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  1. #1
    Join Date
    Feb 2008
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    Economic balance point cut over question…

    Economic balance point cut over question…

    Just went DF. New unit keeping the house cool without humidity this summer. Thanks to all!

    New 14 SEER 4 ton Amana AZ 410A heat pump with coil.

    I have set the cut over to change over to oil heat at 20* outdoor temp.
    Based on my calculations, with a COP of 2.65 at 20* - oil at 3.85 a gallon and .21 KWH

    #2 Oil needs to plummet to roughly 2.65 a gallon to begin considering bring the cut over upwards to 25* or 30*

    Is my math on?

    I used many of the online calculators on this site so I think I am close to reasonable number.

    Thanks again…

  2. #2
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    Jan 2004
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    Your math is correct for economical balance point.
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  3. #3
    Join Date
    Jun 2008
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    By cut over I assume you mean locking out the HP to not run at all. Without knowing the efficiency of your oil furnace I can't confirm the calculations, but it sounds like you have the COP numbers and therefore are on the right track.

    I wouldn't be surprised if your oil furnace has to run at least part time at those lower temps.

    Its been too long since I did COP and EER calculations on test units, so maybe others on this forum can tell us if defrost is accounted for in those COP numbers. If not, that COP number will be lower (negative?) on days when defrost occurs often. If you're in a very dry climate this isn't much of an issue.

  4. #4
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    COP NEVER includes defrost.
    HSPF includes defrost.

    COP is just BTU output per WATT consumed in heat mode. Defrost is not heat mode.

    At 82% efficiecny, oil is still $1.03 per 100,000 BTU output more expensive then the heat pump at a COP of 2.65.
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  5. #5
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    Feb 2008
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    That sat will cut out the heat pump for 90 sec. before it kicks on the oil furnace.

    The furnace is a 5 ton 80% armstrong.

    How often might my heat pump be in defrost mode when it is say 25* outside?
    Unit is on the unshadded South side of the house.

    Long story short, unless #2 goes below $3 I donot even have to think about it... yes?

  6. #6
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    Your correct.
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  7. #7
    Join Date
    Jul 2008
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    are you considering the difference in power usage for the blower? The blower will be running considerably longer for a heat pump at 20 degrees than it would be for your furnace. That might bump the balance point up a bit.

  8. #8
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    NY
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    Blower is a 5 ton 3/4 horse PCS motor.

    How woud I handy-cap the added elcecitcal use of the blower into my cut over numbers?

    Sounds like tall order.

  9. #9
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    A ¾HP PSC blower motor, will consume around 746 watts.(give or take a couple)
    That 746 watts will also add 2,546 BTUs per hour of run time to the air.
    So its a semi wash. It used more electric running longer, but it also added heat to the home.

    It would have a COP of 1. So it would be the same as an electric strip heater.
    You will get 3413 BTUs for every kilowatt it consumes.
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  10. #10
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    Jun 2008
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    Quote Originally Posted by jrongo View Post
    How often might my heat pump be in defrost mode when it is say 25* outside?
    In part it depends on humidity, but I know it can occur at 40 degrees on down. Would love to know from the pros on its perceived or observed frequency at a given temp and humidity.

    Quote Originally Posted by beenthere View Post
    At 82% efficiecny, oil is still $1.03 per 100,000 BTU output more expensive then the heat pump at a COP of 2.65.
    With that kind of BTU cost spread, is there any reason to lock out the HP at 20 degree temp?

  11. #11
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    Thats his economic balanc point. Not his thermal.

    I have some dual fuels in, that teh HP isn't locked out. Its first stage heat no matter what OD temp. If it can't keep up, the stat calls for second.

    Every once in awhile. A customer calls and wants is locked out before the discharge air temp gets cool for them.

    So, its what ever the customer wants. High comfort, or high efficiency.
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