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. 

 blower door
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.

DIY blower door
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.



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!


Page Index for the homemade blower door:

- Why build your own?

- Safety

- Building, wiring, and instrumenting the blower door

- Using the blower door

- Estimating flow rates, ACH, and heat loss

- References

- Comments/questions/suggestions

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:

 DIY blower door mount
The furnace blower is mounted to a piece of
5/8 inch thick plywood.

wiring diy furnace blower
Wiring the speed control switches.
wiring diy furnace blower
Plywood and blower -- ready for mounting. 

---> All the details on how to pick a blower, mount the blower, wire the blower, find a pressure gage, and hook it all up....


Using the Blower Door

The section on Using the Blower Door covers:

- 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.

---> All the details on how to setup and use your blower door to find leaks


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.

---> All the details on how to measure blower door flows, estimate home tightness, and estimate fuel costs...



There is a lot out there on blower doors -- here are few selected references you might want to look at:

Home Energy Analysis: Highlighting the Blower Door Test, FESC
Good explanation of how blower doors work and what you can learn from them

Residential Pressure & Leakage Testing Manual, Retrotec, Inc
An extensive manual on doing blower door testing

Blower Door Basics, Frank Spevak, The Energy Conservatory
Good presentation on air sealing and using a blower door.

Blower Door Basics, Martin Holladay, Green Building Advisor,
Good article on using blower doors.

Blower Door Testing, David Keefe  -- a Journal of Light Construction article on using blower doors

Minneapolis Blower Door  website -- lots of good information and references on blower doors.

Air Sealing in Occupied Homes, David Keefe, Home Energy
Good Home Energy article on air sealing homes. 

The Infiltration Station: Air Leaks in a 1924 House in Downtown Eugene, Bartley, Menne, Vernia
An interesting and somewhat humorous case study on an older leaky home.

Measurement of Infiltration Using Fan Pressurization and Weather Data,  M. H. Sherman, D. T. Grimsrud
Background on the development of the the LBL natural infiltration method.

Pressure Testing for Air Infiltration, NLCPR
Another approach for a DIY blower door.

Pinpointing Leaks With a Fog Machine, Martin Holladay, Green Building Advisor
Discusses using a fog generator to show air leakage paths.


I want to thank Nick Pine and the folks at The Hearth Forum - Green Room for ideas and advice.


Comments, Questions, Ideas?

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.

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