A DIY Blower Door -- Easy to Build
-- Easy to Use -- Cheap
These pages describes how to build a pretty good blower door
for about $30.
You can use it to 1) find air
infiltration leaks into your house, 2) trace your progress in sealing the
house, 3) estimate your infiltration flow at 50 Pa (a blower door
standard) and compare it to other homes, 4) estimate your heat loss due to infiltration,
and 5) estimate fuel saving and CO2 reduction from air sealing your
home.
It
basically does what a commercial blower door does, but less
automatically and less accurately. Also makes one helluva
ventilation fan.
Commercial blower doors are high capacity calibrated fans that are
installed in a door in order to measure how leaky your home is and to
allow easier spotting of leaks so they can be sealed.
Commercial blower door.
A standard blower door test uses the fan to depressurize the house to
50 Pascal (0.2 inches of water). The fans are normally variable
speed to allow them to be adjusted until the precision manometer
(pressure gage) indicates that the pressure difference between the house
and outside is exactly 50 Pa. The manometer computer then gives
the flow rate out of the house.
This DIY blower door uses a multi-speed furnace blower for the fan and
is fitted into a window rather than a door. An inexpensive (but accurate) off
the shelf pressure gage is used to measure the house depressurization.
With a little scrounging, it can be built for inside of $30.
The furnace blower blower door mounted in a window.
The switches allow any of 4 speeds to be selected.
The meter to the right reads the difference between the pressure inside and
outside the house.
If you build one of these blower doors, I'd love to hear how it works out.
I'd also like to hear any experiences you have had wit air sealing your
house.
Safety
There are safety issues that you need to pay careful attention to:
- The blower fan is powerful and can suck all manner of
things into it -- including fingers, hair, curtains, pets ...
- You will need to do a little 120VAC wiring and this
poses an electrocution hazard.
- When using the blower door, you must be careful to turn
off or disable all combustion appliances (furnace, water heater, ...) because
the blower door depressurizes the house and can backdraft these appliances.
- You may be able to get your house tight enough
that ventilation is required for air quality, or that some combustion
appliances may not draft properly.
There are probably other hazards I've not thought of -- its completely up to
you to make sure that what you do is safe.
I take no responsibility whatever for any
property damage or injuries you may sustain while building or using this blower
door, or for any misleading or incorrect information in this article. Do
your homework!
Why Would I Want to Build a Blower Door Rather Than Just Having Real A
Blower Door Test?
The advantages of having your own blower door are along these lines:
- Allows you to take your time finding the leaks -- you
can run it for hours if you like. You can work on sealing leaks over
months if you want to.
- You can measure how much things have improved after
each leak or set of leaks that you seal.
- With a little calculator work, you can 1) estimate your
total air infiltration, 2) estimate the heat loss this leakage causes, and 3)
see how it compares to other heat losses from walls, windows, ... That is,
you can get a full picture of your home heat loss, and know where to best spend
your money and time. At the end of your leak sealing you will know how
much you have actually reduced infiltration and how much heating fuel you will
save.
- Some of the more time consuming variations on the
blower door test that would probably not be performed in a Utility Energy Audit Program
blower door test due to time limitations can be performed at with this door as
its only your time.
- If you are a home builder or contractor, having your
own blower door means you can check frequently as construction progresses to see
how tight things are and spot leaks while they are still easy to fix.
- If you ever want to ventilate your house at a high rate
for whatever reason -- this gadget will blow or suck a goodly amount of air.
One of the main reasons I decided to make a blower door is that I had a
blower door test done by my utility. It was fairly expensive ($200+), was
rather rushed with little time to look for leaks and really understand what was
going on, and in the end the report never came through despite a couple phone
calls. Building my own blower door allows me to take my time finding and
sealing leaks and to get a good idea what the leakage rate is for my house.
If you have a blower door test done by your utility (or other service), they
will likely point out some places where air is leaking, and you can have a go at
sealing them. But, unless you want to pay for a 2nd blower door test, you
won't really know how successful you were.
This DIY blower door does not, of course, have the flexibility, ease of use,
and accuracy of a good commercial blower door, but on the other hand its not
that difficult to build or use, its reasonably accurate, and it costs 1/100th of
what a commercial door costs. If you are in the business of doing
certified blower door tests, you need a good commercial blower door, but this
homemade one may be a good alternative for other folks.
You might also ask why not use existing exhaust fans in the house to
depressurize it? This may be worth a try, but I was not able to get
an measurable depressurization with our exhaust fans or even with our whole
house fan.
Building the Blower Door
Building the blower door consists of these steps:
Find a suitable used furnace blower -- probably from a HVAC dealer who
installs furnaces, and has old furnace pullouts around.
Cut out a board that can be slipped into the window you plan to mount
the blower in.
Mount the furnace blower on the mounting board.
Wire the blower to a power cord such that all four speeds of the blower
can be used.
Buy a suitable manometer to measure the inside of house to outside of
house pressure difference.
The furnace blower is mounted to a piece of
5/8 inch thick plywood.
- Preparing the house for blower door testing (turning of
combustion appliances, taking care of fireplaces, closing and opening the right
doors, ...)
- Setting up the blower door in the window safely and
without leaks.
- Running the blower door
- Finding leaks -- both how to find them and where to
find them. Often the largest leaks are not the main floor windows,
doors, and outlets -- attic and crawlspace/basement leaks can be more
significant.
- Using the manometer (pressure gage) to see how
successful your leak sealing efforts have been.
Figuring Out Air Change Rate, Natural
Infiltration, Heat Loss
This section covers all the aspects of figuring out blower door flow,
estimating 50Pa flow, comparing your home tightness to others, and estimating
natural infiltration rates, heat loss, and fuel costs:
- Estimating blower door flow for each blower speed.
- Estimating flow rate at 50 Pa
- when you can get to 50 Pa
- when you can't get to 50 Pa
- Existing homes infiltration at 50 Pa for comparison to
your home.
- Estimating natural infiltration
- Estimating heat loss from infiltration
- A full example using an example house with all forms of
heat loss including including estimation of infiltration and all other heat
losses, as well as the associated fuel costs and carbon emissions.
If you build one of these blower doors, I'd love to hear how it works out.
I'd also like to hear any experiences you have had wit air sealing your house.
