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  1. #1
    Join Date
    Mar 2008
    Central Maryland
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    Confusion on HP sizing

    I'm really confused.

    Let's take a semi-hypothetical case. Say I have a house that needs 70K BTU/Hr to meet heat loss at coldest design temps.

    The homeowner upgrades from 3 ton HP with 5KW heater strip to 3 ton HP dual fuel with 75,000 BTU 95% gas heater.

    Below the set point, the gas furnace will produce about 71,250 BTU's/Hour.

    But wait a sec. The old heat pump could only produce 36,000 BTU/Hour, and the 5kW heater strip 17,065 BTU/Hr. You can't really add them together, because when the heater strip comes on the HP is already running below optimal temps, and besides it is running a good percentage of the time in defrost mode, but if you DID add them together, you'd only get 53,065 BTU/hr.

    Are contractors (in Maryland, anyway) allowed to undersize for all-electric installations, are all gas furnace installs vastly oversized, or am I missing something?


  2. #2
    Join Date
    Apr 2005
    Tampa, Florida
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    Was the old heat pump carrying the load satisfactorily in the past? How well was it able to maintain temperatures on the extreme days? If it was able to keep up, the gas furnace is probably oversized and the heat load calc is suspicious.

    Not sure what inspectors are like in MD, but in FL there's no real oversight as to the capacity installed (unless it's grossly oversized). Instead the inspectors are more concerned about things like hurricane tie-downs, proper electrical connections, etc. That is if they bother at all. Depends on the jurisdiction.

  3. #3
    Join Date
    Jan 2004
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    In your hypothetical question.
    If the house has a 75,000 BTU loss at design.
    That 3 ton HP and 5 KW strip, never kept the house at design temp at ambient design conditions.

    Of course, neither will that 75,000 BTU furnace, since its not 100% efficienct.

    What brings about this hypothetical question.
    Contractor locator map


    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jul 2008
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    most installers would just add more kw,if there is room in the breaker box to run more power to it.

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