Low Energy Heating Systems

Heating Systems for Low Energy
Buildings
Introduction ..................................................................................................................................................2
The design process........................................................................................................................................4
Energy efficiency ..........................................................................................................................................5
Heat Generation ...........................................................................................................................................7
i) Combined Heat and Power (CHP) ...................................................................................................7
ii) Multiple Boiler Arrangements ........................................................................................................7
iii) Heat Pumps .....................................................................................................................................7
iv) Electric Heating ..............................................................................................................................8
Heat Distribution Systems ..........................................................................................................................8
Heat Emitters ..............................................................................................................................................10
i) Radiators ..........................................................................................................................................10
ii) Natural Convectors ........................................................................................................................10
iii) Fan Convectors .............................................................................................................................11
iv) Underfloor Heating.......................................................................................................................11
v) Warm Air Heaters ..........................................................................................................................11
vi) Radiant Panels...............................................................................................................................11
Hot Water Plant..........................................................................................................................................12
i) Central Calorifier Systems..............................................................................................................13
ii) Central Self-Contained Systems ...................................................................................................13
iii) Local Storage Systems .................................................................................................................13
iv) Point of Use Water Heaters..........................................................................................................13
v) Solar Thermal Water Systems.......................................................................................................13
Summary......................................................................................................................................................14
Heating Systems for Low Energy Buildings
Page 1 of 14
Introduction
The heating of buildings accounts for over 40% of UK non-transport energy use (Figure 1). This
is typically over 60% of domestic energy use, rising to ~80% if water heating included. Given
the current targets for the reduction of energy use and associated CO2 emissions, a good design
of heating system is essential to ensure efficiency and effective use of energy. A major problem
is oversizing of heating systems, which can lead to reduced efficiency in part-load operation.
The energy consumption of for oversized plants can be 50% more than necessary [source:
CIBSE KS:8]. For low energy buildings this might become a greater problem because few
engineers will have worked on a low energy building.
Figure 1 Pie chart showing the breakdown of non-transport related energy consumption in the UK.
There are many options available when designing a heating system. The fundamental
components are:
•
•
•
A means of generating the heat, the heat source.
A means of transporting that heat to where it is needed, the distribution network.
A means of delivering the heat into the space to be heated, the heat emitter.
Table 1 Examples of common heat sources, distribution networks and emitters.
gas
CHP
LPG
solar
oil
biomass
Heat Source
coal
off peak electricity
electricity
wind
Heat pumps, ground or air source
Water: low, medium or high temperature
Distribution network
air
steam
electricity
radiators
ceiling panels
fan convectors
natural convectors
Heat emitter
panel heaters
underfloor heating
unit heaters
storage heaters
high temperature radiant panels
Heating Systems for Low Energy Buildings
Page 2 of 14
The first thing to consider is why a building needs a heating system?
• To keep the building up to temperature when occupied,
• To pre-heat the building before occupancy.
These traditional answers revisiting when discussing low energy / low carbon buildings.
Some of the options available are listed in Table 1. This gives rise to many different
permutations. Whilst heating systems may seem simple, in practice there are many factors to be
considered during the design process, in order to achieve a well-designed system that delivers
the required level of thermal comfort for minimum energy usage.
The airtightness and increased thermal insulation in a low energy building implies that for most
buildings, most of the time, gains from occupants and equipment will account for most of the
losses from the building including ventilation losses. However, because these gains may be
localised within certain parts of the building there is a need to move excess heat around the
building to where it is needed. This achieves two things, firstly it stops occupants having to
dump heat outside of the building, which is wasteful, and secondly it will heat areas of low
occupancy and stop these from triggering the heating system. So an important question to ask is:
How will heat be moved from spaces with high gains to those with low gains?
An example: three models of primary schools, with identical U-values, occupancies and
ventilation requirements. However, one has radiators (or other typical heaters) and opening
windows provides ventilation, another has a mechanical ventilation system with heat injected
into incoming air, exhaust air expelled via the corridors. Another has the same mechanical
ventilation system but with the addition of a heat exchanger.
Annual Heating Energy:
Normal heating + windows = 36.8 MWh
Mechanical ventilation = 34.8 MWh
Mechanical ventilation + heat exchanger = 10.3 MWh
The system with the heat exchanger uses less than 30% of the energy of the standard system.
