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Community and Q&A

DIY Blower Door Testing

NICK KEENAN | Posted in Green Building Techniques on

Recently I saw a Matt Reisinger video where a builder has the house blower door tested, and it doesn’t meet the target ACH50. The thrifty builder sets up his own blower so he can find leaks at his own pace before paying the tester to come back. What he does is put a piece of board foam over the door, seal it with tape, and then put a 1200 CFM* floor dryer into a hole into the floor.  (Something like this: ). Then he goes around the house with a smoke pen and plugs every leak he can find.

Now that’s good, but it’s not very quantitative. How does he know when it’s time to call the tester back? I’ve been thinking about ways of doing my own testing. Obviously it wouldn’t be as accurate as a pro testing rig, but I think there is a way that I could get results that have meaning.

There are two parts to the ACH50 test. The first is getting a pressure differential of 50 Pascals. Which means you have to be able to be able to measure pressure. Fifty Pascal is 0.20 inches of water, so if you had a U-shaped tube with water in it, and one end inside the house and one end outside, when the water level has moved by 0.20 inches you have 50 pascals. The airflow in the fan has to be adjusted to achieve 50 Pascals, and I would do that the same way a pro would, by restricting the air intake to the fan.

The second part is measuring the air flow in the fan, and that’s trickier. My idea is to use a clamp on ammeter, and measure how much current the fan is drawing. The power drawn by a fan is proportional to the square of the air moved**.  By taking measurements with the fan completely obstructed (zero flow) and completely free (rated flow) I get two points on the fan curve, and I can interpolate the rest. I would like to have more points but I can’t think of an easy way to measure CFM directly. This also assumes that the fan rating is honest.

Thoughts on this method?

*(1200 CFM is 72,000 CF/hr, or 3ACH in a 2400 SF house with 10′ ceilings. That may not be enough for every job.)

**(It’s a little counter-intuitive, but fans draw less power when they are throttled down, because they are moving less air and doing less work.)

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  1. this_page_left_blank | | #1

    While it's true that the fan uses less power when restricted, I'd be surprised if the relationship is linear. Do you have a source for this? And you definitely cannot rely on the rating of the fan to be accurate.

    0.2" is a very small increment to visually resolve. You'll be lucky to get +/-25% of true value.

    I think you'd be better off buying a cheap anemometer and a digital manometer. You won't get particularly close to real CFM readings, but at least they will be somewhat linear and repeatable.

  2. Expert Member
    NICK KEENAN | | #2

    >While it's true that the fan uses less power when restricted, I'd be surprised if the relationship is linear. Do you have a source for this?

    It's not linear, it's proportional to the square. Source is:

    1. this_page_left_blank | | #4

      I probably used the wrong word, but by not linear I meant that the relationship would change, not just a single variable equation. It looks like that's not the case, but according to the link, it's proportional to the cube, not the square. I think these equations assume certain parameters remain constant. For example, if you completely blocked the air flow the equation won't work. For the proportionality to hold true, zero air flow would equal zero power, and that is definitely not the case.
      Regardless, given the lack of a reference point to apply the equation to, you're probably still better off with an anemometer. This assumes you really care about the absolute values. If you're just looking to be able to tell when you've reduced air leakage by a particular proportion of its original value, then either way will work.

  3. Expert Member
    Akos | | #3

    I have done budget flow meters with a longish length of straight pipe and a pitot tube anemometer. As long as the straight length is long enough (40x diameter upstream and 20x downstream), it gives reasonably accurate readings.

    You can do this with a length of sonotube taped to a fan and a cheap vane anemometer. It won't give exactly accurate readings, but they will be repeatable as long as you place the anemometer in the same spot

  4. jj1 | | #5

    Hi DCC: Gary Reysa of has detailed instructions re building a DIY blower door for $30-$50 at
    He further provides detailed information re estimating flow rates and ACH50 etc. at

  5. charlie_sullivan | | #6

    You can get a manometer that can read 50 Pa for well under $100. A Dywer "Magnahelic" gauge comes in a 60 Pa range or a 100 Pa range (as well as many other options), and costs $65-75 new. Or for $40 they'll sell you a traditional manometer. Used ones on ebay are cheaper.

    Using fan power to measure flow is an interesting idea. Note that the motor efficiency varies with operating point, so it's not as simple as using the theory of the fan itself. An ECM fan would be nice, both to control it, and because the high efficiency means that most of the input power you are measuring is actually going to rotating the fan, not lost energy in the motor. Even then, the theory of the fan can be more complicated if you are holding pressure constant as you vary flow, rather than having pressure and speed vary together. So I'd want to either have a way to calibrate the setup, or use a fan that has data available.

    Fantech has great data on their website, including a calculator which allows you to put in different operating points in CFM and Pa and see the power input required. And here's an ECM fan model that can do up to 2250 CFM at 50 Pa: The FKD 14 XL EC Mixed Flow Fan. Only thing is, it costs, $1600. And a bunch of your time to build it into a door, make the calibration curves, etc. So maybe buying a blower door isn't such a bad deal after all.

  6. Expert Member
    NICK KEENAN | | #7

    It seems the tricky part is measuring air flow. On this page Bill Pentz goes through the ASHRAE testing process, which is to use a pitot tube in the duct:

    I found this comment interesting:
    "One other fairly serious concern with test pipes is the ASHRAE and Dwyer Instrument test protocols clearly state that the air needs tested at different levels inside the pipe then averaged. Testing just the center gives a maximum airflow that drops significantly as we test closer to the pipe walls. Personally, I choose to just test the center with each test done using exactly the same test pipe to provide a good consistent result between units. I also know that if I test at 0", 1", 2" and the center at 3" then average will give me much less airflow."

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