the day.
www.euroheat.co.uk
Court Farm Business Park, Bishops Frome,
Worcestershire WR6 5AY
Pre sales: 01885 491112
Technical: 01885 491117
Reception: 01885 491100

IN1173 Flue System Planning Guide Multifuel and Pellet Stoves

Transcription

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