Even just the ability to move heat form the classrooms to the unoccupied corridors as in the case
of the mechanical ventilation only system uses less energy. Thought needs to be given to
ventilation strategies in low energy buildings and what happens to the heat in the air.
Returning to the question of pre-heat phase, a low energy building should need less heat
because it will still have suffered a smaller reduction in temperature overnight due to the
increased airtightness and insulation levels. Ignoring the question of boiler efficiency for the
time being, it is generally more energy efficient to keep the pre-heat phase as short as possible,
as the heat loss from a building is proportional to the internal-external temperature difference.
However, for an airtight well insulated building this becomes less important as the heat cannot
escape as easily. This implies that a long pre-heat phase can be used and potentially reducing
the peak load. Hence we can say:
A low energy building can afford a longer pre-heat period and a smaller heating system.
Ideally a reasonable portion of the excess cost of producing a low energy building will be repaid
through the reduction of running costs and installation of smaller systems. Another observation
is that because the annual demand for heat will be much lower than usual the capital cost of the
Heating Systems for Low Energy Buildings
Page 3 of 14
system per unit heat over the lifetime of the system is likely to be much higher. This suggests
that capitally expensive systems (e.g. underfloor heating) might not be a sensible choice if
working to a tight budget.
In general we can conclude that the heating system for a low energy building needs to take into
account the fact that it is sited within a low energy building, hence:
A heating system for a low energy building is not just a normal system made smaller, there are
implications for changes to current practices.
The design process
Design involves translating the needs of the client into proposals and specification of specific
products. There are two main characteristics that define the design process. Firstly, evolution of
a design through a series of stages during which the level of detail is increased. Secondly,
iterative cycles of design where the proposed design is trialled, tested, evaluated and refined.
Feedback is therefore a major contributor to the design process as shown in Figure 2.
Figure 2 Representation of the design process, adapted from CIBSE KS:8.
The aim of the design process is to deliver thermal comfort, most clients do not ask for a
heating system as part of their design brief, their focus is on what the system delivers. Although
there are many factors to consider, thermal comfort is fundamentally about how people interact
with their thermal environment. In order to provide good levels of thermal comfort throughout
the occupied areas of the building knowledge is needed of occupancy levels, internal / solar
gains and building usage.
The four main environmental factors that influence thermal comfort and hence productivity are:
• Air temperature
• Relative humidity
• Mean radiant temperature
• Air velocity.
Heating Systems for Low Energy Buildings
Page 4 of 14
Different people have different preferences with regards to temperature and ventilation.
Comfort metrics such as percentage people dissatisfied can be used to check that heating
systems are suitable for the required situation.
Figure 3 Example of temperature gradients created by different types of heating systems. Adapted from
CIBSE Guide F.
To promote thermal comfort and productivity large temperature gradients (both vertically and
across rooms) should be avoided. Figure 3 shows different temperature gradients created by
different heat emitters. Ventilation strategies should also be chose to avoid the creation of
draughts as this can have a negative impact on thermal comfort. Thought should be also given
as to the effects of glazing and solar gains of the mean radiant temperature felt by occupants in
the room. The placement of the heating systems requires some thought as the radiant
temperatures and air velocities can vary for different locations within a space.
Also as shown in Figure 3 some heating systems such as warm air can lead to temperature
stratification in space. This mans that the inside temperature at high level can be higher than
that used in the heat loss calculations and therefore the heat loss through the roof or ceiling can
be higher than expected. A correction for the type of heat emitter and the height of the space
will need to be applied to ensure estimates of heat loss are feasible, for example a 5-15%
increase in the fabric heat loss is expected for a low level warm air system in a space 5-10m
high. Further details can be found in CIBSE Guide A. This is part of the reason why roof Uvalues are lower than those for external walls: hot air rises.
Energy efficiency
•
•
•
•
•
Incorporate the most efficient primary plant to generate heat / hot water. But also take
into account whether such a source leads to a high or low overall system efficiency.
Optimise the use of renewable energy sources (FITS and RHI can help finance this).
Ensure that heat / hot water s distributed effectively and efficiently.
