# Locating the Neutral Pressure Plane

I am not an energy Rater .. I am an architect

I am curious if any "Home Energy Pros" make an effort to locate the Neutral Pressure Plane (NPP) of a house that is being blower door tested?

Views: 4497

### Replies to This Discussion

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.

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.

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

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

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.

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

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

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

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

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.

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).

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

## Featured Forum Discussions

### 7,200 Normal Homeowner Minds Blown by Proof Is Possible Tour, Private Market Demanding Proven Home Performance

Started by Corbett Lunsford in Marketing. Last reply by Robert DV on Thursday.

### Strange IR Image

Started by Larry Nissman in General Forum. Last reply by Jason Raddenbach on Thursday.

### BPI Supports Strong Consumer Protection in Residential PACE Financing Programs

Started by Quinn Korzeniecki in General Forum Feb 10.

### Hero Pace Program?

Started by Horace Douglas Hunt, Jr. in General Forum. Last reply by Ryan Miller Feb 9.

### BPI February Newsletter & New Logos

Started by Quinn Korzeniecki in General Forum Feb 9.

• View All

## Latest Activity

jackie harsha added a discussion to the group Marketing Energy Efficiency

### Informative video about energy and its implications

Like many, I’ve been struggling with the differences between what consumers say and what they…See More
yesterday
jackie harsha joined James Sayers's group

### Marketing Energy Efficiency

Sharing ideas, tools and examples of promoting energy efficiency to consumersSee More
yesterday
Ajmain Azraf posted blog posts
Friday
Robert Post added a discussion to the group Job Board

### Hiring Residential BPI BA Energy Auditors In Philadelphia Area

Please reach me if interested and we can plan a call to discuss this opportunity. This is a full…See More
Friday
Robert Post joined Diane Chojnowski's group

### Job Board

This group is for posting jobs related to all aspects of the home performance industry including…See More
Friday
John Fulton posted a photo

### Roman Blinds For Living Room

Perfect Blinds to cover Long Window. Roman Blinds for your Drawing Room, Seating Space, Corner…
Friday
Robert Post posted a blog post

### Hiring BPI BA Professionals in Philadelphia Area

Please reach me if interested and we can plan a call to discuss this opportunity. This is a full…See More
Thursday

## Events

• ### Better Buildings Peer Exchange Call: Making an Impact: Low-Income Energy Efficiency Programs

February 23, 2017 from 1pm to 2:30pm – Eastern Time

## Home Energy Pros

Welcome to Home Energy Pros – the unique digital community by and for those who work in the home energy performance arena.

Home Energy Pros was founded by the developers of Home Energy Saver Pro (supported by the U.S. Department of Energy) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network. Please honor our Guidelines