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Managing humidity in a tightly constructed ventilated house

Dave Brooks | Posted in Mechanicals on

In a tightly built, newly constructed house in Zone 5, we’re experiencing higher than desired humidity. As expected, the air conditioning does not run often enough to effectively dehumidify, even in the peak heat of an Illinois summer. I’d value your recommendations.

Here are some specifics:

Including the basement we’re conditioning 3,300 square feet with a 3.5 ton two-stage Rheem, with a 5 ton ECM drive. The system can hold temperature easily even on extreme days with Stage 1, which is set up for 700 CFM. With the ventilation rate on our Ultimate Air 200 DX RecoupAerator as low as 30-50 CFM, indoor RH will hover in the 55-60% range on muggy days with the AC fan set to Auto. If we leave the fan on continuous–which is our desire–the humidity will go even higher.

I’m just about to conclude we need to add a whole house dehumidifier. What are your thoughts? If we go this direction, I’d love to hear recommendations about specific models. The one I have my eye on is the Honeywell DR90.

Thanks for your insights.


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  1. GBA Editor
    Martin Holladay | | #1

    It's possible that your air conditioner is oversized; the only way to tell would be to perform a cooling load calculation. But even if your air conditioner is oversized, I doubt that it is the main cause of your problem.

    One possibility is that your HVAC contractor included an outside air duct connected to your return-air plenum -- a technique that I call a "lazy man's central-fan-integrated supply ventilation system." If that's the case, it would explain the difference in indoor humidity levels when you switch the fan from "auto" to "continuous." If you discover that your house has this unnecessary (and counterproductive) outdoor air duct, block it off -- after all, you have a RecoupAerator for ventilation.

    Assuming that your home doesn't have any egregious air leakage issues, the answer is to install a dehumidifier. I would start with a $250 stand-alone unit, because it's cheaper than the alternatives.

    P.S. It occurs to me that there is one other possibility: if your RecoupAerator shares ductwork with your forced-air system (rather than having its own dedicated ventilation ducts), your problem could be due to an interaction between your air handler fan and your ventilation system. There are lots of ways to duct ERVs, and most of them are wrong. If your system is ducted in such a way that the air handler fan depressurizes the fresh air duct connected to your RecouAerator, then every time the air handler fan turns on, you might be ventilating your house at a very high rate. For more information on ducting HRVs and ERVs, see Ducting HRVs and ERVs.

  2. Dave Brooks | | #2

    Thanks for your thoughts.

    The AC system is sized according to Manual J. Blower door came in 1.14, but I've done additional sealing since testing. There is no outside air duct other than ERV intake and exhaust. The ERV integrates with the system as described in the RecoupAerator manual for the shared forced air option.

    Do you have any recommendations for avoiding mold on a stand-alone dehumidifier? I've never used one that didn't become moldy.

    The stage one air handler fan speed can be slowed 10%. Would you recommend this, assuming our contractor has no concern for icing? Even with this, I'll still have to come up with a way to dehumidify without excessive cooling in transitional seasons.

    Here’s another wrinkle: With the dryer and water heater both on and the furnace blower cycled off, I can feel air being drawn in through the ERV exhaust. Should it have a backdraft damper? The idea of unfiltered, unconditioned air gaining easy entry here is concerning, and is no doubt adding to the moisture load. The RecoupAerator can be set to have 20, 40, or 60 CFM of positive pressure for summertime, but that’s not enough to overcome the exhaust and I hate to waste energy during the times when there is no exhaust.

    Thanks again for your help.

  3. GBA Editor
    Martin Holladay | | #3

    When you operate exhaust appliances (your clothes dryer and your power-vented water heater), the makeup air has to come from somewhere. If it wasn't coming in through your RecoupAerator, it would be coming in through other cracks in your envelope. Air in always equals air out.

    For more information on dehumidifiers, see this article: All About Dehumidifiers.

    I looked up the RecoupAerator installation advice, and the company provides four ducting options, including the one shown in the illustration below. I don't like it. If your ducts look like this, it would be easy for the air handler fan to suck in more outdoor air through the RecoupAerator than the device is programmed to introduce.

    It's always better for an HRV or an ERV to have dedicated ventilation ductwork rather than to share ductwork with a forced-air system.


  4. Dave Brooks | | #4

    In the RecoupAerator manual, our system is installed as described in Option 4. I'm guessing your concern for the air handler overcoming the ECM motor of the ERV still applies.

  5. user-723121 | | #5

    In Minneapolis we cool a similar square footage with a 2 ton, 16 SEER AC. The humidity level stays at 50% or less. We set at 78F and even on the warmest days the 2 ton unit just coasts. Our house is reasonably energy efficient, about 2.2 BTU/SF/HDD. I have 2 building customers (in Mpls.) with similar sized homes and a bit more solar gain and there I specified 2 1/2 ton units. They keep the house cooler than I would but the 2.5 ton AC works nicely.

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