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  1. #1
    Join Date
    May 2009
    Location
    Seattle
    Posts
    3

    2-stage AC with single-stage furnace

    I know the general rule is that you don't install a 2-stage AC with a single-stage furnace, but everything I read seems to point to humidity issues with this setup. Here's my situation:

    I live in the Seattle area and am looking to install central AC so we can keep the house closed up most of the time due to health reasons in the family. The summer climate here is exceptionally mild with average highs of 75 degrees and humidity levels of around %50. The house is four years old, is reasonably well built and very tight. The existing furnace is an 80% single-stage Bryant that's more than fine for our mild winter temps (our highest winter gas bill has been $150 including water heating). Replacing the furnace at this point makes no sense economically.

    I've now had two bids that suggest getting a 2-stage AC to pair with my furnace. The vast majority of the time this would run at the lower setting, extending runtime and improving air circulation throughout the house, which is an issue at 75 degrees. Both companies say they do this on a regular basis when a customer needs to run the AC througout the summer. Only on the handful of 85 degree days (<10) we get every summer would the second stage kick in. Major humidity control doesn't appear to be much of a concern during the summer.

    What do you guys think? Will this cause me any major problems?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,774
    You won't remove much moisture in first stage without the blower slowing down.

    Since they say they do this commonly.

    Ask for references that you can go and visit, and talk to them. And feel how comfortable it is or isn't in their homes for yourself.

    There are add on devices that can be put on your furnace to slow the blower.
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  3. #3
    Join Date
    May 2009
    Location
    Seattle
    Posts
    3
    So, it's really just a moisture issue with the high relative CFM rate? In that case, it may be fine since summers here are very dry (contrary to what most people think). I think air circulation to push cooler air back to the second floor is a larger problem most days.

    Good idea to talk to some people they've done this for, and I'll ask about the blower add-on.

    Thanks!

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,774
    With the higher air flow. It removes very little moisture.

    And, when you cool 75F 50%RH air, to 72F. If no moisture is removed. It becomes 70F 60%RH air.

    With the lower moisture removal, you could end up with 55%plus RH.
    Last edited by beenthere; 06-03-2009 at 04:52 PM.
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  5. #5
    Join Date
    May 2009
    Location
    Seattle
    Posts
    3
    Actually increasing the humidity level might be an issue. If it just stayed constant at the 40-something percent we normally experience during summer it wouldn't be an issue. Sounds risky. Thanks very much for the info.

  6. #6
    Join Date
    Jun 2009
    Posts
    8
    Listen to BT, he is 100% on the mark. You can not dehumidify with exessive blower speed. Period.

    Lowering your RH (Relative Humidity) will increase you Comfort Level temp.

    Dehumidifier also helps.

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