Include effective controls on primary plant and distribution systems to ensure that heat
is only provided when and where it is needed and at the correct temperatures.
Be responsive to changes in climate, internal and solar gains, occupancies and usage.
CIBSE guide F states that designers should:
• Select fuels and tariffs that promote efficiency and minimise running costs.
• Consider de-centralised heating and hot water generation plants on large sites to reduce
standing losses and improve load matching.
• Locate plants to minimise distribution losses.
Heating Systems for Low Energy Buildings
Page 5 of 14
•
•
•
•
•
Ensure distribution systems are sized correctly to minimise pump and fan energy
consumption, also to reduce potentially increased maintenance costs.
Insulate pipework, valves etc to reduce distribution losses.
Ensure the base-load is provided by the most efficient plant.
Utilise condensing boilers where feasible and appropriate, these have the benefit that
they can be more efficient at part load than at full load and are therefore useful in small
installations.
Consider heat / energy recovery where feasible.
Remember: In a low energy building people and incidental gains should be considered a main
source of heat.
Figure 4 Illustration of typical efficiencies for different low temperature hot water (LTHW) boilers at
different loads. Adapted from CIBSE Guide F.
With ever tightening building regulations and the drive for more energy efficient buildings, the
levels of insulation present in walls and roofs is increasing. As the level of insulation in building
elements increases heat loss through those elements is decreased, and hence other heat loss
routes become more important. In highly insulated buildings heat loss through infiltration is a
major heat loss path accounting for up to 50% of heating load in small buildings. Ventilation is
also a major consideration and in buildings where high levels of ventilation are required;
offices, schools etc the use of heat recovery should be considered as a possibility.
As we have discussed oversizing can cause system inefficiencies, increased maintenance and
shortened lifespan. Therefore there is the need to address load diversity, not all peak loads will
necessarily occur at the same time so thought needs to be given to the correct sizing of heating
systems. Thought should also be given to the use of the building in question, the attire of the
people within the spaces and the level of activity of the occupants as these will have an impact
upon how the space should be heated and hence the peak loading calculations.
Heating Systems for Low Energy Buildings
Page 6 of 14
Peak loads may only occur for short periods of time, hence there are benefits to several smaller
systems rather than one large system. The primary system should be as efficient as possible
such as a condensing boiler if appropriate. Modular systems also have the benefit that extra
modules can be added at a later date if more heat is required due to changes in building use.
Heat Generation
There are several different ways heat can be generated for transportation to the space where it is
required. Some of the more common ways are detailed below.
i) Combined Heat and Power (CHP)
Combined heat and power (CHP) has a wide range of applications in buildings and can be used
to provide a large reduction in CO2 emissions. An important consideration when thinking of
CHP in a building is when the heat will be needed. Typically in the summer months there is
little demand for heat compared to the winter. This can have an impact of the sizing of the CHP
plant if the plant is too large energy is wasted (see Figure 5).
Figure 5 Example of possible implementation of CHP and loading when combined with a typical boiler
plant. Source: CIBSE guide F.
ii) Multiple Boiler Arrangements
Multiple boiler arrangements can be used to better match the demand for heat more closely and
hence improve energy efficiency (Figure 4). These may comprise an integrated package of
modules or independent boilers including CHP or Biomass (Figure 5). As load increases
individual modules are independently switched on. Since each boiler is operating close to its
own design load, efficiency is maintained. The overall plant can therefore provide an improved
part load efficiency. Careful sequence control is fundamental to the correct operation of this
system. In some instances such as for low temperature systems, it can be economic to specify all
the boilers in a multiple arrangement as condensing. However, in most instances it is more
economic to specify the lead boiler as condensing with high efficiency boilers to top up. This
minimises capital cost while keeping overall plant efficiency.
iii) Heat Pumps
Heat pumps can in theory produce a high coefficient of performance (CoP) when operating at
low temperature differentials (see Figure 6). Heat pump have found wide spread use in
applications where low-grade heat is available as a resource. Heat pumps are available in a
number of different forms and exploit different sources of low-grade heat.
Heating Systems for Low Energy Buildings
Page 7 of 14
Figure 6 Effect of temperature on heat pump CoP. Source: CIBSE guide F.
