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Using a Differential Manometer To Measure Stack Effect

Andris Skulte | Posted in Energy Efficiency and Durability on

Can a differential manometer be used to measure stack effect? I’m thinking one hose would be left on the first floor (or in the basement), and the other would be in the 2nd story.

Will the elevation difference of the open ends affect the reading? Or does that net out, since it’s an open system on both hoses? 

Any suggestions on a budget model to buy? Or would an inclined water manometer do the trick? 

Or even the iPhone’s built in barometer? (100 Pa is about 27′, and I’ve heard it measures stair climbing, so it might work)

I’m thinking of using it to see pressure differences when balancing an HRV, as well as a virtual ‘blower door’ by using our kitchen vent on high, and then measuring the effect of opening/closing/sealing things… 

Thoughts?

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Such a setup would measure the pressure differential between the two locations, but that can’t be directly converted to a rate of air movement (like cubic feet per minute) without a lot of other information. I don’t think the pressure differential alone would be particularly useful.

    Bill

  2. Andris Skulte | | #2

    Hi Bill,
    Thanks for chiming in. Sure, it won't be as accurate as a proper blower door test, but should give some actionable data.

    It'll start with getting pressures though! And maybe a smoke stick... Looks like some iPhone barometer apps read down to .01 hPa (10 Pa), so at least it's something... Any suggestions on a real gauge that won't break the bank? None of the $40-$100 ones on Amazon get down to single Pa unit resolution. I'd prefer digital, but maybe a magnehelic with a ±.25"H2O range will be it...

    Here's my reasoning - I had a blower door test done a decade ago (1610 cfm @ 50Pa, roughly 90 sq inches equiv leakage area). If I crank up the 600 cfm kitchen exhaust fan and bath fan, I hope to get a noticeable pressure drop in the house. I could crack a window open (known size, add leakage area) and see what the pressure difference is, and get a rough order of magnitude on the leakage as I open or seal things up...

    https://www.greenbuildingadvisor.com/article/zonal-pressure-diagnostics-part-iii

    By doing zonal pressure diagnostics, it should show (grossly) where the leakage is worst. If I take an attic crawlspace (gable vents on our dormered cape) vs 2nd floor differential pressure reading, it'll show proportionally what the leakage area is (what comes in, must go out) compared to the inside. Same with the basement vs 1st floor. Is most of my leakage open sill plate to the basement? Or top plate / walls to the crawlspace attic? I figured it'll help steer what I need to improve next.

    I was also curious where my neutral pressure plane is, as well as if it makes sense to run the HRV unbalanced against stack effect (I'm thinking exhaust intakes on the first floor and fresh air delivery to the 2nd floor bedrooms).

    Or maybe it's all just an exercise in sharpening a pencil since I should be sealing off the sill plate, band joist, top plates anyway...

    1. Expert Member
      Zephyr7 | | #5

      I would look at the Magnehelic gauges, which are commonly available on Ebay with inches of water scale (which you can easily convert to other units if needed).

      I agree that if you have a particularly leaky room, it would run slightly lower pressure compared to other areas, but the differential will be affected by the aperature of whatever connects the spaces. I think you'll find it difficult to actually get useable data here. A blower door test doesn't look for leaks based on pressure, you look for leaks using a smoke stick. The pressure reading just gives you a sort of whole building leakiness number at a specified pressure to allow for comparisons with other places.

      While the theory that a leaky room will have slightly lower air pressure due to the leaks is true, I think this falls into the old joke of "theory is like mist on glasses: obscures facts" :-)

      Bill

  3. mikemorr | | #3

    Larry Janesky does it in this video: https://youtu.be/a3rKGMIqWis

  4. Deleted | | #4

    Deleted

  5. Expert Member
    Peter Engle | | #6

    I don't think this will work, at least in still air. The stack effect is caused by the buoyancy of warmer air rising and cooler air falling. But it's all air. When you run your tubing for the manometer, it is exposed to the same conditions as the air in the rooms and the air inside the tubing will rapidly approach local room temperatures. Therefore, the air in the tubing will have the same buoyancy as the air in the rooms. There will be no net pressure difference at the manometer, regardless of where you place it in the house.
    You could do this measurement by using outdoor air as a reference though, because it will generally have better mixing and little or no temperature stratification. Measure the basement compared to outside and the upstairs compared to outside. Best would be to use two manometers and take the measurements at the same time.
    You can certainly use your kitchen fan as a poor man's blower door as you describe above. With a constant forced pressure difference, you can use the manometer to check differential pressures from room-to-room, or any or all rooms relative to outdoors. With some diligence, you could certainly find the areas with greatest leakage and aim to seal those areas first. A smoke stick will come in very handy while you are doing this as it can help pinpoint the exact locations of many leaks.

    1. Jon R | | #8

      I agree - there had to be some airflow from basement to upstairs in the video in #3. This airflow restricted by the closed basement door created the pressure difference.

      In a perfectly still/sealed house (no flow) with the basement open to upstairs, it wouldn't measure any upstairs/basement pressure difference - so measure interior to exterior.

  6. Jon R | | #7

    > it'll help steer what I need to improve next.

    I suspect that it's easier and more effective to fill the house with smoke, pressurize it and then look where the most smoke is coming out.

    Some will say "but leaks at the neutral pressure plane don't matter". But considering wind, they do matter.

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