# Fun with Neutral Pressure Planes (NPP)

Greetings

First of all......

* As we upgrade and build new....

I think it is time to eliminate atmospherically vented appliances from the breathing zones of our homes

* I realize that the effects of wind and mechanical equipment can and often do overwhelm Stack Effect.

My current favorite online resources for visualizing "Stack Effect" are.....

*John Straube's Article

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

*John Klote's Article

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

*Bud Poll's Worksheet

I plan to post some Illustrations and see if you folks agree with my current thinking about the location of Neutral Pressure Planes

Meanwhile...

Does anyone else have suggestions for online links concerning  "Stack Effect"?

Views: 5487

### Replies to This Discussion

Bud, I think the key to how a tube or a chimney chase relates to the NPP is to look at the point (altitude) where the "tube" "connects" to the outdoors

Here is an Illustration of a tube as the only significant opening

I say the NPP will be at elevation 12 feet ...

the point where the tube "connects" to the outdoors

this is assuming that the enclosure is heated, the house has already burped and the tube is full of 70 degree air...

If that were true, then a chimney extending a couple feet above the roof peak would move the NPP outside the house, and that isn't the case. It merely moves it upward according to the chimney air flow and how much that reduces the interior pressure.

But why is there a fish in the house?

Once you follow a chase or tube from one end or the other, adding or subtracting the stacks of air, you end up with the stack pressure across that path.  I would assume we could also use the equation, as we need to assume the inside (path) and outside (path) temperatures.  Once we have the pressure across the path we can divide it in half and view half as pushing air in and the other half as pushing air out, and a NPP in the middle.  I'm shaky on how the pressures set up across a uniform path so I tend to view it as a large chamber with one opening high and the other low.

Bud

Bud, did you notice the "red herring" in my previous illustration?

I continue to believe that the barometric pressure at the point(s) where our enclosures "CONNECT" to the outdoors is the key.

This is where the pressure is applied or resisted.

I am attaching a John Straube quote and illustration.

Assuming the outside section of tube is filled with outside temperature air and the inside section of tube is filled with inside air, start at the bottom of the outside tube and subtract the air pressure as you move up to the opening.  Since the air in the tube is the same temperature as outside, when you reach the opening the pressure inside the tube will be the same as outside.

You now have your reference pressure, also the NPP.  If you calculate up or down from the opening on the inside, inside the tube or not, you increase of decrease a different amount than outside, thus the pressures above and below the NPP.

Bud

Yes, but that's true only of a hermetically-sealed cylinder, not a leaky house.

Oh! Red Herring. I get it (I guess). I couldn't identify the fish.

Robert, Humor me here.....

If the tube were the only "significant opening"....

Imagine that the house is extremely tight (< 1 ACH-50)

and the cross sectional area of "the tube" is say 4 times greater than all the other openings combined.....

would you agree that the NPP is near elevation 12 feet?

It's an interesting thought experiment (does it have a real world application?).

I don't think you'd be correct even if the house was hermetically sealed, because this is not a horizontal opening but a chimney.

A 4:1 leak area ratio would still allow some stack effect flow, somewhat slowed by the U-turn in the "chimney" and the restriction of the other openings, but that would effectively reduce house interior pressure such that, assuming the other leaks to be balanced high and low, the NPP would rise.

Assuming 10' effective chimney height, that tube would create a pressure differential of about 2.5 Pa.

the real world application ...would be for locating and sizing the intentional openings (intake ports & exhaust ports) of a house to manipulate the location of the NPP

I am extrapolating my thought experiment from the John Klote paper

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

in particular Example 3

I suspect that, with such a tight house, a 2.5 Pa draw would move the NPP to the ceiling.

I don't follow how this example applies to locating and sizing intake and exhaust ports, if those are the ports of an HRV/ERV which, if barometrically balanced would not alter the natural NPP. Or are you referring to passive "holes" in the envelope?

In any case, such horizontal "ports" wouldn't be chimneys unless there was a significant offset and channel flow within the envelope.

My take-away from Klote-Example 3

(and the entire Klote Paper)

is that the height of the NPP is strongly influenced by...

the elevation of the most significant Opening(s)

In a "tight house" I would think that the Significant openings ARE the Intentional Openings.

The intentional openings might be ....

Passive openings like Aldes Air inlets

or the termination of an exhaust fan (off or on)...

or the intake portal for "supply ventilation"

or an open basement window

or an open clerestory window

or they might be the exterior connection points of an HRV/ERV

I realize that wind and mechanical effects can overshadow stack effects

however, it seems to me that it would be better to work "with" stack (or container) effects rather than "against" them.

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