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Why does opening the HVAC closet door affect duct blaster number?

user-979926 | Posted in Energy Efficiency and Durability on

Recently I conducted a duct blaster test on a house with an interior HVAC closet. While conducting the test I had accidentally left the door to the closet open. After doing all my sealing on the system I tested the ducts again only to find the number hadn’t changed. However when I closed the HVAC closet door the number miraculously dropped. Now this closet is where the air handler is located sitting on top of a box built return so there is no way that I can see how shutting this door can effect the duct numbers. The entire return and plenum was sealed off so there were no leaks in the closet area. I know I’m missing something here. Please assist.

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  1. Dana1 | | #1

    The air handler is leaking air as are some of the ducts. Air leakage at the air handler is either going out or entering via the closet door.

  2. user-3116650 | | #2

    Agreed. If you are doing a supply side only test, you are seeing the number change due to the tightness of the closet door.

    As for the tightness of the ducts not changing at all after sealing - What is the number that you are getting? and how much does it actually change when you close the closet door?

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