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Community and Q&A

Supply-only ventilation with dehumidifier ?

user-1126830 | Posted in Mechanicals on

The house will be a 2200 square foot house in Zone 4A. At this time the house is planed to be as tight as I am able to get it with in reason.

My question is, is there any reason not to use the fresh air option on a stand alone whole house dehumidifier such as the Honeywell TrueDry 90 as the ventilation source ?

I ask as I know one of the bigger IAQ problems in the home will be humidity.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not in favor of the routine installation of a dehumidifier in most climates. Dehumidifiers are energy hogs, so I'd be reluctant to install one unless you are addressing a specific problem.

    If your home is well designed, there is no reason to assume that you will have an IAQ problem or a humidity problem.

    I assume that your home will include an air conditioner. In most cases, your air conditioner should handle your humidity issues.

    Be careful that you don't overventilate, especially if you choose to install a dehumidifier. Overventilation plus a dehumidifier is an energy disaster.

  2. user-1127834 | | #2

    Agreed too: We have seen 20-40 dollar mo expense adders to those things.

    And if your HVAC-HW bundles together, there can be very VERY low speed blower operation, in a DeHumidifcation programmed process as a result if making HW for a time in that HW heating as a 'mode'. This can be because a much lower (what's called ) suction pressure to an air-conditioning-mode air-coil in a forced-air unit is achieved with very much lower than normal blower speed used when the compressor operates at a higher (whats called) head pressure for higher temperatures to make usable HW, again , by programmed , etc, for a lifestyle . (detail available)

    The Programming can be selected and changed by the homeowner as lifestyle changes with HVAC-GT-HW-Reclaim systems since 2007. Larger air-coils to blower speeds in low cooling stages allow for wringing water out of the air as Mark says: (in to ) assumed home will have air conditioning.

  3. homedesign | | #3

    I live in a very tight.... not-so-big house in North Texas.
    I believe that in a very tight not-so-big a humid climate ... there will be a few times in the shoulder seasons that a Dehumidifier is necessary.
    My stand -alone dehumidifier (I think I paid about $275) is running right now
    this morning it was 72 outside and 77 RH
    no reason for the AC to kick on ....
    soon it will be warm enough that the AC can do the job ... and I will not need the Dehumidifier again until a few days next fall

  4. user-1022459 | | #4

    Whole house ventilating dehumidifiers can be an efficient choice when properly applied to a house. If you are ventilating mechanically in a climate with humid air for a portion of the year, a whole house ventilating dehumidifier (such as the Honeywell unit you are considering) is a great choice.

    The previous posters are correct that dehumidifiers cost money to operate, but if you have excess moisture in your house - you need to remove it. There are many dehumidifier choices today ranging from low cost consumer units to high efficiency whole house dehumidifiers. You have chosen a high efficiency whole house dehumidifier that will have a lower life-cycle cost (will cost much less to operate) than a cheap department store dehumidifier. I suspect that the previous posters are referring to the cost to operate a cheap department store dehumidifier in their posts above.

    A properly sized A/C system should do a good job removing excess moisture when it is running. As John states above, the A/C system does not remove moisture unless there is a call for cooling from the thermostat. Unfortunately the need for moisture removal and cooling are not always coincident and the A/C system is a poor (inefficient) choice for moisture removal if cooling is not needed.

    The dehumidifier you suggest will automatically control the moisture level inside your house independent of the A/C system. That dehumidifier is much more efficient at removing moisture (in pints/kWh) than an A/C system. With the dehumidifier you will not have to over-cool your house to dehumidify it. That dehumidifier will also provide supply ventilation for your house - including filtration and dehumidification (if required) of the air before it is introduced into your house.

    As we improve the tightness and insulation of houses, we reduce the (sensible) cooling load in the house, but we do not reduce the (latent) moisture load in the house (as much). The ventilation air and occupants will bring/create moisture inside the house which must be removed. A high efficiency whole house dehumidifier will handle the moisture removal, allowing the A/C system to keep the temperature comfortable without having to over-cool in order to dehumidify.

  5. GBA Editor
    Martin Holladay | | #5

    Note to GBA readers: for those who don't know him, it's worth mentioning that Tim O'Brien is an employee of a manufacturer of dehumidifiers. Tim O'Brien is the vice-president of engineering at ThermaStor.

  6. green654 | | #6

    But what happens in the winter when the air is dry and dehumidification didn’t necessary but ventilation still is? Do these ventilating dehumidifiers have the ability to run ventilation without dehumidification?

  7. user-2310254 | | #7


    Note that this is an old thread, so you may want to create a new one. At my last house, the controller for my Ultra Aire ventilating dehumidifier could be set to ventilate without dehumidifying.

  8. hotandhumid | | #8

    I'm in the same boat as you. I am replacing my upstairs HVAC/heat pump with all new system and new ducting (old ducting in attic was R-4). I wanted to add dehumidification, as well as fresh air since we are foamed and over insulated in the attic. Can I use the dehumidifier ( which will be ducted into heat pump hander) to bring in fresh air-it has an option to duct it that way. I was told the humistat has an option/damper to close off outside air when weather is too humid or not good.


    Hot and Humid

    Hot and Humid

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