Technical / Extra Edition

Is It Better to Test Ducts with a Positive or Negative Pressure?

Colin Genge
Colin Genge

A very common complaint about duct testing is that the covers get blown off the registers.

Most code language leaves the choice of pressurizing or depressurizing up to the tester, but experience has shown that the positive pressure made sealing registers much more difficult, and could triple the time to perform a test.

This has influenced most new codes, such as North Carolina, which makes a point to state that a single point depressurization test is sufficient. It also means that inexpensive Saran Wrap could be used to seal registers, making it much easier to remove, and less likely to cause damage to paint or ceiling surfaces.

All states allow results under depressurization, with the exceptions of Washington, Delaware, Idaho, and California. If you’re testing there, I suggest doing a depressurization test until you know it will pass, and then quickly turn around your fan to pressurize. This way, if you get significantly different readings you may be able to attribute it to blown register seals, for easier problem solving.

If your state’s regulations have the option of duct leakage to outdoors, and you go the pressurization route, an easy way to avoid blowing register covers off is by pressurizing the house first then reducing the duct to house pressure to zero.

How do you avoid blowing register seals?

Publication date: 8/20/2012

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