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  1. #1
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    Jan 2007
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    Heat Pump Auxillay Heat Question

    I have 20 k Backup auxillary heat for my 4 ton Trane XL 16i heat pump.
    I had the HVAC company break it down for 10k down to 20 degrees and full 20k below 20 degrees.
    Do you think this is the most efficent way or should I just let the whole 20k come on at once as it will recover a lot faster?
    Will full 20k use more electricity for 10 minutes as oppossed to running 10k for 25 minutes?

  2. #2
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    Quote Originally Posted by wscsteve View Post
    I have 20 k Backup auxillary heat for my 4 ton Trane XL 16i heat pump.
    I had the HVAC company break it down for 10k down to 20 degrees and full 20k below 20 degrees.
    Do you think this is the most efficent way or should I just let the whole 20k come on at once as it will recover a lot faster?
    Will full 20k use more electricity for 10 minutes as oppossed to running 10k for 25 minutes?
    I'm not sold on this idea.

    20 kw for 10 minutes is the same cost in your electric bill as 10 kw for 20 minutes.

    How many kw will be used when the heat pump is in defrost? If it's not the full 20 kw, you may feel cold air blowing from the registers while the heat pump is defrosting.

    Good luck.

  3. #3
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    Electric resistance heat makes and exact number of btus (I think 3,142 without looking?) per watt. So if all you had was electric heat, it would make no difference.
    Since this is secondary heat, if your strips come on in stages, the heat pump will be providing more of the heat. Since it is more efficient, you will be saving a few dollars there.
    In addition, if you are on a demand type meter, you will probably save some additional $$ by setting a lower demand peak rate.
    I would set it up the same way you have it now.

    I see I did not fully answer your second question:
    Will full 20k use more electricity for 10 minutes as oppossed to running 10k for 25 minutes?
    The answer is yes, but the question is incorrect. You would have to run 10k heat exactly twice as long as 20k heat to produce the same number of btus. (10 minutes Vs. 20, not 10 Vs. 25) The only (very slight) cost savings would be the blower motor would run a shorter period of time, but if the heatpump is operating, it would run the same.

  4. #4
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    Staging the heaters as stated is a good idea,not sure if 20 F is where it should be,can't tell from here.

    When the heat pump needs the aux. heat the ideal thing is for the aux to come on and then stay on or cycle off,but leave the heat pump running.So your staging can better accomplish this then bringing on 20KW at the first sign of a need for aux. heat.

    Hopefully you have aux heat locked out above a higher temperature,when it's not needed at all.

    Bottom line is the heat pump is always producing heat for les cost per btu than the strips,so you want it to run as much as possible,when there is a demand for heat.

  5. #5
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    By splitting it, you allow the heat pump to provide the most heat it can, instead of having the strips satisfy the stat and shutting down the heat pump.
    As for the temp choice you made. May or may not be good temps.
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  6. #6
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    Thanks for all of your help. I may raise my temperature to 30 * to see if that works better. The HVAC company told me to try it for awhile and that they might need to change temperature later.

  7. #7
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    Quote Originally Posted by gary_g View Post
    How many kw will be used when the heat pump is in defrost?
    ??????????????????????????????????????????

  8. #8
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    Since its split 10 x 10.
    He should have 10KW during defrost.
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  9. #9
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    Quote Originally Posted by beenthere View Post
    Since its split 10 x 10.
    He should have 10KW during defrost.
    If so, he will feel cold air from the registers while he is defrost.

    With 10 kw aux during defrost:

    34,130 = 1400 x 1.08 x Delta T
    Delta T = 22 degrees.

    If the cold supply air from the a/c coil is 48 degrees, then he will get 48 + 22 = 70 degree air from the air handler during defrost.

    I don't see where splitting the 20 kw in half gains anything. 20kw for 5 minutes is the same kw usage as 10 kw for 10 minutes. Maybe I'm missing something.

    Take care.

  10. #10
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    Gary,

    As beenthere stated

    "By splitting it, you allow the heat pump to provide the most heat it can, instead of having the strips satisfy the stat and shutting down the heat pump."

    To expand a little on that,as the outdoor temp drops the HP needs a little help,not a lot.If you gring on 20KW the system will likely cycle off ,when you want it to keep running.

    20KW will also give you high supply air temps,followd by much cooler when it cycles off,not good for comfort.

  11. #11
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    Quote Originally Posted by dash View Post
    Gary,

    As beenthere stated

    "By splitting it, you allow the heat pump to provide the most heat it can, instead of having the strips satisfy the stat and shutting down the heat pump."

    To expand a little on that,as the outdoor temp drops the HP needs a little help,not a lot.If you gring on 20KW the system will likely cycle off ,when you want it to keep running.

    20KW will also give you high supply air temps,followd by much cooler when it cycles off,not good for comfort.
    OK - a little help for the heat pump. Got it. Thank you.

    With this set-up, could all 20kw come on when the heat pump is in defrost or does it have to be only 10 kw?

    Thanx.

  12. #12
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    It would take some creative wiring, and a relay to do that.
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  13. #13
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    Quote Originally Posted by beenthere View Post
    It would take some creative wiring, and a relay to do that.
    Or, as in my Carrier system with the TSTATCCPRHO1-B Thermidistat control, I can select how much heat provided by Thermidistat control during heat pump defrost cycle. Also, obviously the heat must be in two sections. In duel fuel applications the selection choices apply and can be used to select low or high heat if furnace is 2 stage. Yes, W1 and W2 need not be connected to the fancoil to enable this function.

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