Air source heat pumps may be used to extract heat from outside air or from ventilation exhaust
air. When outside air is used as the heat source, the CoP tends to decline as the air temperature
drops (i.e. winter). Also problems in high humidity areas such as the UK can include icing of
the heat exchanger often requiring the heat pump to be run in reverse to de-frost, this reduces
the CoP. Air to Air heat pumps supplying heating only (typically only winter use), using outside
air as the heat source in the UK can have a relatively low CoP. Their main advantage is that
they are available for relatively small capacities compared to say Biomass boilers. They are also
simple to control and unlike pellet stores for don’t constituent a safety hazard.
Ground or water source heat pumps extract heat from the ground, bodies of water such as lakes
at ambient temperature. These heat sources have greater specific heat than air and provided
there is sufficient mass, the temperature of the heat source should remain fairly constant over
the year.
iv) Electric Heating
Various systems are available with outputs up to 5 kW; these include panel radiators, natural
draught and fanned electric convectors. These systems are generally:
• Inexpensive to install
• Can reduce space requirements
• Require little maintenance
• Provide quick response to controls
• Are highly efficient but can have high CO2 emissions
• Are suitable for intermittently heated areas
• Have high running costs.
Due to the high unit cost of electricity, high levels of insulation and good central/local control
of the system is required for the efficient use of electric heating. Electric storage heaters can
take advantage of low electricity costs at night. They also have a low capital cost, are easy to
install and are maintenance free. However, their main disadvantages are the limited charging
capacity and the difficulty of controlling output, often leading to expensive daytime re-charging.
Heat Distribution Systems
The characteristics of a heat distribution system can have a profound effect on the thermal
performance and energy consumption of the system. The main sources of inefficiency are:
•
incorrectly sized pump and fans resulting in high energy consumption.
Heating Systems for Low Energy Buildings
Page 8 of 14
•
•
•
The position of plant rooms and the subsequent length of pipework influences both
capital and running costs by increasing heat losses.
Inadequate insulation on pipework / ducting.
Part loads resulting in reduced efficiency.
As heating demand reduces due to increased levels of insulation and air tightness, the
importance of heating distribution losses increases. There is a need to choose the heat
distribution system carefully depending upon the type of emitters that will be used, the size of
the distribution network and the presence of any other processes that require heat or hot water, a
comparison between different heat distribution media is shown in Table 2.
Table 2 Characteristics and comparisons of different heat distribution media.
Medium
Characteristics
Pros
Cons
Low heat capacity, low
density, small temperature
difference between supply
and return so large volume
needed.
High heat capacity, high
density, large temperature
difference permissible
between supply and return.
Smaller volume required
than air.
Low temperature hot water
systems operate below
~90°C and at low pressures
that can be generated by an
open or sealed expansion
vessel.
Medium temperature hot
water systems operate
between ~90-120°C with a
greater drop in water
temperature around the
system. Pressurisation up to
5 Bar.
No heat emitters
needed
Direct heat, no other
medium or heat
exchanger needed.
Large volume of air
required, hence large
ducts, fans can have
high energy usage.
Small volume required
so small ducts, requires
less space.
Required heat emitters
to transfer heat to the
space.
Generally recognised
as simple to install and
safe to operate. Use
with condensing boilers
to maximise energy
efficiency.
System temperatures
limit output.
Higher temperatures
and larger temperature
drops give smaller
pipework, which may
be advantageous on
larger systems.
Pressurisation requires
additional plant,
controls and safety
requirements.
HTHW
High temperature hot water
systems operate at
temperatures over 120°C,
sometimes up to 200°C.
Even greater temperature
drops around system, with
pressurisation up to 10 Bar.
Higher temperatures
and temperature drops
give even smaller
pipework
Safety requires all
pipework must be
welded to the standards
required for steam
pipework. Unlikely to
be a cost-effective
method of heat transfer
except for over long
distances.
Steam
Exploits the latent heat of
condensation to provide very
high heat transfer capacity.
Operates at high pressures.
Principally used in hospitals
or kitchens where steam is
required.
Use of latent heat of
condensation allows
large transfer of heat.
Higher maintenance
and water treatment
requirements, extra
safety requirements.
