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Community and Q&A

Is a dehumidifier needed in a tight house?

Ben Rush | Posted in Mechanicals on

I’ve read several times that it’s common for tight, well-insulated homes to need supplemental dehumidification, as the AC doesn’t run enough to dehumidify the home.
As a person who is especially sensitive to humidity, I’m very interested in this.
I wonder if an equal level of comfort can be achieved more economically with a higher temp. set point but lower humidity. (In my case, Chicago area- warm and humid in the summer).
How can we intentionally affect humidity more than temperature- or vice versa- when dehumidifiers and ACs seem to be basically the same device, with different packaging?
Isn’t it inefficient to operate a dehumidifier? Doesn’t it lower the humidity- but raise the temperature- causing the AC to work more? It seems comparable to operating a window AC on a dining room table instead of installed in a window. it would dehumidify the air, but the added warmth (ok, technically not “added” warmth. More like relocated warmth, maybe…something about latent vs. sensible heat- which is over my head- but you get my point) would remain in the house instead of going outside. No one would operate a window AC inside their house- but isn’t it actually pretty similar to running a dehumidifier? On the other hand, with the window AC installed in the window as per normal, it won’t run enough in a tight, well-insulated house to provide sufficient dehumidification. We’re back where we started- with an adequately cooled- but overly humid house. (Granted, you probably can’t have a “tight” house with a window AC unit, but that wasn’t the point.)
For that matter, I could run some heaters in the house in summer. Then the AC would have to run more to compensate for the added heat- which would also cause it to dehumidify more. (Ok, that sounds really crazy. Evan crazier than a window AC on a dining room table.)
Can you help a non-physicist to wrap his head around this? If at all possible, please don’t use words like “latent” and “sensible” without explaining them at a high school level. 🙂
Thanks!
Ben

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    I have a tight house (1.1 ach), and it require supplemental ventilation. To introduce outside air while controlling humidity, I use a ventilator/dehumidifier. There is an energy cost associated with drying the incoming air, but it is offset by the home's overall efficiency.

  2. Ben Rush | | #2

    Steve,
    Thanks for chiming in.
    How humid is it where you live?
    What indoor humidity level are you aiming for?
    What sort of ventilator/dehumidifier do you have?
    Thanks,
    Ben

  3. Steve Knapp CZ 3A Georgia | | #3

    Atlanta is a mixed humid zone, so humidity control is important. I have an Ultra Aire 90H in combination with a Panasonic FV-04. In combination, the two units provide the CFM spec'ed for the structure. (FYI. The Panasonic delivers about half the CFM defined in its data sheet.) Humidity stays between 40 and 50 percent most of the year but can drop into the low 30s in the winter. I use inexpensive humidstats (from HD) to keep an eye on levels throughout the year.

    In my installation, we are dumping the conditioned fresh air into a storage room that is next to the HVAC return. The new air is mixed with the recirculating air and ducted throughout the house. The HVAC fan is always on in the low-speed position. FWI, this is not an optimal installation. You might be able to do better.

  4. John Clark | | #4

    One can build a HVAC system that's comprised of dual-stage compressor along with a variable speed air handler.

    There's also inverter driven compressors.

    Then you have the ductless mini-splits which from what I've read are better at modulating. From my own personal experience (Panama) they work well at removing humidity without increasing the temp.

  5. User avatar
    Dana Dorsett | | #5

    To convert a pound of liquid water to a pound of water vapor at the same temperature takes ~970 BTUs of additional heat. Converting a pound of water vapor to a pound of liquid water releases that heat to something- usually air, with standard dehumidifiers. A mini-split or air conditioner releases that heat into the outdoor air. A room dehumidifier releases that heat to the room air.

    Yes, that sensible heat from the room dehumdifier adds load to the AC, but it's only incrementally more load than if the AC were removing that humidity/heat directly.

    Almost all mini-splits have a "dehumidify" mode to bring down the humidity efficiently & quickly with minimum of sensible cooling, but it does cool the air off slightly when in dehumidify mode. Most mini-splits don't control to either a temperature or humidity set point when in dehumidify mode- it won't stop if it gets too cool (or too dry) indoors without human intervention to change the mode to heating or cooling.

    The Daikin Quaternity series mini-splits will control to separate humidity & temperature setpoints, and can even dehumidify with NO sensible cooling. But they aren't as efficient in heating mode as most the cold-climate mini-splits suitable to a Chicago climate.

    An ERV doesn't dehumidify unless the outdoor air is drier than the indoor air (as measured by dew point or wet bulb temp), but it does limit the amount of moisture coming in on the ventilation air when it's more humid outdoors than indoors.

    The little Panasonic ERVs don't cut it in Chicago for winter use- they'll ice up and self-destruct if you try (and it won't be under warranty), but would be fine from April through October. An HRV would be more robust, and you could lower the ventilation rate during periods of high outdoor dew points to lower the size of the latent load introduced by the ventilation air.

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Ben,
    Here are links to two articles that may help you:

    All About Dehumidifiers

    Air Conditioner Basics

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Ben,
    To answer your original question: In most cases, an ordinary air conditioner should be all you need to control indoor humidity in Chicago.

    If your house is unusual in some way, or if you are particularly sensitive to high indoor humidity levels and require very low indoor humidity levels for medical reasons, you can supplement your air conditioner with a simple stand-alone dehumidifier. While this dehumidifier will add a little bit of heat to your house, the air conditioner will easily be able to handle the additional heat load.

  8. Ben Rush | | #8

    Thank you all. It's making sense.
    Ben

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