Locating the Neutral Pressure Plane

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Permalink Reply by Robert Riversong on September 12, 2012 at 6:47am

I just found a formula for approximating the house depressurization of an exhaust fan:

depressurization ≈ 50 x (CFMfan/CFM50)^1/0.65

This roughly agrees with my test results.

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Permalink Reply by Robert Riversong on September 5, 2012 at 10:53am

In the link provided, Straube uses a variation of the formula I posted.

Your simplified formula, which BSC teaches in their "summer camps" (see page four on this PPT link)

http://www.buildingscienceconsulting.com/presentations/documents/XV...

loses 2% accuracy for each 10°F delta-T.

So, even with your example of 16' total stack height and 35° delta-T (total pressure differential of 4.04 Pa), if you got a reading of exactly half total delta-P at the floor, that would put the NPP at 8.62' up rather than 8'. You could say that a 7½" error is OK, but at a 70° delta-T that error increases to 1¼" feet which is significant.

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Permalink Reply by Bud Poll on September 6, 2012 at 11:31am

Hi Robert,

I checked the link and they do use the same formula.  I see they are referencing the NPP, but it applies to any height.  In their example they are starting with a assumed NPP in the middle, not always the case.

As for accuracy, I haven't had time to review your equation, but plus or minus a foot is a lot more accurate than cracking a window open and feeling how much air is entering or exiting.  That is what they explained in my auditing class.  In any case, the accuracy we need is dependent upon what we are looking for.  A high, middle, or low, location tells us how balanced or unbalanced our leakage is.  If the location then changes a lot or a little when a window is opened we can next estimate how much total leakage we have.  And last, if we have no stack effect to play with on a given day we can use the exhaust fans to see where one then two place the NPP.

Thanks for the input,

Bud

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Permalink Reply by Bud Poll on September 6, 2012 at 3:55pm

Robert, assuming both equations came from similar sources, I was wondering what additional assumption they made to bring it down to Pa = 0.0067 x H x▲t

Just playing with the numbers it seems they assumed a very low ▲t or at least that is what I get working your equation backwards.  If the .0067 were adjusted a bit, then the plus and minus error might be better centered around some average outside temperature, thus a smaller error.

Bud

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Permalink Reply by Robert Riversong on September 6, 2012 at 4:52pm

If you assume 35° delta-T and about 2,000' elevation (atmospheric  pressure of about 13.7 psi), then the simplified formula, using 0.0067 as the constant and delta-T°F, roughly approximates the curve of the accurate formula.

It's a very rough, quick and dirty, approximation of stack effect pressure.

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Permalink Reply by Don Fugler on September 6, 2012 at 5:31pm

As someone said, the easiest way to check for neutral pressure plane is with a smoke stick on a calm day. Crack a casement window or door, run the smoke up the small crack. Does the smoke come in or go out? Sometimes you see the NPP within the height of that window or door. If it is all "in" on the first floor, go upstairs and do this again. If flow is all "in" as high as windows go on the top floor, then the NPP is very close to the top floor ceiling.

I have done hundreds of neutral pressure plane tests with sensitive manometers but the technique described above will be almost as accurate and much faster.

Don

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Permalink Reply by Bud Poll on September 6, 2012 at 6:42pm

Hi Don, it sounds like you attended the same auditing class I did :).  But the back of the hand method lacks a professional look and isn't accurate enough to judge how much the NPP moves with a given amount of added leakage area or exhaust venting.  We have the tools so I like to use them.

You mention using your manometer to locate the NPP, may I ask how you used it?

Robert, I have no problem using the better formula, I just like to understand how these come about.  And even here in Maine, elevation does vary.

With the numbers we gather and calculations we do, there is a balance between accuracy and consistency.  Since accuracy is not always possible, at least being consistent gives us a reference.

Bud

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Permalink Reply by Don Fugler on September 6, 2012 at 6:54pm

The NPP testing that I did was in the early eighties. We simply measured the cross-wall pressures through any wall orifice or opening we could find, and then graphed the pressure differences against height to find the P=0 intersection. On a calm day, the NPP levels on all four sides of a house were within a metre of each other. On windy days, there was little or no consistency or repeatability. These were primarily two storey houses with a full basement, and lots of basement leakage. The NPP was in the top half of the first floor for many of the houses. Older houses usually had it higher on the second floor. If you can discern differences of a meter or so in the NPP in your added leakage area tests, then my prediction is that it is calm enough to do it accurately with a smoke pencil.

But then, I like using the garbage bag airflow test and find it almost as repeatable as a flowhood.

Don 

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Permalink Reply by Bud Poll on September 6, 2012 at 7:10pm

Don, I always chuckle when I have spotted a cold draft pouring in with my IR camera and then I catch myself testing it with the back of my hand. 

Thanks for the explanation, essentially the same.

Bud

Permalink Reply by Cory Chovanec on September 17, 2012 at 12:40pm

John-

Not somthing I try to locate but it I think it could be done witout too much trouble. With a gauge and some tubing a guy could go to different parts of the home (lowest point to highest and anything in between) and just take a house with reference to the outside pressure reading. When you record zero with the gauge configured this way you may find your nuetral pressure plane. Haven't verified this but I think it would work.

 

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Permalink Reply by Robert Riversong on September 17, 2012 at 12:46pm

You could do that, but you'd have to drill a lot of holes through the envelope, since you have to compare interior pressure to outside pressure at the same height (without opening windows which will change the NPP height).

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Permalink Reply by Don Fugler on September 18, 2012 at 5:53am

Bud, CMHC tested the flow vs. pressure performance of a Saskatchewan loop back in 1987. The report is likely still around but essentially the looping just reduced the flow rate compared to a straight inlet, by introducing a 180 degree turn. The test results did not bear out the theoretical advantages of a column of cold air sitting in the vertical tube.

As well, cracking a window to test for neutral pressure plane does not unduly change the NPP location, based on the experience we had testing. In a particularly tight house, where even such a small opening might be critical, you could send out a metal capillary tube through the weatherstripping or tape the crack when doing the test. Windows and doors are really the only NPP measurement access points that you have.

Don

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