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Community and Q&A

Humidity and negative pressure

jdclassen | Posted in Mechanicals on

A customers home (zone 4 non-marine) has condensation problems (summertime) on a particular HVAC duct. Upon inspection the %RH was at 60%. There is a whole house dehumidifier hooked in with the HVAC System (Fan runs at low speed continuously) Bathroom exhaust fans are backflowing and letting in a significant amount of heat, and when opening the front door one can feel the hot air entering the house. There is no ductwork in the attic, it is all in the basement.

I have thought about disconnecting the Dehumidifier from the HVAC (running it independently) and letting the fan cycle with the A/C rather than constantly on.

I still can’t sort out the negative pressure issue.

Any thoughts on the issue.

Many Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds as if you have correctly diagnosed the problem: the house is at negative pressure with respect to the outdoors, and lots of humid outdoor air is being pulled into the house through cracks. This is leading to high indoor humidity levels.

    It takes a home performance contractor skilled in pressure diagnostics to determine why the house is being depressurized. A blower door technician who has been trained by RESNET or BPI may be able to help.

    If a leaky supply duct is installed in a closed ceiling cavity, and the ceiling cavity connects with a leaky rim joist, then supply air may be forced outdoors through rim joist cracks, depressurizing the house. I'm not saying that's what's happening in your house -- it's just one example of a mechanism that isn't obvious.

    Good luck with your detective work.

  2. jdclassen | | #2

    I am BPI Certified (yet not as experienced as some), however I have not encountered this issue in the past, and needed some additional brain power.
    I have set up a date for a blower door test, I am trying to think through how to track down where the leak in the HVAC must be in order to be causing a negative pressure. I have run a thermal scan of the house without significant findings. I am also thinking I need to test for the neutral pressure plane, however that will change when the mechanical systems turn on and off.
    I will update with my Blower door testing etc.

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