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Thread: Air to Air Heat Pump in cold temperatures

  1. #1
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    Air to Air Heat Pump in cold temperatures

    I live in South Dakota and currently, it is 11 degrees. I recently had to completely rebuild my home. I used Spray Foam insulation and have an Air to Air Heat Pump with electric backup.

    Prior to the rebuild, the heat pump could not keep up with the cold temps so Auxiliary Heat always kicked in when it was this cold. Now, the auxiliary heat only kicks in during the defrost cycle. The heat pump is providing enough heat to keep the house at temperature.

    My question is efficiency. If I switch to emergency heat, the air handler runs for a short time to heat the house perhaps 5 minutes). When on normal heating, the heat pump runs and heats the house but it runs for about 3 times the time to heat the house and the compressor goes on and off several times during the heat cycle. During normal use, the compressor draws about 4 KVA. Emergency heat draws about 15 KVA.

    Would it be more economical to continue to run the Heat Pump for 15 minutes verses running the Emergency heat for 5 minutes? Another factor that can't be measured is wear and tear an the heat pump.

  2. #2
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    Run the heat pump.
    UA Local 32 retired as of Jan 2020

  3. #3
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    Up here, in Northern MINNESOTA it is common to have a heat pump and a gas furnace. The tipping point with gas is about 20 degrees. At that temp, the gas furnace takes over for less cost than the heat pump.

    If my math is right, you pay for 15KVA for 5 minutes on electric heat, and 12KVA (4 X 3j for the heat pump. Sounds like running the heat pump is the way to go. If it gets much colder, you might need to reassess.
    If God didn't want us to eat animals... He wouldn't have made them out of MEAT.

  4. #4
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    Home Energy Products: 170 DW Hwy, Belmont, NH 03220
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    What do you have for a heat pump. Wondering the efficiency rating.

  5. #5
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    You need to look at the C.O.P data in the install manual and that will give you actual performance numbers.

  6. #6
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    Why go COP? If you know the runtimes, compare the AMPS being used. Then you can convert to wattage and cost for each scenario.

  7. #7
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    The scenario isn't a constant.

  8. #8
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    I'm in manitoba just north of you.
    The heat pump will be more efficient at 10 degrees than strip heat. Probably cop of around 1.5 depending on make_model.
    So keep running it
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

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  10. #9
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    Quote Originally Posted by MADHVACTECH View Post
    You need to look at the C.O.P data in the install manual and that will give you actual performance numbers.

    Might also take into consideration equipment runtimes with relation to equipment service life.

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  12. #10
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    I once talked to an engineer about this question. His belief was a HP compressor was likely to last
    longer if kept running because the compressor oil would be in circulation and not as likely to migrate.
    They will always be more efficient than strip heat even when they can't satisfy the load.
    I had a HP in N Ill. I kept it running but often worried I was wearing it out. Most of the analysis of HP's don't
    consider mechanical costs and how many techs fall short on proficiency when understanding them.

    The HP I had was in a house I built and got a special deal on electric rates. At the time the electric co, was pushing all
    electric houses. A few years later a consumer group sued and I lost my rates.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

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  13. #11
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    Don’t use emergency heat mode unless your heat pump stops heating your home.
    Switching your thermostat to emergency means: just using your backup heat (if I am correct). The heat pump works as an electric heater, or it is using the backup heat.
    The backup works more efficiently, but less efficient than the heat pump pulling in heat.
    Emergency cases used for normal operation are strongly not recommended for any systems. Heating ones, which are most reactive, is the best case where not to use an emergency for normal operation. Soon you might need a repair team to go and check your system, I believe.
    Use emergency heat only if your heat pump isn’t heating your home at all.

    Edited: if you already invested in the backup, it should switch on without your action, I guess, and keep the system going. With no need for emergency.

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