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  1. #1
    Join Date
    Oct 2008
    Location
    Indiana
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    78

    Debating gas vs electric backup for heat pump

    I am getting ready to replace a system in my home. Presently I have a heat pump with a gas furnace backup, both about 20 years old. For the new system I am seriously considering going to electric heat backup instead of gas.

    My thinking is that with gas backup, I can't run both heat pump and gas simultaneously, so once the temp drops below the point where the heat pump can heat the house by itself, then I am never using the heat pump. Whereas with electric, I can run heat pump and electric heat together, so I can use the heat pump down to a much lower outside temperature.

    My utility cost is about $1.10 per therm for gas, and $1.45 per therm for electricity.

    Does this make sense?

    Is there such a thing as a system that uses all 3 fuel types? In other words use the heat pump as the primary, electric strip for additional heat down to a certain outside temp, and gas heat for very low outside temp and during heat pump defrost cycles?


    -jim-

  2. #2
    Join Date
    Oct 2008
    Posts
    24

    *where ya from*

    just curious what type of region u live in, this info will allow me to give u a much more informative reply

  3. #3
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    At the lower temps that the furnace would run, the HP would be less effecient than the gas. The strips are always less efficient than gas.

  4. #4
    Join Date
    Oct 2008
    Location
    Indiana
    Posts
    78
    Location is central Indiana. With my electric rates it seems as though the heat pump is more efficient than gas at least down to 0.

    This will be a 4-ton heat pump. These are some sample numbers I came up with:

    At 0 degrees, the heat pump puts out a net of 18,000 btu/hr for a cost of $0.15/hr. If the total need for heat at that temp is 54,000 btu/hr (rough guess) then I can get 1/3 of my heat from the heat pump. The other 36,000 btu would come from the strip heat at a cost of about $0.52/hr, total cost to heat is $0.67/hr.

    Using gas, 90% efficiency, I need 60,000 btu/hr in, at a cost of $0.66/hr. So by my calculations, at any outside temp above 0 deg F, it is cheaper for me to heat with electric backup than with gas.

    -jim-

  5. #5
    Join Date
    Sep 2002
    Location
    I don't know
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    2,903
    Quote Originally Posted by Daltex View Post
    At the lower temps that the furnace would run, the HP would be less effecient than the gas. The strips are always less efficient than gas.
    Heat strips are always 100% efficient.
    A heatpump with a COP above 1 will always be more efficient than a furnace.
    Max furnace AFUE = 95%

    Efficiency is not to be confused with capacity or cost of operation.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  6. #6
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,903
    ......
    Last edited by amd; 11-04-2008 at 08:37 PM. Reason: Double post
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  7. #7
    Join Date
    Oct 2008
    Location
    Indianapolis
    Posts
    172
    One other thing to consider is...Does your current electrical panel have the capacity to handle electric strip heaters. If not upping the capacity would be an additional expense to factor in your decision making.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,726
    I wouldn't think Indianna would have many house that need a 4 ton heat pump for cooling, and only a 60,000 BTU furnace for heating.
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  9. #9
    Join Date
    Oct 2008
    Location
    Indianapolis
    Posts
    172
    Quote Originally Posted by beenthere View Post
    I wouldn't think Indianna would have many house that need a 4 ton heat pump for cooling, and only a 60,000 BTU furnace for heating.
    Orginal poster did not give square feet of space to be conditioned or if a load calculation was done. I live in central Indiana and am heating and cooling 6400 square feet with 4 ton of A/C and a +90% gas furnance. Load calculation's where done.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,726
    Quote Originally Posted by Indy View Post
    Orginal poster did not give square feet of space to be conditioned or if a load calculation was done. I live in central Indiana and am heating and cooling 6400 square feet with 4 ton of A/C and a +90% gas furnance. Load calculation's where done.

    What size is your furnace.
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  11. #11
    Join Date
    Oct 2008
    Location
    Indianapolis
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    172
    Quote Originally Posted by beenthere View Post
    What size is your furnace.
    125,000

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,726
    Quote Originally Posted by Indy View Post
    125,000
    Ok.

    My point was, he was trying to calculate operating cost.
    Using disproportional sized equipment.

    He is comparing a 4 ton HP, to a 60,000 BTU furnace for operating cost.
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  13. #13
    Join Date
    Oct 2008
    Location
    Indianapolis
    Posts
    172
    I agree 100%.

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