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Community and Q&A

How to duct a Whole House Dehumidifier?

Richard Marks | Posted in Mechanicals on

I am building a 6000 sq ft house in Alabama. Downstairs is about 4000 of those sq ft. On one of the two A/C units downstairs, we are installing a whole house dehumidifier. My installer will install however I want but he typically pulls air from the return trunk, and returns into the supply trunk bypassing the air handler. Is this the best way to do it?

I’ve read some prefer to have a dedicated vent in the house to pull in air to WHD and then return to the supply trunk after the air hander. Thanks.

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

    Richard. If construction is not finished, what makes you think you need a dehumidifier? If your house and basement are well constructed and the HVAC system is correctly sized, you may not need dehumidification.

    Also curious about your floor plan. How are you managing 2000 square feet above grade with a 4000 square foot basement?

  2. User avatar
    Dana Dorsett | | #2

    Are you sure you even the whole house dehumidifier?

    In a tight house with a right-sized (or better yet, right -sized and modulating) air conditioner(s) there is rarely a need for a whole house dehumidifier, unless you insist on very high ventilation rates.

    Oversizing the AC such that it runs at a low duty cycle is where poor latent load management usually creeps in, prompting the whole house dehumidifier band-aid. The exceptions would be houses designed for very low sensible loads, such as a PassiveHouse, etc.

  3. Richard Marks | | #3

    In Alabama, we have high humidity. Often in fall, winter and early spring, we have no need for heating or cooling so HVAC doesn't run unless I set thermostat into the 60s but then we all get cold. Hence, I want a dehumidifier. Steve, we will have 2 units downstairs - 5 tons total for downstairs. Plan is to put WHD on the 3 ton unit.

    Back to my original question - how should it be ducted and why?

  4. Steve Knapp CZ 3A Georgia | | #4

    Richard. I live in Georgia and use two tons to heat and cool 3,200 square feet. Your HVAC sounds massively oversized if your home is being built with proper air sealing and insulation. But it's your house.

    You need to install the dehumidifier according to the manufacturers design guidelines to preserve your warranty.

  5. Richard Marks | | #5

    Does anyone have a substantive answer to my question? I'm sure there are several ways to duct a WHD that won't void a warranty. What I'm asking is for the preferred way to duct a WHD and why?

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

    1. There is a good chance that your cooling system is oversized. This can lead to comfort problems. An oversized air conditioner will do a worse job of dehumidification, not a better job of dehumidification, compared to a right-sized unit.

    2. Most homes don't need whole-house dehumidification unless (a) the HVAC contractors did a poor job of equipment sizing, or (b) it's a very high-performance house that needs very little cooling so that the air conditioner runs rarely.

    3. If you want to buy a whole house dehumidifier, follow the installation instructions provided by the manufacturer. The illustration below comes from the installation instructions for the Therma-Stor Ultra-Aire.

    -- Martin Holladay


  7. Richard Marks | | #8

    I was previously concerned about units being oversized. However, I've been to the installer's office, looked at his inputs on the Manual J, and seen the output results. Everything looked correct (SHG, facing W, window sizes, overhangs, etc). Only thing he did was round up on tonnage. For instance, upstairs Man J calls for 2.1 tons. Since the units I'm looking at only come in whole tonnage, installer says to go up to 3 tons.

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