Air
Water
LTHW
MTHW
If the site requiring heat is a disparate collection of buildings or perhaps one large building
complex there is the option of have a centralised heating system and distribution network or a
de-centralised collection of independent heating systems. Both types of system have benefits
and drawbacks a comparison between the two is show below in Table 3, the most efficient
Heating Systems for Low Energy Buildings
Page 9 of 14
combination of heat distribution media and system will depend upon the site under
consideration.
Table 3 Comparison of centralised and de-centralised systems.
Centralised
Capital cost per unit falls with
increasing capacity of central plant.
Capital cost
Space requirements
System efficiency
System operation
Capital cost of distribution is high.
Space requirements of central plant
and distribution systems are
significant, particularly ductwork.
Large, high flues needed.
Central plant tends to be better
engineered, operating at higher
system efficiencies, due to higher
more stable loads.
As load factor falls, total efficiency
falls and loses can be significant.
Convenient for some institutions to
have a centralised plant.
Distribution loses can be significant.
System
maintenance
lifetime
Central plant tends to be better
engineered, more durable.
and
Fuel choice
Less resilience if no standby plant
provided.
De-centralised
Low overall capital cost, savings made
on minimising the use of air and water
distribution systems.
Smaller or balanced flues can often be
used.
Flueing arrangements can be more
difficult in some locations.
Energy performance in buildings with
diverse patterns of use is usually better.
May require more control systems.
Zoning of the systems can be matched
more closely to the occupancy patterns.
Can be readily altered and extended.
Equipment tends to be les robust with
shorter operational life.
Flexibility in choice of fuel, boilers
can be dual fuel.
Plant failure only affects local services.
Fuel needs to be supplied throughout
the site.
Better utilisation of CHP, etc.
Boilers tend to be single fuel.
Heat Emitters
Different types of heat emitter have different output characteristics in terms of the ratio between
radiant heat and convection. Where there is a high ventilation rate, the use of radiant heating
systems results in lower energy consumption. Fully radiant systems heat occupants directly
without heating the air to full comfort temperatures.
i) Radiators
Radiators have a convective component of 50-70%, they are cheap and easy to install. Control
is simple but can be slow to respond. Local control is available through the use of thermostatic
valves.
ii) Natural Convectors
Natural convectors have a convective component of ~80% and tend to produce a more
pronounced vertical temperature gradient, which can result in inefficient use. These are best
used in well-insulated rooms with a low air change rate and relatively low ceilings. The
convectors themselves are often unobtrusive being incorporated in to the floor or skirting or a
room.
Heating Systems for Low Energy Buildings
Page 10 of 14
iii) Fan Convectors
Fan convectors like natural convectors are suitable for well insulated rooms with a low air
change rate. These can be directly incorporated into mechanical ventilation systems. Fan
convectors respond very rapidly to control and can have a variable output allowing for greater
control. The high cost of electricity means that these are not the most economical heating
system, they also have a higher maintenance requirement than other systems and can be noisy.
iv) Underfloor Heating
Underfloor heating usually consists of a low temperature warm water distribution system set
into the floor slab. This type of system gives a slow thermal response and it better suited to
spaces where there is continuous occupation. As the heat source (the water) is at a relatively low
temperature compared to other systems there is a level of self-regulation. As the room warms up
the temperature difference between the air and the floor decreases as does the heat output.
Generally operating between 35-45 °C, these systems provide an ideal opportunity to use
condensing boilers due to the low return water temperatures. Heat pumps are equally
advantageous in underfloor systems due to the low temperature of the water required. There is a
high thermal inertia with underfloor systems and therefore are less responsive to local heat
gains. Underfloor heating is difficult to control in buildings with fluctuating gains such as a
school. If the building is up to temperature at occupancy the underfloor heating system can be
turned off but the high thermal inertia of the building slab in which it resides will continue to
emit heat until it reaches the same temperature as the room. There is unlikely to be any use for
this heat and the room temperature will require the occupants to dump the heat by opening
windows etc. If this occurs then low energy aspirations will not be realised. Underfloor heating
systems are therefore best suited to situations where there is consistent internal gains and
preferably constant occupancy, otherwise a sophisticated control system is required to predict
when systems need to be turned off prior to reaching heating set points.
v) Warm Air Heaters
Warm air systems have a quick thermal response but can promote temperature stratification.
