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Indoor winter humidity level affects required heat set-point for comfort?

David107David107 Member Posts: 1,250
I have heard that keeping indoor humidity levels in winter around 40% will make for greater comfort--partly in the sense that it will seem warmer at the same set point. It occurs to me as I'm looking at putting in a new boiler, that, in addition, adding a whole house humidifier to our attic central AC might allow a lower set point and save heating $$. (Like in summer, we can feel cooler with lower humidity levels.)


  • SuperTechSuperTech Member Posts: 154
    Unless you can keep the humidifier in a closet, and pipe the steam line coming off the humidifier into the attic and the ductwork it's not going to be a good idea.

    Humidifiers require water supply and drain lines that can't be installed in an area that is subject to freezing temperatures such as an attic. Fortunately a nice steam humidifier like the Aprilaire 800 can be installed like I described.

    It won't just seem warmer, air with a higher humidity level provides a more comfortable warmth than dry air.

    Your ducts are for A/C only? What provides heat to your home? A boiler and baseboard/radiators?
  • David107David107 Member Posts: 1,250
    edited February 12
    I should have mentioned that my attic is now insulated with closed cell foam (about R38)--a conditioned space within 3-5 degrees of floor below. Ducts AC only. HW boiler in basement; main/2fl cast iron rads; basement steel baseboard. Is there any way to measure potential heat savings? I was thinking with 40% humidity--instead of the usual 20-25% in winter--set point might be dropped maybe two degrees--a guestimate of course.
  • SuperTechSuperTech Member Posts: 154
    Sounds good to me. That foam insulation is a blessing for us HVAC techs that have to brave the attics in the summer.
    I recommend an Aprilaire 809 steam humidifier. Only with a steam humidifier will you be able to achieve your target R.H percentage. And I agree that you should feel at least 2-3 degrees warmer with 40% relative humidity. And you will have less static electricity in the house and nosebleeds related to dryness will be eliminated
  • David107David107 Member Posts: 1,250
    So the humidifier would be in the attic and piped into the air handler? Unfortunately the AC system doesn't reach the basement, small 1st floor half-bath or under-insulated side porch, but in general this would likely be a big improvement.
  • SuperTechSuperTech Member Posts: 154
    Typically I see these units installed on the side of a vertical air handler/furnace or on the wall next to the unit. The steam line and nozzle is usually installed in the supply plenum. A humidistat is provided with the unit, but if you have a thermostat like the Honeywell 8000 series that can be used to control it as well.
  • David107David107 Member Posts: 1,250
    Great idea. I wonder if the introduction of water into the duct system might create conditions for mold and therefore require an annual cleaning. And I guess the humidistat, if connected to a thermostat, can also turn the AC on and off based on humidity more than temperature.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 3,524
    edited February 12
    @David107 adding a humidifier is a great idea but.............they require quite a bit of maintenance....not a free ride.

    The good news is even if your ductwork doesn't reach certain areas the humidity will. Moisture transfers by vapor pressure not by air flow.

    In the winter the indoor air is very dry. You may have trouble getting to 40% but the improvement is likely worth it

    Your thermostat could fire up the ac on high humidity but you would need duct reheat to prevent overcooling don't want to do that, it's just for commercial controlled spaces, labs, manufacturing etc.

    A humidity high limit located 10ft downstream of the humidifier wired to shut the humidifier down on high humidity is a good idea and prevents duct flooding.
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