Using the Homemade Blower Door

This page covers using the DIY blower door made from a furnace blower.

It goes through the process of installing the blower "door" in a window and setting it up.  How to search for leaks.  And, how to take pressure measurements that will allow you to calculate infiltration rates and heat loss due to infiltration.

diy blower door

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Using the Blower Door

Please don't do anything with your new blower door fan until you read through the safety note and read through this section and read through some of the references ...  This is not just the pro forma make the lawyers happy boilerplate -- there really are safety hazards if you use the blower incorrectly.

Install the blower in the window:

Find a good window with enough working room around it and with a clear space outside the window so the fan flow will not be restricted.  Be mindful of things that might get pulled into the fan when its running.  Make sure there is a power outlet nearby.  Remove any window screens before installing.

blower door in window
Blower door installed in the window frame. 
Note how the tube from the pressure gage goes out through a hole in the blower mounting door -- this
tube must extend several feet so that it is not effected by the blower and is in a sheltered area.

Seal the furnace blower into the window so that there are no air leaks around the edges of the plywood mount board.  I use wood spacers to force the mounting board against one face of the window frame as shown in the picture below.  Use something like masking tape to seal any remaining leaks -- this may be easier to do from outside.

Shimming the blower door in place
The wood spacer or shim being put in place to lock the mounting
board against one face of the window frame.
The tube going outside from the manometer is also visible.

 Be very careful not to let anything be drawn into the fan -- be particularly careful of curtains.

Since the furnace blower is not variable speed and has only 4 fixed speeds, its not likely that any of these speeds will give the standard 50 Pa of depressurization.  But, noting the pressure drop for at least two of the speeds, its possible to interpolate a fairly accurate flow rate for 50 Pa. 

Before you turn it on:

In general, any path for air to follow from inside the home to outside that is not a normal infiltration path should be closed. 

Running The Blower:

Once you have double checked all the cautions above, you can turn the blower on.  I'd start with low speed, and once its running, go around the house and make sure nothing is amiss.  Then you can step it up to a higher speed.  Then take another tour around the house to make sure nothing is amiss.

Finding Leaks

I've not really seriously gotten started on using the blower door to find leaks in my house yet, so this section is mostly reports from others.  I'll add my own experiences later.

Before you start looking for leaks, note the pressure differences you get on all fan speeds so that you have a record of what the house started at.   Also make a note of any parts of the house you have closed off or sealed up pet doors -- that is, note anything in the house configuration that would effect the pressure differences so that you can recreate this configuration when you retest after sealing leaks.

To find leaks, set the blower door to a speed that gives a good depressurization -- this will make the leaks more obvious.  Some references recommend a depressurization of about 30 Pa, but this is not at all crtical.  

blower door manometer
Pressure gage during the test indicating about 0.15 inches
of water or about 38 Pa.

These pressures, by the way, are not going to pop your ears or anything like that -- they are pretty modest.

With the blower running, start looking for leaks.

Most people report that using the back of the hand is about as good a way to find leaks as any.  You can wet the back of the hand with a wet cloth to make it more sensitive. 

There are also devices like smoke pencils or chemical smoke puffers that generate a little smoke that will indicate airflow leaks.  With the the blower door depressurizing the house, an area that is leaking will stream the smoke quickly away showing where the leak is.  This video from Minneapolis Blower Door shows some smoke testing for leaks.

Where To Look For Leaks

Inside the main living area, the usual places to check are: 

For the reasons listed just below under "High and Low", I would also check things like outlets and baseboard for interior walls.  These interior walls may have openings through the top plate and into the attic (eg for wire or plumbing), and the stack effect will tend to pull in air from the room, up through the interior wall and up into the attic.  The blower will result in the reverse flow -- that is, into the room.

One thing we found some time back was that our dog door was a major leaker.  We replaced it with this dog door, which seals much better...  In starting our blower door tests, I simulated our old pet door by pushing the flap open on our new door, and then just letting it settle back to where the blower flow kept it part way open -- this change alone dropped the manometer pressure by 25%! 

tight pet door

As you find and fix leaks, go back and check the pressure gage and see if you can see any difference.  Either way, you will probably want to keep a record of what leaks you fixed and how much they effected the pressure gage reading. 

High and Low

For most homes (they say) the leaks listed above, while they may be significant, are not likely to be the largest leaks in most homes. The biggest leaks will more likely be from the living space up into the attic and from the outside into the basement or crawl space (eg leaks along the rim joist just under the flow).  The reason for this is that the house is the stack effect that the heated air in the house creates -- warm air wants to rise up to the attic, and this pulls cold air in down low.   The stack effect will result in a slight negative pressure in the lower part of the house which tends to pull outside air in, and a slight positive pressure up high in the house which tends to make air flow into the attic or outside.

If you have combustion appliances that are subject to backdrafting, you should work on the attic (high) side of this equation first, and then move to the basement or crawl space side.

Up in the Attic

Venturing into your attic can uncover a lot of evil leaks.  Here are some of them:

attic sealing
Sealing a pluming vent with foam in a can.

Clearly, this is all stuff you want to do before adding more insulation to the attic as the new insulation will just make it more work to do the sealing.

One thing to note is that if the ceiling on the living space is not well sealed from the attic, that a powerful attic exhaust fan can make these leaks much worse by depressurizing the attic and will likely do more harm than good.

There are some good references on measures to take in the attic here...

Basement and Crawl Space

Some of the things to look for in sealing the basement or crawl spaces:

sealing the rim joist
Sealing the rim joist.

Some links for basement, crawl space, rim joist sealing:

If your efforts using the blower door are successful, bear in mind that some combustion devices may have more trouble with developing a good draft to run properly.  If in doubt, an HVAC technician can run a check on your combustion appliances to make sure they are not back drafting combustion products back into the house.  Gas appliances with blower forced draft or sealed combustion should not be a problem.

Using a Fog Machine and Blower Door to Find Leaks

A theatrical smoke or fog machine located in the house along with a blower door turned around to pressurize the house can be used to find air leaks.  In this case, the fog machine is started, blower door turned on, and the leak spotters go outside and look for the smoke escaping the house.  The effect is apparently quite dramatic.

This GreenBuildingAdvisor story gives some more detail...


Again, I will add to this section as I progress through my leak exploration process.

Any Comments?....


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Gary February 22, 2013