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Community and Q&A

High Humidity Despite Whole-House Dehumidifier

green654 | Posted in General Questions on


We completed building our home a couple of months ago. We decided late in the game to add and ERV onto each furnace. As a result, it was tied into the existing duct work (I know this isn’t ideal but we didn’t feel like ripping up the walls). On the system that services the second floor, we added a whole house dehumidifier. In the basement, we have two free standing large dehumidifiers.

On very hot, humid days, the humidity upstairs is significantly higher than the first floor. Since we have the whole house dehumidifier upstairs, the humidity levels should be much lower. Currently the humidity upstairs is ready
ing close to 70%. Way too high! On the first floor without the whole house dehumidifier, it’s 50%.

The HVAC contractor has come out and tells me everything is working correctly and is set up right. But it’s not. The humidity upstairs  should never be this high.

Any ideas on what could be the issue? A malfunctioning dehumidifier? Anything else?

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  1. AlexPoi | | #1

    Is your dehumidifier only running when the furnace is running?

    Hard to tell without a duct diagram.

  2. charlie_sullivan | | #2

    There are lots of things that could be wrong, but one thing that could be part of it is that during the summer, the "reverse stack effect" means that cold air inside the house sinks and flows out of leaks near the bottom, and replacement air is sucked in through leaks in the envelope at the top. For example, if you have a vented attic and recessed lights in the ceilings upstairs, those often leak hot, humid outside air from the attic into the house. Attic hatches are another common leaks but there can be lots more.

    Did you have a blower door test? Is your attic conditioned or vented to the outdoors?

  3. Expert Member
    NICK KEENAN | | #3

    It could be that it's working as designed, which is what the HVAC contractor is checking, but that the design is flawed.

    Tell use more about what is going on. Do you have a temp/humidity gauge inside and out? You can get one like this for ten bucks on Amazon:

    Telling us the humidity is "too high" doesn't really help us.

    Also, what is the outdoor humidity and temperature?

    Does the whole house dehumidifier have its own fan, or does it only run when the HVAC fan is running? Does it have a humidistat setting, and what is is?

  4. Expert Member
    PETER G ENGLE PE | | #4

    With the ERV, whole-house dehumidifier and HVAC system all using the same ducts, there are all sorts of possibilities for crosstalk and unexpected air flows. Even with its own fan, a dehumidifier that is cut into the main ducts can just recycle its own air through the ducts without ever treating the room air. There are many other possibilities. It would take careful observation of the system in operation to catch them all. You will probably need someone local who is not related to your HVAC contractor to figure this one out. Your location, a duct diagram and equipment identification might help us to give you some more specific advice.

  5. user-5946022 | | #5

    Start with the simple stuff first.
    1. Get duplicate humidity sensors. Everywhere you are monitoring humidity should have two sensors. Does not matter if one is unconnected and inexpensive - you just want to be sure you humidity sensor is not wildly off.
    2. On the upstairs HVAC system, do you have the option of running the fan constantly? If so, try that for a day or two. That will help cycle the air through the upstairs space.
    3. Have you checked your dehumidifier to be sure it actually runs? Once confirmed, try turning it up. If it is a dial, turn just a bit more each time. See if that has any impact on humidity.
    4. Do you have a humidity monitor in your attic? Is the attic vented or conditioned?

    Note also that if you recently moved in, and the HVAC was recently turned on, there is a significant amount of moisture in the building materials that will dissipate in the first few months. Is your overall reading trending down when adjusted for weather/exterior humidity/temps?

  6. joshdurston | | #6

    Is the temperature way lower upstairs?

    Is the humidifier driving infiltration? Either by drawing in OA directly (some dehumidifiers have a outdoor air inlet), or by causing the furnace fan to run and lose/gain air in the leaky attic ductwork?

    Remember the R in RH is for relative.
    Use this calc to compare dewpoints

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