Safe Room HEPA Filter - The Preparedness Podcast

Safe Room Fan
September 6, 2004
Using the information based on a “Fan in a Can” Safe Room filter from
(alpharubicon.com), I set out to create my own safe room fan/filter unit. The result was a
successful creation of a fan assembly and safe room test. Here’s how I did it:
The materials needed for this fan assembly:
1- Squirrel cage fan from Grainger, part number 4C447
(2011 Note: This fan is now discontinued, but pulled 135 CFM @ 0.500-In. Static
Pressure; the Dayton 7063-3277 is an exact duplicate)
1- Power supply cord, 12 feet, from Grainger
2 – Cable/cord connectors
1- 30mm ammo can w/ good gasket, inside dimensions: 8.75” x 17.25” x 14”
1- Tube silicone sealant
3- Wire nuts, 16 gauge
6 inches of wire
4 – bolt assemblies, 1/2 inch length (bolt, nut, lock washer, flat washer)
1 – HEPA filter, 99.97% efficiency (I used a Holmes HAPF 600)
I ordered the fan from Grainger, along with a power cord because one does not come with
the fan. I also ordered 2 cord connectors to provide stress relief for the wiring
connections.
Construction
The green wire on the power cord goes to the ground connection on the fan, I had to use a
short length of wire for this; the other two wires can go to either fan lead, as it’s an AC
motor. Once this was wired up, I plugged it in for a test. I used wire nuts so that field
repairs could be effected easier than if I had soldered the connections. I used one
cable/cord connector on the fan housing (there’s a pop out on the fan housing for it) for
stress relief.
One side of the fan’s squirrel cage was higher than the other and was scrapping the intake
housing. Closer inspection showed that the motor was not mounted evenly. I removed
the intake cowl and, using a Torx driver, I loosened the cam nut and removed the cage
from the shaft. I adjusted the fan mounting screws so that the motor was more parallel
with the housing, but it was still canted. However, the shaft was long enough to provide
plenty of adjustment on where the cage sat, so I just lowered it enough so that it didn’t
scrape the housing.
Placing the fan in the location where it was going to be in the ammo can, I marked the
holes for the bolts with a marker. I drilled two holes and then lined up the fan on the
outside to mark the other two mounting holes. This was necessary because I wasn’t able
to access the other holes to mark them. This worked out fine, as the fan’s mounting
frame is symmetrical. Once the holes were drilled; I did a quick mounting check with the
bolts to ensure that everything lined up properly. Since everything lined up, I grinded
down the holes to remove burrs and moved onto the next step of cutting the exhaust hole.
I drew a line from caddy-corner holes so that an X was formed. This X allowed me to
center the cardboard template I used to outline where the hole was going to be cut.
(Note: The only tools I had available to me were a drill and a jigsaw. There are other
ways of accomplishing cutting in metal, but I used what I had.) I drilled a half-inch hole
and using a jigsaw and a metal-cutting blade, cut out the square hole for the fan exhaust.
After a quick check to make sure the hole was the size I wanted, I grinded the edges to
remove burrs. I made the hole of the exhaust slightly smaller than the fan’s output for
two reasons. First, I didn’t want the hole to be too big and have problems making an
airtight seal, and second, I wanted a small lip in order to attach duct tape to.
Cutting the hole for the filter was next. The only filter I found that was worth using was a
99.97% HEPA filter from Holmes. Not only was it a 99.97% efficiency, but it was also
rated to last 12-18 months and had a built-in rubber gasket on the bottom that would aid
in creating an air-tight seal. It’s larger than I wanted, but with limited availability in filter
sizes, I decided to see if I could make it work. The problem was that the filter was so
large that I would need to remove a portion of the support brace on the side of the ammo
can in order to have room to tape the filter to the can. This caused some concern, as I
wasn’t sure if it was possible to remove this without compromising the integrity of the
airtight seal.
In the image below, you can see two outlines, the inner outline is the hole size that
needed to be cut out and the outer outline is the size of the filter. I figured that I would
need at least an extra inch of support removed in order to have enough space to
effectively tape the filter to the can.
Looking at the support brace more closely, I found that it was held in place by four pairs
of spot welds, and I hoped that it also wasn’t glued into place. Drilling 4 holes at each
corner for the jigsaw blade, I proceeded to cut the hole for the intake. Once the hole was
cut, including the small bit of support brace that needed to be removed, I was able to see
if the support brace was glued; luckily it was not.
Now that I knew that the support was only held in place with only spot welds, it made the
prospect of removing the part that I didn’t want more likely. But, the trick was to do it
without cutting through the skin of the ammo can. I tried using the jigsaw, but it only
took two seconds to determine that it wasn’t going to work. I ended up pulling out my
Dremel tool and used a cutting wheel to delicately cut through the support without
damaging the ammo can itself. I used a screwdriver to lift the support away from the
ammo can enough so that I could cut it without cutting the can.
Once this was all done, I grinded down all the edges and used a wire brush in the drill to
clean off the gunk and rust that was under the support. You can see the result in the
image below. The diagonal line that is in the upper right corner of the hole is a jigsaw cut
I made to break the spot weld. The cut was still well within the outline of the filter
housing, so the integrity was not compromised.
