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How Outdoor Temperature Affects Dehumidifier Run Times

thegiz | Posted in General Questions on

Hi, hope everyone is doing well. This question is to make sure I’m using a dehumidifier correctly. I live in the suburbs of nyc which means humid summers and cold winters. In the summer I run my ac which is central air with a vent into my basement as well as a stand alone dehumidifier in the basement at 60. Now that the temp is beginning to drop, do I turn off my dehumidifier for the fall/winter and then wait until temperature rises. In other words do you need a dehumidifier in cold weather. Isn’t cold weather dry? You also don’t run into the problem of condensation. I read that once temps drop under 60 you don’t need a dehumidifier. Is that correct?

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  1. Jon_R | | #1

    Most dehumidifiers have a humidistat - which means it turns itself off when it isn't needed.

  2. Expert Member
    BILL WICHERS | | #2

    I leave my basement dehumidifier running 24/7 all year -- it cycles automatically to maintain the target humidity level I set which is around 40% or so. It runs a little less in the winter, but it stills runs. Exactly how much it will run for you depends on how much moisture gets into your basement. I recommend you just leave the unit connected but set a reasonable humidity level for the dehumdifier to try to maintain for you.


    1. aunsafe2015 | | #4

      Out of curiosity, why do you keep it set to 40%? Personal preference, or do you view it as having some worthwhile benefit over the more commonly recommended 50-55%?

      1. Expert Member
        BILL WICHERS | | #6

        It's the same target I shoot for in the rest of the house. Comfy and low enough to ensure no mold issues. The other reason is that humidity levels are notoriously difficult to measure reliably, and are especially difficult to control and hold to a narrow band. Even though I set "40%" with the humidistat as a target, I typically see around 40-50% with seperate meters, so there is a pretty big window -- just because I dial in "40%" on the dehumidifier doesn't mean that's what the humidity level in the space will actually be.


  3. arnoldk | | #3

    I also leave my dehumidifier on year around and let it cycle on and off based on the humidity level I have set, 50% in the summer and 40% in the winter.

    I find in the winter my cheap builder grade window will get condensation once the temperature gets below freezing but having the dehumidifier running help reduce the amount that we get on the window.


  4. GBA Editor
    Kiley Jacques | | #5

    Here are Allison Bailes's "Three Goldilocks Conditions for Dehumidification":

    1. Cold weather. When it’s cold outdoors, you don’t need a dehumidifier because you can control the indoor humidity with ventilation. Cold air is dry air, you know.

    2. Hot weather. When it’s hot and humid outdoors, you may not need a dehumidifier because the air conditioner will be running and it should do all or most of the dehumidification you need. But that’s changing as homes get more airtight and better insulated. When we build a house with a really good building enclosure, we reduce the amount of time the air conditioner needs to run and that makes controlling humidity with just the AC a bit harder. This is especially true for smaller homes, like apartments and condos.

    3. Mild humid weather. When the temperatures are in the 60s and 70s Fahrenheit and the outdoor humidity is high, keeping the indoor conditions comfortable will probably require a dehumidifier. It’s too cool for the air conditioner to run but too warm for the outdoor air to be able to dry out the indoor air.

    1. Expert Member
      NICK KEENAN | | #7

      I'm not sure I agree with #1. Sure, in cold weather ventilation reduces the interior humidity, and a leaky house can be quite dry. But in a tight house a dehumidifier has two advantages:
      1. The dehumidifier lets you set a target humidity and hold it. Ventilation won't do that unless you have a humidistat controlled ventilation fan.
      2. A dehumidifier is a highly efficient space heater, with a COP of 2+. The latent heat that is removed from the water is kept in the interior. With ventilation, all of that latent heat -- much of which came from the interior to begin with -- is lost. Plus the air that is introduced has to be warmed. So if ventilation is not otherwise needed a dehumidifier is considerably more energy efficient.

      1. charlie_sullivan | | #8

        That comparison depends on whether the ventilation is done with HRV, no heat exchange, or ERV, and what the heating source is. In the shoulder seasons--when you are mostly likely to want some dehumidification, the COP of an air-source heat pump will be much higher than 2, so it can be worthwhile to ventilate and heat, rather than dehumidify, if you have an HRV.

    2. Expert Member
      NICK KEENAN | | #9

      In all of those scenarios, if you have a dehumidifier with a humidistat, and leave it on all the time, it will only run when needed and will always run when needed.

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