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  1. #1
    Join Date
    Mar 2008
    Jackson, NJ
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    RH - Forced Hot Air ?

    I'm going to start a new thread here as I want to be respectful of the original poster on a diff't topic.

    Originally Posted by Jopopsy
    I don't mean to hijack this thread (and if I am i will happily post a new topic), but am I reading this right? Heat Pump or Gas Furnace, neither will affect your humidity in the air by warming the air up. What affects the humidity is the amount of air infiltration you get during the course of the winter (and I will assume you get higher infiltration when the system is running do to the sucking power of the blower).

    I always thought that forced hot air systems dried out the air - its almost a universal complaint about those types of systems. Hence the whole (baseboard - moist heat argument).


    Slight hijack so may post new topic if further info is needed but you seemed to misread Been's statement. He says the RH will change (decrease as temp increases) but the actual moisture content of the air remains the same. Infiltration of the dry outside air will decrease both the RH and the actual amount of moisture necessitating a humidifier.

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    #37 Today, 01:16 PM
    Professional Member Join Date: Jan 2004
    Location: Lancaster PA
    Posts: 27,906

    Leaky supplies and returns can cause infiltration of outdoor air. So in those cases, a forced air system dries out a house more then a hydronic, or electric baseboard system.

    If the air entering the furnace has 50 grains of moisture per pound at 70°F, the air leaving the furnace at 140°F still has 50 grains of moisture per pound.

    And when that 140° air cools to 70, it will have the same RH as the room air.

    You can't destroy moisture.

    So in affect, my prior home (McMansion) w/ 2 Forced Hot Air Furnaces, horrible windows, and massive cathedral ceilings was probably leaking like a sieve all over the place?

    I could watch the humidity in my house drop from 55% to 20% over the course of a couple of days in extreme cases. I especially watched this during those months when we would get 1 60 degree day (open the windows and let some of that new air in), then watch it sink like a rock as it got back to normal temps outside (30s). The RH in the house would go up to 50 or even 60 w/ the house temp around 65 to 68. I'd close the windows when the inside temp would drop below 65, and a couple days later all that humidty would be gone and my Aprilaire 400 would be running non-stop every time the fan kicked on. Which was pretty much 80% of the time given my cathedral ceilings and under-sized system.

    I don't miss that house at all.

    Yesterday I closed all the windows in my new house around 8pm. It was 62 outside and 72 inside. This AM I got up to it being 51 outside. Inside was still 70. I'm so much happier in this new house.

  2. #2
    Join Date
    Jun 2004
    4H: Hot, Humid Houston H.O.
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    Yes I believe infiltration is the difference. It could be focused on the ductwork, or could be all over the place. This is a subject a little outside normal HVAC training I believe, at least until you get to advanced topics. I am a homeowner who is trying to keep informed on these topics as they affect me more than many.

    Regards -- Pstu

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