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  1. #1
    Join Date
    May 2005
    Posts
    47

    Dual-fuel break even point

    I built a new house 5 years ago with two dual-fuel systems with a 95% efficient furnace for the lower level and an 80% for the upper level. The heat pumps are both 19 SEER. At the time, I made a spreadsheet showing that the heat pump was always cheaper than the furnace at the then-current utility rates. (Assuming that the temperatures didn't get so cold that the heat pump couldn't meet the required output.)

    I hadn't reexamined the cutover points again until now. In those five years, gas prices have gone down while electric has gone up (at least in Virginia). At my current rates, for the 80% furnace, the break-even point is 41 degrees, and for the 95% furnace, it's 63. Time to reprogram the thermostats!

  2. #2
    Join Date
    May 2005
    Posts
    47
    Here's the spreadsheet I use. The furnace calculations work for anyone, but the heat pump calculations are specific to my coil/outdoor unit combo.

    https://docs.google.com/file/d/0B1W5...it?usp=sharing

  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,002
    Quote Originally Posted by kcb203 View Post
    I built a new house 5 years ago with two dual-fuel systems with a 95% efficient furnace for the lower level and an 80% for the upper level.
    The heat pumps are both 19 SEER.

    I hadn't reexamined the cutover points again until now. In those five years, gas prices have gone down while electric has gone up (at least in Virginia). At my current rates, for the 80% furnace, the break-even point is 41 degrees, and for the 95% furnace, it's 63. Time to reprogram the thermostats!
    $/ Therm ________ 0.83 ___ 0.83
    Efficiency ________ 0.80 ___ 0.95
    Net Cost/Therm ___ $1.038 __ $0.874

    With electric rate at $0.12/ kW the break even points are 30'F and 47'F.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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