The units can be directly fired so distribution losses are reduced, but the systems are often noisy
and require large lengths of ductwork, which increase fan power requirements.
vi) Radiant Panels
Radiant panels can be supplied with medium or high temperature water or steam and have a
radiant component of ~65%. These are best suited for use in large spaces with high air change
rates. They have the benefit that they heat the occupants and building fabric rather than the air
within the building. Due to the temperature of the panels care needs to be taken to ensure the
occupants cannot burn themselves. The height of the panels is important, if they are positioned
too low then overheating can occur, whereas locating them too high can result in increased
energy consumption due to reduced comfort levels and longer running times.
Heating Systems for Low Energy Buildings
Page 11 of 14
Table 4 Comparison between different heat emitters.
Design points
Radiators
Output up to 70%
convective.
Occasionally limit on
surface temperatures.
Natural convectors
Pros
Good temperature
control.
Balance of radiant and
convective heat gives
good thermal comfort.
Low maintenance, cheap
Quicker to respond to
control.
Skirting or floor trench
convectors can be
unobtrusive.
Cons
Fairly slow to respond to
control.
Slow thermal response.
Can occupy more floor
space.
Can get higher
temperature stratification
in space.
Can be noisy.
Higher Maintenance.
Occupies more floor
space.
Fan convectors
Can also be used to
deliver ventilation air.
Quick thermal response.
Underfloor heating
Check required output
can be achieved with
acceptable surface
temperatures.
Unobtrusive.
Good space temperature
distribution with little
stratification.
Heat output limited.
Slow response to control.
Quick thermal response.
Can be noisy.
Can get considerable
temperature stratification
in space.
Unobtrusive.
Low maintenance.
Slow response to control.
Quicker thermal
response.
Can be used in spaces
with high air change
rates and high ceilings.
Need to be mounted at
high level to avoid local
high intensity radiation
and discomfort.
Can be direct fired
units.
Warm air heaters
Low temperature
radiant panels
High temperature
radiant panels
Ceiling panels need
relatively low
temperatures to avoid
discomfort.
Can be direct gas or oil
fired units.
Check that irradiance
levels are acceptable
for comfort.
Hot Water Plant
As we can see from Figure 1 the heating of hot water is a large proportion of the total energy
use of buildings in the UK. In a low energy building where heating energy usage has been
minimised this proportion grows. Once we reach Passivhaus-style efficiencies it could well be
greater than the space heating demand. As a major contributor to the total energy use of a
building and potentially a stumbling block towards achieving a truly low energy building equal
thought should be given to the design of the hot water system as is given to the heating system.
The key will be an accurate idea of the true hot water demand. It is worth comparing the effort
that is undergone to estimate heat loss from the building via a thermal model against that used
to predict hot water demand – usually just a value plucked from a table!
Hot water plants should always be sized correctly thus minimising capital and running costs.
Reducing temperatures saves energy and reduces the risk of scalding, but this should not be
achieved at the expense of safety. To avoid multiplication of legionella, hot water should be
stored at a temperature of 60 °C and should be distributed such that a temperature of 50 °C is
achieved at the tap within 1 minute. This requires thought when considering low energy design.
In practice for all but domestic properties this means that either water is circulated centrally
around the building to maintain its temperatures, electric or trace heating is placed along the
pipework. Both these approaches will use substantial amounts of energy and this indicates that
unless hot water use is high, point of use might be better.
Heating Systems for Low Energy Buildings
Page 12 of 14
The main types of hot water system are:
•
•
•
•
Central calorifiers, supplied by the main heating system (see previous sections).
Central self-contained gas or electric.
Local storage, gas or electric.
Local point of use, usually electric.
i) Central Calorifier Systems
Where hot water loads are high and distribution systems are compact, a well controlled central
boiler / calorifier plant can operate reasonably economically. However, where hot water loads
are not substantial, separate heating and hot water plants will be more energy efficient
especially during the summer months. Combination boilers can provide an energy efficient way
of achieving both heating and hot water in a small (typically domestic) centralised system. Hot
water is heated almost instantly on demand, although interaction between the heating and hot
water may occur in winter if the demand is high.
ii) Central Self-Contained Systems
Self-contained central hot water systems are typically more efficient than systems combined
with the main heating. This is because standing losses are lower and the poor part-load
efficiencies characteristic of boilers sized for the full heating duty are avoided during summer
operation. Relatively high efficiency storage water heaters are commonly used and condensing
versions offer a further high efficiency option. Electric immersion heating is generally only
economic (due to high electricity costs) where there is adequate capacity for off-peak storage.
