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.
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.
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.