Fun with Neutral Pressure Planes (NPP)

Replies to This Discussion

Permalink Reply by tedkidd on April 9, 2012 at 7:50am

Great thought provoking thread!

To the last question, as I think you've illustrated, npp is relative to where leaks exist more than the shape of the total structure.  Assuming the house is airtight I'd guess the flue, being the only leak, is the equalizer.  Where ever it ends, the pressure between inside and outside will equalize - so that's the neutral wrt outside.  (Put this house completely under water and it fills to the pipe.)

If there were just leakage to outside to the height of the pipe, then neutral is probably at 3ft.  

Now in a regular house with some leakage throughout, that pipe becomes a leakage supply that low in the stack.  This makes me think dominant leakage drives npp.  (Open a low window and NPP goes up.  Open a high window and it goes down)

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Permalink Reply by John Brooks on April 9, 2012 at 10:34am

Ted,

thanks for commenting

I confess that I do not know the answer.

I have a "guess"  that is very different from yours

 

This particular example is more complex than I first imagined

 

I also tried to imagine this as a "house" and "snorkel" under water just as a thought experiment

I am not so sure that the "underwater" analogy works.....

The "underwater analogy" would not predict the location of NPP in the other example Illustrations that I posted(the one window & two window examples)

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Permalink Reply by Bud Poll on April 9, 2012 at 10:43am

Hi John, Ted,

A perfectly sealed house seems to leave us in limbo as there would be zero air flow.  If some exchange managed to occur via the flue, then the NPP would have to be located near the middle of the flue pipe to provide the same ▲ pressure on each end.  But I'm guessing as well. 

The better question would be to consider some uniform leakage and define the location of the NPP in that regards.

Ted, you said "(Open a low window and NPP goes up.  Open a high window and it goes down)"  That should be the other way around?

Bud

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Permalink Reply by John Brooks on April 9, 2012 at 11:20am

Ted,

Your comment about dominant leakage driving the location of the NPP is consistent with the John Straube and John Klote Articles.

I agree with Bud that the NPP will move closer to the larger opening

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Permalink Reply by John Brooks on April 9, 2012 at 11:42am

Bud,

I think some of the examples in the Klote paper come close to what you are suggesting....

They show a narrow slot that is the full height of the enclosure in addition to the other openings that are being considered

To me the narrow slot is analagous to "uniform leakage"

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Permalink Reply by John Brooks on April 9, 2012 at 6:03pm

Here is my illustration of Example 3 from the John Klote Paper "...Analysis of Stack Effect"

I think the intention of the 1/4" x 100 ft tall opening is to simulate "uniform leakage"..

My take from this example:

The relatively Larger Openings (10 square feet each) have more influence on the location of the NPP than the Not-As-Big (2 square feet) "uniform leakage" opening

here is the link to the John Klote Paper

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

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Permalink Reply by Bud Poll on April 10, 2012 at 12:30am

You've got it John.  Now, if another window were opened, we know the NPP would move, but how does it move?  We can predict where it will move to, but all of the pressures will have changed.  What causes the change in the pressures? 

Bud

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Permalink Reply by tedkidd on April 10, 2012 at 4:56am

Hey, you guys bail on me? I don't see npp moving the way you do yet and need more explanation.

My take is a bigger opening reduces/releases pressure, therefore moving the npp away from it, can someone help me see why this is the opposite of what others see?

Also, wouldn't opening an upstairs window make the basement go more negative?

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Permalink Reply by Bud Poll on April 10, 2012 at 7:17am

Ted, you've got it:

"a bigger opening reduces/releases pressure" correct, if that opening is above the NPP.

"Also, wouldn't opening an upstairs window make the basement go more negative?" Also correct, in fact it makes the whole house go a bit more negative.

Example, a balanced house with the NPP right in the middle.  Let's assume a cold day with +4pa at the top and -4pa at the bottom.  That's a total of 8pa of stack pressure from the earlier equations.  If we open an upstairs window as you state, air will escape and the pressure inside the whole house will go down, let's say 2pa.  That would give us a +2pa at the top and -6pa at the bottom, so we will have retained our total 8pa of stack pressure, but the NPP will have moved closer to the top, where we opened that window.

Bud

Permalink Reply by John Brooks on April 10, 2012 at 7:18am

Ted,  thanks for the dialogue

I think that opening a window upstairs may or may not cause the basement to go more negative.

It depends on the season and where the NPP was before the window was opened.

In some homes the NPP can be above the 2nd floor ceiling during the heating season.

 

I don't have a quick explanation for your other question....

I will work on something...meanwhile

Have you had a look at John Straube's BSD-104?

http://www.buildingscience.com/documents/digests/bsd-014-air-flow-c...

 

maybe his illustrations will help

Permalink Reply by Tom Mallard on April 12, 2012 at 7:50am

Comparing the standard flow diagrams for a home to one of using a windproof layer of insulation board on the outside and having the space between studs & joists part of the airflow path.

A use for the idea was with a solar gain wall, but you can heat/cool the air other ways. Then, adding thermal-mass using pvc pipes in the open space now without insulation in the flow path to have the heat-cool last over a day-night, this reduces energy to maintain comfort zone for any system.

If you review the standard flow with this in mind, infiltration to open windows & doors & all, the floors are connected to ceiling vents, that is either down the wall or up into the roof close below the weather-proof top with a return to the wall and floor loop.

This really changes the overall flow path and thermal opportunities so adding the thought to this example, which is a huge volume so interested in that effect and what flow would be.

 

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Permalink Reply by Bud Poll on April 21, 2012 at 1:29pm

John, since we are having fun with Neutral Pressure Planes I'll ask this question here.

I was following a link on Allison Bailes' blog: http://homeenergypros.lbl.gov/profiles/blogs/a-home-energy-rater-or...   lots of great links there.  But I went to this one: http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/42...

This is a guest post by Paul Raymer.  There is a ventilation chart and a quick example below it that states a 60cfm fan would be appropriate for their sample home, either intake, exhaust, or a balanced solution like a HRV.  My question is, if 60cfm is correct for an intake or exhaust remedy, would the balanced solution need to be 60cfm or only 30cfm?

The nature of the NPP is that it will adjust and reduce a 60cfm exhaust to only 30cfm.  Or it will do the same to a 60cfm intake.  But a balanced solution doesn't shift the NPP, so a 60cfm HRV would result is an actual 60cfm flow rate.

Bud

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