Locating the Neutral Pressure Plane

Replies to This Discussion

Permalink Reply by Robert Riversong on September 4, 2012 at 12:14pm

I think you need ultra-violet light to see it ;-)

Actually, the neutral pressure plane would be at the highest ceiling (or air barrier boundary) when under negative pressure.

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Permalink Reply by John Brooks on September 4, 2012 at 12:36pm

Let me try again.

I'm not really trying to locate the NPP when the house is at Negative 50

I just wonder if there is a diagnostic method to determine the "natural" NPP of a house.....

the location of the NPP on a calm day, windows & doors closed and with mech ventilation off?

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Permalink Reply by John Brooks on September 4, 2012 at 12:47pm

I know af an analytical method ....

when the size and the location of the significant openings are known.....

http://fire.nist.gov/bfrlpubs/fire91/PDF/f91013.pdf

 

I am curious about a diagnostic method ...

when the size and location of significant openings are unknown

 

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Permalink Reply by David Meiland on September 4, 2012 at 2:17pm

Seems like a simple job, although I could be wrong. The theory is that the lowest areas of the house are negative wrt outside, and the highest areas are positive. Somewhere between them there's no difference, inside wrt outside. So, you'd set up your blower door, throw the green house outside, connect it to the manometer... then physically raise and lower the manometer inside the house until you found a reliable zero. That's the NPP.

In practice I bet the NPP is hard to find, because the pressure differences are probably slight (needle bounce would become an issue) and the location of the NPP would change with slight breezes, temperature movements, and so on. And, it's probably different in different parts of the house, too.

If you used some sort of data-logging over time, you might get a somewhat clear picture.

If I get a chance, I'll experiment with it here.

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Permalink Reply by Robert Riversong on September 4, 2012 at 2:35pm

I suspect you could determine the NPP more accurately (and easily) on a cold winter day with no wind by using a smoke stick.

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Permalink Reply by Bud Poll on September 4, 2012 at 3:05pm

Hi John, I did come up with a method, similar to "add a hole" for ZPD.  You establish a pressure reading above or below the NPP directly from inside to outside, like through a window.  Then you open another window a specific amount ( one sq ft for example) and see how much your test location changes.  A bunch of questionable math and it will tell you the NPP is "X" distance from your test point.  It's crude, but better than trying to locate it with a manometer.

David, when you toss your green hose out the front door, no mater where you go inside the home you will be reading the delta p at the front door.  As you walk upstairs the house pressure decreases at about 3.5 pa per foot, but the air inside the green hose cancels out the change so it looks like you are still right where you started.

Even though the "add a hole" approach is crude, you can also estimate the CFM50 from your results.  That's a leakage test with just a manometer.

Bud

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Permalink Reply by Bud Poll on September 4, 2012 at 3:52pm

John, I got ahead of myself.  The "add a hole" is needed for the second equation for estimating the CFM50. To locate the NPP is much easier.

Calculate the stack pressure for that day and divide it by the top to bottom height of the house.  You end up with a ▲p per foot of stack pressure inside the house.  Then take a pressure reading somewhere, ceiling to attic or front door.  Use the pressure per foot to calculate how far up or down you need to move to be at the zero point or NPP.

Bud

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Permalink Reply by David Meiland on September 4, 2012 at 4:29pm

Bud, it sounds like you're saying that a pressure tap outside the front door is not the way to measure outside pressure. What would be better?

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Permalink Reply by Bud Poll on September 4, 2012 at 5:13pm

David, placing the reference tap outside the front door simply means all measurements taken inside are wrt outside the front door.  In a way it is an advantage to be using the manometers we do, as they ignore the barometric pressure changes by comparing reference to input.  When we run tubing all over the place, the weight of the air inside the tube becomes part of the process.

If you run the reference tube out the front door and then take the manometer up to the second floor (16' up) , the input port is now looking at 16' less air pressure.  But the weight of the air in the reference tube, also 16' is also subtracting from the reference.  The result is you are right back at the front door.  If your tubing just came in from a cold trunk, then there is a bit of a difference due to warm air inside and cold air in the tube.  For a 35° inside to outside ▲t, it is about 1/4 pa per foot elevation difference.  Over 16' that would be 4pa. 

If you ran your reference tube out the front door of a split level and it hung down 6' to the ground, your reference would still be just outside the front door where it penetrated the envelope and not at the end of the tube 6'down. 

Bud

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Permalink Reply by David Meiland on September 4, 2012 at 10:29pm

Bud, sounds like you (though not me) could easily account for the weight of the air in the tubing. So back to John's question, how to find the NPP?

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Permalink Reply by Bud Poll on September 5, 2012 at 6:48am

David, accounting for the weight of air is what stack effect is all about.  Start here: http://myenergyworkshop.homestead.com/hot-air.html

As for locating the NPP without Robert's smoke stick (not really a bad idea) here is an example.

On a cold day with a good ▲t there will be a measurable amount of stack effect, plus pressure at the ceiling and minus pressure at the floor.  The total top to bottom stack pressure can be easily calculated.

Pa = 0.0067 x H x ▲T

H = building height in feet

T = the Fahrenheit inside and outside temperature difference

This is the total stack pressure for that temperature difference.  If the NPP is right in the middle of the house, then half will be a positive pressure at the ceiling and the other half will be a negative pressure at the floor.  But houses don't always set up with half and half so here is what you do.

Calculate the the total pressure and divide it by the total height you used.  Let's assume 4 pa total stack pressure over 16 feet of height.  Divide 4 by 16 and you get 0.25 pa per foot.  Take a pressure reading under the front door, say 3 pa.  Divide 3 by 0.25 and you get 12.  That says the NPP is 12 feet above where your reference tube exited the envelope.

What good is this, turn on an exhaust fan and check the reading at the front door again.  If it didn't move, you have a leaky house.  If it went to 4 pa or higher, you have a tight house.  Getting familiar with the numbers will enable you to make some judgement calls and if you mastered the add-a-hole math, you could even estimate the CFM50.  I thought about this method when someone was posting about getting all of their equipment onto a plane to test a house at a remote location.  A very simple app and this would be a quick test.

Bud

Permalink Reply by Robert Riversong on September 5, 2012 at 9:31am

Your formula (Pa = 0.0067 x H x ▲T) is for H = distance from NPP.

And that's just a rough approximation. It will be accurate with very small delta-T (98% @ 10°) but not with larger delta-T (86% accuracy at 70° difference).

The actual formula is 

where:

ΔP       = available pressure difference, in psi

C         = 0.0188

a          = atmospheric pressure, in psi

h          = height or distance, in ft

To        = absolute outside temperature, in °R (°F + 460)

Ti         = absolute inside temperature, in °R (°F + 460)

Or, to get the pressure in Pa, substitute 1893 for C x a

 

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