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  1. #1
    Join Date
    Oct 2008
    Posts
    4

    Question with Carrier Natural Gas burner and Air source heat pump

    Hello, I have been having a minor issue with my carrier furnaces and air source heat pumps that I have been trying to figure out. I am not sure if it is actually an issue with the thermostat (I am leaning toward this being the cause) or the furnace/air source pump. What is happening, is even though the outdoor temperature has not reached the cutoff threshold for the air source pump to shut down, every 12-16 hours the system calls for auxiliary heat and burns the natural gas for about 2-3 minutes. This actually happens on both systems setup in my home. At the time, I have the temp settings set to remain at one temperature for heat, and the air source heat pump when it operates keeps the temperature steady, when the natural gas cycle kicks in, it is normally already at the setpoint temperature. The main problem with it operating this way, is when I switched the gas off on the furnace, it prevents the heat from running unless I manually shut off the system and start it back up again, at which time the air heat kicks in and keeps it heated until the next gas cycle is attempted.

    I do not have the model # for the thermostat at the moment, but the same thermostat is on both systems, and both have the exact same issues. If I have the gas turned on on the systems, it does not have issues where it will remain trying for the gas heat after its cycle, cutting off the heat until it is manually turned off etc. The whole idea of using this setup was to not need to run the gas as much during the warmer cool months...but it is annoying me that it is not running only on the air source heat when the temp is still steadily 25-30 degrees above the cutoff for the air source pump, and the pumps are keeping the temperature up where it is supposed to be otherwise.

    Is it a normal operation for it to run a cycle on gas to basically test the backup heat or something? Any help would be great, I was just wanting to see if it is a normal issue before contacting the furnace installer regarding it.

  2. #2
    Join Date
    Jun 2004
    Location
    St Paul, minnesota
    Posts
    1,245
    more than likely furnace is set up to run when your heat pump goes into defrost, so you won't get really cold air out of the registers. when a heat pump goes into defrost it reverts to the a/c mode & keeps the outdoor fan off to remove frost from the outdoor coil. depending on the outside temperature. the colder it is outside, the more frequent the defrost cycles will be. hope this helps.

  3. #3
    Join Date
    Jun 2007
    Location
    south carolina
    Posts
    176
    sounds right to me in addition to that tho it will continue in gas until the cycle has been satisfied and next cycle if its warm enough will be your heat pump again.

  4. #4
    Join Date
    Oct 2008
    Posts
    4
    Thank you for the information, it makes more sense to me now...at what temperatures is it normal for the units to need to run a defrost cycle? I appologize if that should be more of common knowledge... normally it has done this cycle in the mornings when it has been 30-40 'F, and I have noticed it in the evenings as well when it gets down to those temps. Also, maybe a dumb question, but is the defrost cycles the main reason for that low temperature shut off? Right now, I know it is set at 10'F for that. Thank you for the quick responses!

  5. #5
    Join Date
    Jun 2007
    Location
    south carolina
    Posts
    176
    negative low temp shut of is becuase between 30 and 40 degrees your heat pump looses your efficency and gas heat is needed. The defrost is becuase when your in heating your outdoor unit is in air conditioning so the coil freezes from time to time and needs to be defrosted.

  6. #6
    Join Date
    Oct 2008
    Posts
    4
    OK, thank you again for the reply.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,599
    The low temp shut off is to be set to the temp where the HP uses more electric then what it cost ot use gas, oil, or electric resistance heat.

    As far as efficiency, many heat pumps are still more efficient at 10°F then electric strip heaters, and some times gas or oil.

    We seldom lock out a HP here.

    Your contractor should have done a balance point graph to set up the proper lock out temp.
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  8. #8
    Join Date
    Oct 2008
    Posts
    4
    Quote Originally Posted by beenthere View Post
    The low temp shut off is to be set to the temp where the HP uses more electric then what it cost ot use gas, oil, or electric resistance heat.

    As far as efficiency, many heat pumps are still more efficient at 10°F then electric strip heaters, and some times gas or oil.

    We seldom lock out a HP here.

    Your contractor should have done a balance point graph to set up the proper lock out temp.
    For that setting on my thermostat, it allows me to go down to 5'F or OFF, and allows only 5' increments up to I believe 55. The contractor had set it to 15'F but instructed me that I would probably be able to lower it some if it doesn't seem to make the air source run too heavily etc, which I did and figured I would try it at 10'F to see how it goes...it has not gotten that cold here yet, so am not sure if I could lower it more, need to increase etc. As far as I know, he didn't do a balance point graph. Is this something I could catch on to do my self, or a trade secret?

  9. #9
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    I believe the balance point is determined by a load calculation. Did your installation dealer do one to determine equipment sizing?

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,599
    You would need the load calc for your house, plus the performace specs of your heat pump.
    You can get the ARHI specs online, and do your own head loss calc if you want.
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