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  1. #1
    Join Date
    May 2012
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    Question Duel fuel (NG + Heat Pump) optimum crossover point.

    I have a 80% gas furnace and a Goodman DSZC16 16 seer heat pump. I live in Eastern Washington.

    Our system is currently configured to switch to natural gas somewhere around freezing temp.

    However looking over the Goodman literature it seems like switching below freezing would save more money.

    I am paying about $1.00 per therm delivered and $0.0731 per kWh.

    Check my math on this.
    1 Therm x 80% = 80,000 BTU = 80 MBh
    $1.00 / 80 = $.0125 per MBh (NG)

    DSZC16036 (30 degrees outdoor) = 26.2 MBh, 2.39kW
    2.39 kW = $0.174
    26.2 MBh (NG) = $0.3275

    So even at 30 degrees outside temp the heat pump is significantly cheaper then the gas furnace? Can you run a heat pump when it is below freezing outside? Am I missing something here, too many assumptions?


  2. #2
    Join Date
    Jan 2009
    Saint Joseph, MI
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    SO you're showing a COP of 3.2 at 30F. Where did you get that data? I wonder if that's steady state, or if it factors in energy for defrost.

    Although looking at your insanely low electric prices (almsot 1/2 of my summer rate), I'd say that even if defrost cost you 20% effciency, you're still well ahead.

    Too bad you don;t have a Honeywell Visionpro IAQ thermostat (I assume you have the white Rodger Goodman branded communicating stat). THe VPIAQ allwos you to select two setpoints so you create a range. THe lower limit can be set to the economic balance point. The upper limit can be set at around the thermal balance point where the unit should start falling behind.

    Ideally what you want is to run the heat pump most all the time above maybe 20F, and have het furnace occassionally upstage for short periods of time to then return to the heat pump. I'm not sure if your thermostat will do this.

    No problem running below freezing. Unitl my electric rate went up I ran mine down to 25F and it did well. It was a little oversized, but the air was still reasonably warm. Later I had the furnace locked out above 45F and the heat pump locked out below 35F.

    Some thermostat will allow the unit ot ru at a reduced airflwo when it get below a certain temperature. Not a bad idea for comfort. You lose just a little effciency, but I think it's worth it. I'd do that under 40F if it's an option.

  3. #3
    Join Date
    May 2012
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    Thread Starter
    [QUOTE=motoguy128;13367621]SO you're showing a COP of 3.2 at 30F. Where did you get that data? I wonder if that's steady state, or if it factors in energy for defrost.QUOTE]

    The numbers are from here... (Page 24)

    I believe that I have a Honeywell VisionPRO 8000 thermostat. I'm not sure what it's capabilities are.

    I appreciate your feedback and will try running the heat pump to a lower temperature next winter. 20 degrees sounds like a good start.

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