The system needs to be controlled and sized to prevent expensive daytime top-ups.
iii) Local Storage Systems
Small-decentralised gas fired storage water heaters close to the point of use can significantly
improve efficiency since the standing and distribution losses are greatly reduced. The problems
associated with legionella are also normally reduced with localised systems. Capital and
maintenance costs are likely to be higher, but this is usually more than compensated for by the
increased efficiency. This type of system is ideally suited to situations where there are short
periods of high demand such as catering and sports facilities.
iv) Point of Use Water Heaters
Evidence suggests that point of use water heating is extremely economical where the demand is
low, e.g. offices without catering facilities. Capital cost and delivered energy consumption is
generally low. Water softening may be required to prevent scaling of units, adding to cost but
reducing maintenance.
v) Solar Thermal Water Systems
The addition of solar thermal water systems to a building can reduce the fossil fuel energy used
to heat water. It is unlikely that a solar thermal system will be able to provide sufficient heat all
year round but it will dramatically reduced energy costs in the summer months and pre-heat
water in the winter months thereby reducing water-heating costs. Capital cost is high but the
renewable heat incentive (RHI) provides a financial incentive for such systems. Solar thermal
units require space that could be used for other renewable systems such a photovoltaics
therefore thought needs to be given to the use of the building and which system will give the
greatest benefit. One important issue for non-domestic properties is that solar hot water will still
require the use of trace heating or continuous pumping so as to guard against legionella as
described above, greatly reducing its carbon credentials.
Heating Systems for Low Energy Buildings
Page 13 of 14
Summary
Heating is often considered a simple, basic system, however there are many options and many
different permutations to be considered. Most buildings require heat but different building
types, uses and locations have different heating requirements. A holiday cottage on Dartmoor
has different heating requirements to a city centre ground floor flat. A department store has
different heating needs to a large warehouse DIY store. The construction of the building is also
important as is the level of internal and solar gains. A school of heavy weight construction with
long unoccupied periods will require a different heating solution to a school of lightweight
construction. It is important when designing the system to understand the needs of the client and
the type and usage of the building in order to choose and appropriate heating system.
The choice of internal and external design conditions can have a substantial impact on the initial
system loads and subsequent system performance. These are fundamental part of heating load
calculations and the choice should be very carefully considered. For example the difference
between assuming an internal-external temperature difference of 21 K (-1 °C to 20 °C) and one
of 25 K (-4 °C to 21 °C) for a particular building is nearly a 20% increase in the heat loss an
hence an increase in heating requirement. It is also important to consider what system
performance criteria are required and the level of control that is acceptable. Establishing the
required system performance criteria at the briefing stage is one of the most crucial tasks in the
design process. It is vital that clients and designers have a thorough understanding of what
conditions are required and what can practically be delivered. For example the difference
between a specified internal condition of 21 °C ± 1 °C and 21 °C ± 2 °C can have a
considerable impact on the buildings energy consumption, control choice and system
performance. The closer the control the more expensive the system, if the design conditions can
be relaxed (within reasonable limits) the system can be simpler, cheaper and use less energy.
Therefore it is important to have a dialogue with the client to understand their specific
requirements and make them aware of the possible need to change current practices in order to
achieve a low-carbon building.
One key question is, how will the system be controlled. In any modelling it might well have
been assumed that all rooms experience perfect control. But in the real building if a room does
not have a temperature sensor how will this control be achieved? In reality occupants may well
open windows to dump heat while the system is still injecting heat into a space due to
insufficient control (the thermostat may be in a different cooler space). If this happens then it is
unlikely you have a low energy building.
The heating system, the controls and the occupants are not adjuncts to the low energy building:
they are as important as the insulation in the walls.
Heating Systems for Low Energy Buildings
Page 14 of 14