The next thing that had to be cut into the ammo can was the opening for the power cord.
I didn’t have a drill bit large enough to make a hole large enough for the cable/cord
connector, so I drilled a hole for the jigsaw blade and then chewed away metal to enlarge
the hole. Once the hole was almost big enough, I used the conical grinding stone on the
drill to enlarge it just enough so that the connector would fit. Once finished, I again
grinded the edges to remove any burrs.
After all of this was completed, I washed the entire ammo can to make sure that it was
free of dirt and any oily residue that might be present; I didn’t want to take any chances
with the airtight seal. Once the ammo can was dry, I installed the cord connector and
then removed the wires from the fan. This allowed me to pass the cord through the small
hole in the connector. Leaving enough slack in the cord, I reattached the wires to the fan
and tightened the connector.
Just before mounting the fan, I put a liberal amount of silicone around the face of the
mounting plate of the fan and bolted it to the can. At this point the unit was assembled.
All that remained was to silicone around the power cord coming into the can, both inside
and outside the can.
At this point, I let the silicone cure overnight before testing the unit.
The finished unit:
Testing
Now that the unit was fully assembled, it was time to test it. In order to do so, I had to
not only attach the filter, but also create the entire safe room in order to test whether it
was going to provide enough positive pressure in the area that I had selected as the safe
room.
First though, I needed to attach the filter to the fan assembly. Using Nashua 324A foil
tape, I taped the filter over the intake hole, making sure that the tape had a good seal all
around. Once that was in place, it was ready to go. Before bringing it up to the safe
room, I plugged it in again to see what the airflow was like with the filter on. The amount
of air that it was pushing out was dramatically reduced.
The area that I had selected to be the safe room of the house was the master bedroom,
master bathroom and master closet. I chose this area of the house for the logical reasons
of it having a bed, toilet, sink, storage space and it was on the second floor. There was
only one other area that I could have used as a safe room, but it was vastly smaller and
didn’t offer the same advantages as the master rooms.
The total cubic feet of the selected area is 4,296. I was a little concerned that the fan
wouldn’t be able to pressurize this size of area, so testing was the only way to be sure.
One of the reasons I selected this area was that I could seal off the closets to make a
smaller area to pressurize, if needed.
The entrance to the master bedroom has “French Doors,” or double doors, that open into
the bedroom. The only way to seal off the room and still be able to get in and out would
be to take off one of the doors. So, off it came. After the door was off, I cut the plastic
and taped it around the door. I also taped it along the floor, but I did not tape a large
section of it as I needed to go in and out of the safe room for testing purposes. My plan
was to tape it only if the room failed to pressurize.
I placed the fan assembly with its taped on filter just outside of the plastic and cut a hole
in the plastic where the exhaust was and taped it into place. I then plugged the fan in and
crossed my fingers. I thought it was doing great as the plastic was bowing out from the
room nicely when I realized that the air conditioning was running. So, I turned that off
and went back inside the safe room.
At this time, I had not put plastic over any of the air or fan vents in any of the master
rooms. I had wanted to see how well the fan would pressurize before sealing these.
Without any vents sealed, the pressurization was minimal; it took several minutes for the
plastic to go from a neutral to slightly taut. I figured that this wasn’t good enough, so I
started sealing vents. My thought was that if I had sealed all the vents and still had poor
pressure, I’d have to tape up the rest of the plastic around the door as I assumed that it
was leaking quite a bit of air.
There are 3 main vents in the master bedroom itself, two HVAC vents and one air return.
I sealed these and noticed a dramatic improvement in the pressure on the plastic. At this
point I had an idea hit me and I turned on the bathroom fan … the pressure on the plastic
immediately ceased and when I turned the ceiling fan off, the pressure immediately
returned. This was enough to convince me that I had enough pressure to accomplish the
job. Once all of the vents were sealed and the plastic around the door was fully taped up,
I would have a fully pressurized safe room.
The only thing left to do was conduct the “vanilla test.” After placing an open bottle of
vanilla immediately next to the filter, I re-entered the safe room and starting sniffing. The
motor itself made the air smell ‘electrical,’ like the smell that comes from the
KitchenAide mixer when I’m mixing dough, so it was tough trying to determine exactly
what smells I was detecting. After about 5 minutes of not smelling vanilla, I reached
under the plastic and brought the bottle inside and placed in front of the exhaust so I
could see what it would smell like mixed with the motor smell. The vanilla was quite
easy to discern, therefore, there was none coming fan and filter assembly! Yea!!
I let the fan run for four hours and then checked it. The motor was hot, but nothing else;
the air coming out of the unit was cool, the ammo can was cool, the power cord was cool,
and since the motor has a automatic thermal shutdown feature, it must have been running
within normal tolerances.
While I let the fan run, I measured and precut all of the plastic that I would need to seal
up the safe room. Once I turned off the fan, I found that I was able to store everything
that I’d need to create the safe room: plastic, filters, foil tape, duct tape, and extra plastic
sheeting.
With the successful conclusion of this test, I now know for a fact that the fan will
pressurize my safe room and keep my family safe!
Afterward: about the only thing that I’d do different if I built another unit, would be to
rotate the intake hole 90 degrees so that the intake of the fan had a more direct shot
through the filter.