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  1. #14
    Join Date
    Sep 2009
    Arnold mo
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    There was a pro on this site a little while ago that said his company insulates ductwork in attics with spray foam. You may want to look into that in conjunction with an 80% furnace. You are likely losing 30% efficiency on your system due to the ductwork being in the attic, so use the price difference between the 80% & the 90% and get your increased efficiency from air sealing & insulating that duct system. Sounds good in theory at least.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............

  2. #15
    Join Date
    Jun 2005
    SW Wisconsin
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    Here is what I come up with on sizing:

    1) Observation of current furnace:
    At 32F outside (per HVAC-Calc the low temp for our area is 10F) and inside set to 69F, it runs for 5-6 minutes, waits 13-15 minutes, then does it over again.
    If you can dress so a temperature swing of a few degrees within the comfort zone is okay, I'd install a room thermostat with a temp swing setting and then go with a single stage furnace.

    You can control the off time & on time to a more efficient cycling ratio in both the heating & cooling modes.
    At 32F outside & Inside set to 69F, it runs for 5-6 minutes, waits 13-15 minutes, then does it over again.
    Of course, that is what I call short cycling; runtimes are too short with too many cycles per hour; it wastes energy reheating the plenum & duct system & cycles off about the time it is reaching its steady-state efficient heat delivery condition.

    I have mine set to run about one cycle per hour at night with around 14 to 18-miniutes of runtime each cycle. In the day-time the off times may be 2 to 4-hours & when the sun is shining it may not cycle until evening.

    At 32-F outdoors or somewhat lower, with the TH Set at 67-F it drops about 3-F before it cycles on. It appears to be very efficient at those settings; using that or a similar room t-stat setting I'm perfectly comfortable, I don't know why I'd ever need a 2-stage furnace.

  3. #16
    Join Date
    Feb 2012
    Post Likes
    Thread Starter
    Ended up going with a 80% 75k VS 2-stage. To vindicate the HVAC-Calc and other data below, the house is cozier than ever, and even at 17F outside I have only seen it running on low heat. Thanks for the help back then.

    HVAC-Calc estimates:
    Infiltration set to 'average': 25.5k cool and 44.2k heat
    Infiltration set to 'excellent': 23.1k cool and 34.6k heat
    (house is standard vinyl-sided split-foyer built in 1992 with a vapor barrier, but a few leaky spots.)

  4. #17
    Join Date
    Aug 2011
    Lewes, DE
    Post Likes
    Systems are less efficient and will fail sooner when running on low stage constantly

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