Effective Zone Pressure Diagnostics

ZPD illustration

The Accuracy of Zone Pressure Diagnostics (ZPD)

A few weeks ago while presenting ZPD training to many New Hampshire weatherization folks, the auditors and crew leaders had a keen interest in learning how to lower the uncertainty of the “Results for Initial Zone Configuration” of the RED ZPD tool. We measured all the field test data we needed and then entered input choices in the RED tool, reducing the uncertainty with each step. On my drive back to Maine after this training, it occurred to me this would be a good topic for a newsletter.

The uncertainty results are near the bottom of the screenshot of the tool. We suggest you aim for an uncertainty of 50% or less to ensure a reasonable level of accuracy.
In the screenshot example, the uncertainty of “Zone-to-house” is 46%, “Zone-to-outdoor” is 25%, and “Through zone” is 41%. This means the respective “Leakage @ 50Pa” results (390, 1040, and 340) are reasonably accurate. For example, for the “Zone-to-house” leakage, the value is 390 + 180. Stated another way, there is a very high chance the actual leakage is within a range of from 210 to 570 CFM50 (the center of this range is 390).
On the other hand, if the uncertainty were doubled, or 92%, the range increases to from 30 to 750 CFM50 (the center of this broader range is still 390). It is clear the accuracy of the answer is significantly reduced.

Reducing ZPD Uncertainty

There are a number of ways to reduce uncertainty if you find your test results for a house are showing uncertainty values higher than 50%. It won’t always be possible to get your uncertainty values below this, but we have found if you follow these suggestions you will succeed most of the time.

1) For the “Initial Zone Configuration” measure and enter the leakier of the two pressure boundaries. For example, in the screenshot above, the “House wrt zone” pressure of -41Pa is entered. If the leakier boundary were entered instead, “Zone wrt outdoor” with a probable pressure of -9Pa, the uncertainty values drop to 33, 20, and 30%, a significant reduction. It is always important to measure the boundary pressure difference, rather than just subtracting it from the pressure difference between the house to the outdoors created by the blower door.

2) Maximize the pressure shift between the Initial Zone and Modified Zone configurations. Try to make this shift at least 15Pa. The pressure shift in the example above goes from -41Pa for the “Initial Zone Configuration” to -14Pa for the “Modified Zone Configuration” when the attic hatch is opened, a shift of 27Pa. This is a substantial shift. If you need to make your shift larger, try to open a hole or a door wider. When doing a ZPD test to an attic, if there are two attic hatches, open both rather than only one. 

3) Create the temporary hole, the modification, in the tightest of the two pressure boundaries.This rule has been followed in the example above. 

4) For the “Modified Zone Configuration”, measure and enter the pressure boundary that you are NOT modifying when you open a hole or door. For the example above, the “Hole/door [is] located between the ‘zone and House'”, so this suggestion means you should measure and enter the “Zone wrt outdoor” pressure difference in the “Modified Zone Configuration”, probably -36Pa. This lowers the uncertainty values slightly from 46, 25, and 41% to 44, 23, and 40%.

A Few More Tips

A) Avoid ZPD testing in windy conditions. Not only does this rule hold for standard blower door testing, it holds for ZPD testing; the wind significantly increases uncertainty. The RED ZPD tool “knows” the wind condition if you enter three “Baseline pressures (wrt outdoors) at 5 sec intervals [Pa]” in the “Use Advanced Inputs” section of the tool. If these three baseline readings are different, say +2Pa, -1Pa, and -3Pa, it indicates windy conditions, so uncertainty increases. On the other hand, if the three readings are -1Pa, -1Pa, and -1Pa, it indicates no wind, so uncertainty is much lower.

B) Use baselines when doing ZPD testing, preferably the auto-baseline feature of your manometer. In the example screenshot above, notice that the “BD off” values for the “Initial” and “Modified Zone Configuration” are “0”. This is because the auto-baseline feature of The Energy Conservatory DG-700 was used. This feature incorporates the averaged baseline into the pressure difference measurements for us so we don’t have to enter it; it is accounted for automatically. Using baselines in your ZPD measurements, especially during extreme cold or warm outdoor temperatures, can significantly impact results.

C) Use two digital manometers if you have them available. This allows you to use the auto-baseline feature for the manometer at the blower door and for the one used for the zone-related pressure testing without having to switch manometers. This makes your ZPD testing faster and easier.

And Finally . . .

A Bit of History: ZPD originated in the late 1990s thanks to Michael Blasnik, Jim Fitzgerald, and others. In 2001 ZPD was studied by top weatherization researchers who determined it is a valid method of determining the leakage through designated pressure boundaries. This report, An Investigation into Zone Pressure Diagnostic Protocols for Low Income Weatherization Crews, is the detailed analysis of the 2001 study.
Since the inception of ZPD, its use in the filed has varied, waxing and waning with changes in weatherization standards, available training, and the usefulness of calculation aids. Based on our data of your visits to the RED website and requests for ZPD training, we think the use of ZPD is increasing in the WAP and home performance sectors. We hope that one reason for this is the ease of use and accuracy of the RED ZPD tool.
The Certainty of Uncertainty: An uncertainty value of 50%, or even 20%, might seem quite inaccurate. Well, to put this in context, it is important to know that all the measurements we take and tests we do each day in weatherization are uncertain; normally we just don’t calculate uncertainty or think about accuracy. Just so you know, some people do. At RED we think it is important for you to be aware of the uncertainty of the RED ZPD tool calculations. 
The National Institute of Standards and Technology (NIST), a division of the U.S. Department of Commerce, published Technical Note 1297 in 1994, titled Guideline for Evaluating and Expressing the Uncertainty of NIST Measurement Results. We don’t expect you will read this technical document before you use the RED ZPD tool the next time, but we want you to know that we have you covered; we use this method for our uncertainty calculations.
Rick Karg

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