Duct testing in a conditioned crawlspace

Concerning duct testing in crawlspaces (this is for new houses for Energy Star, but will be the same for existing houses):

I have a house on a conditioned crawl with ducts in that crawl. There is an exterior hatchway into the crawl, and no intentional connection between the house and the crawl. When performing a total duct leakage test, no problem, just open the hatch before the test. When performing a duct leakage to outside test, the crawlspace ducts never experience the effect of the blower door, so the test is inaccurate, with leakage to outside numbers that are too high.

We know the numbers are too high in a case like this, and we have been able to simulate the problem in a house with a basement by taping the basement door while doing the leakage to outside test. Just today, we tested 2 identical houses that we had sealed while under construction. The 2 identical houses - 1400 ft2 on a basement making them 2800 ft2 of conditioned space - had building leakage of 620 CFM50 and 650 CFM 50. Duct systems are entirely within the shell. Both had a leakage to outside numbers of 20 and 26 CFM25 range. We then taped the doors to the basements and got numbers of 82 and 92 CFM25 as leakage to the outside. In these cases, off by a large factor from the real 20-25 number.

Without cutting a hole in the floor or installing a second blower door in the crawlspace, do you have any ideas?

Ed Minch

Tags: conditioned, crawl, duct, testing

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Permalink Reply by Ed Minch on November 25, 2011 at 3:15pm

It does, but even though:

"According to the RESNET Formal Interpretation mentioned above: The measured outside leakage shall be deemed to be 0 CFM25 for each system the meets the following criteria: 100% of ducts and air handler located in conditioned space boundary, 100% visible, and not located in an enclosed space such as a chaseway, interior or exterior wall or roof cavity or in an enclosed floor cavity."

EStar says you have to test BOTH total leakage and leakage-to-outside on every house, with a few exceptions some of which you mentioned. I think V 3 has overreached on a lot of topics, this total leakage number being one of them.

Here is another topic: Next time you do a duct-leakage-to-outside test, when the test is done, leave the equipment running with the leakage number showing on your manometer. Then pull the tape off the registers one at a time. Check the leakage number on the gauge. If the house is reasonably tight (let's say 4 ACH50), the number won't change even with all registers open - you didn't have to tape!

Ed Minch

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