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  1. #1
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    Oct 2008
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    Questions re heat pump efficiency at low temps

    What factors determine how well a particular heat pump will perform at low outside temperatures? Is the COP vs temperature curve likely to be pretty similar across the board, or do some units do significantly better than others when outside temp is low? Is refrigerant choice a factor?

    What's a reasonable minimum outside temperature to be able to run a heat pump?

    I am getting ready to replace 2 units (central Indiana) and would like to choose units that will do well in heat mode at low temps. Some manufacturers publish the curves but some don't.

    Thanks,

    -jim-

  2. #2
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    No.

    Not all heat pumps have the same curve.

    Compressor, outdoor coil, and indoor coil combined determine the heat output.

    Generally, with a time and temp defrost board. As long as the COP is 1.5 or greater, the heat pump will be efficient to use.

    Once the COP drops below 1.5 with a time and temp defrost board. It cost about the same to heat with just the strip heat.
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  3. #3
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    Jun 2007
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    Quote Originally Posted by beenthere View Post
    Once the COP drops below 1.5 with a time and temp defrost board. It cost about the same to heat with just the strip heat.
    For my GSH14 Goodman heat pump with time and temp defrost board, COP = 1.61 at -5F.

  4. #4
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    As long as its 1.5 or above. With the new(been code for a while now) 15 minute max allowed defrost time. Its still more efficienct to let the HP provide heat, then to just use the strip heaters.

    Just at -5°F, time and temp has wasted defrost times.
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  5. #5
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    Indiana
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    Quote Originally Posted by beenthere View Post
    No.

    Not all heat pumps have the same curve.

    Compressor, outdoor coil, and indoor coil combined determine the heat output.

    Generally, with a time and temp defrost board. As long as the COP is 1.5 or greater, the heat pump will be efficient to use.

    Once the COP drops below 1.5 with a time and temp defrost board. It cost about the same to heat with just the strip heat.
    --------------
    How much variation are we talking about? Is R410A better or worse than R22 for low-temp heat pump operation?

    So is it a matter of just digging through each manufacturer's performance data, or are there some specific design characteristics to look for? I guess I'm having trouble understanding why there would be much variation between units.

    Yet another question. If I have electric backup heat then I can run the strip heaters and heat pump simultaneously, correct? Does doing that hurt the efficiency of the heat pump? I am assuming the electric heat elements are upstream of the coil, therefore the coil is going to be trying to add heat to air that's already been heated by the resistance heat.

  6. #6
    Join Date
    Oct 2008
    Location
    Bellevue, WA
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    194
    I was really impressed by the low temperature efficiency numbers of the York heat pump I got. Not that it gets that cold in Seattle but since I have natural gas heat as a back up I figured that at around 2.5 COP it would be more economical to switch to gas. With the York the heat pump can theoretically provide economical heat all the way down to 10F (lower design temp for our area).

    Where we reach the balance point and how well the defrost in our damp climate works are still an open question. So far I've only see the unit go into defrost once. It surprised me since it was a clear sunny morning and outdoor temp was 44 degrees. A check of the weather channel reported relative humidity of 84%. It was sort of exciting seeing all the steam come up from the unit. I was expecting the Hogwarts Express to appear

    The unit has "demand defrost" which intellectually makes sense (even though I can't claim to fully understand it ); we'll see how well in does in reality. I'm guessing your winters are dry so even though it's colder demand defrost might not be as important.

  7. #7
    Join Date
    Aug 2008
    Location
    Seattle, WA
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    Quote Originally Posted by jstjohnz View Post
    Yet another question. If I have electric backup heat then I can run the strip heaters and heat pump simultaneously, correct? Does doing that hurt the efficiency of the heat pump? I am assuming the electric heat elements are upstream of the coil, therefore the coil is going to be trying to add heat to air that's already been heated by the resistance heat.
    No, it's the opposite. The system needs to extract as much heat as possible from the coil. The strip is DOWN stream of the coil.

  8. #8
    Join Date
    Oct 2008
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    OP, the easiest way to say this is you DO NOT WANT that electric coil to come on at all unless you're at a COP of less than 1.5 ideally.

    Now, say your COP at say 5F(not accurate, but works for the purpose of my example) is = 1.55, but your heatpump is then producing say 20mbh of heat at that point. Lets say its a 3 or 3.5 ton unit.

    Well, your location might have a winter design temp of 5F, BUT at the design temp your homes heat loss is 24mbh.

    Well, in that case, efficiency wise, you dont run that elec coil until below 5F, but, by then, you've already gone below your homes balance point and your house is now getting colder because you're losing heat faster than your heatpump is bringing more in.

    So, in that case, you need to bring your electric coil on sooner (aka at a higher temp).

    Maybe at 10F your house is losing 23mbh, and your heatpump at 10 can bring in 23mbh, then THAT is your balance point and at any temp below that you will need to run the coil a little to keep your house temp up.

    That DOES NOT mean STOP running your heatpump at 10, it just means add coil. Your t stat should do that auttomatcally because it'll see that the heatpump cant hold the room temp up any more.

    However, say at 2F your heatpumps cop is 1.45, then at 2F or maybe 3F is where you would program the heatpump to just not run anymore at all and only use the coil because thats the point that you're wasting more than you're saving.

    Again, those temps are not accurate to any heattpumps and in reality are prob a little lower tthan that, but for my example i hope it made sense.

    Tthis should also explain why noone on this forum can tell you where to set yours at because we don't know your homes heat loss.

    If anyone sees a problem with my explaination, let me know cause I'm a tad sick and not at 100% this minute, but been doing hvac design for 5 years now. I'm an engineer.

  9. #9
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    You set your aux to come on at a temp just above the thermal balance point.
    That temp could be 40 or 20°, or some other temp.


    As long as the HP's COP is above 1.5 for a time and temp defrost, you leave the HP run with the aux.

    Below 1.5, the defrost cycles cool the house enough that the HP will cost more to let it operate.

    Exception being, if the contractor has sized teh aux that teh HP has to run also at design.
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  10. #10
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    Thats what i said right? lol

  11. #11
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    Quote Originally Posted by Rhizzlebop View Post
    Thats what i said right? lol
    Not quite.

    You opening statement said the HP shouldn't come on at all, unless the COP was less then 1.5.

    In reality.
    The aux must come on long before that.
    Your post made COP and thermal balance point sound related, and they are not.
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  12. #12
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    Oct 2008
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    Quote Originally Posted by beenthere View Post
    Not quite.

    You opening statement said the HP shouldn't come on at all, unless the COP was less then 1.5.

    In reality.
    The aux must come on long before that.
    Your post made COP and thermal balance point sound related, and they are not.
    I think you meant to say "....electric coil shouldn't come on at all...", but i got what you meant,

    If you did mean what I think, then, yes thats what i meant, don't bring the coil on till cop is at 1.5.

    Why would you bring it on "long" before then? i am not following you there.

    Also, i reread my big post below and I don't see how i made balance point and cop related. I actually tried to clearly explain how any why they are separate and independent numbers. Thats what my intention was anyway, butt I'm tired so who knows how I'm possibly misrepresenting it.

  13. #13
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    Because at 17°F outdoor temp(some can be as high as 35&#176. many heat pumps can't heat the house. COP is still 2 or better.
    The house needs aux heat at that point. No matter what the COP is.

    COP is works better to determine when to lock out the HP.
    Thermal balance point to determine when to allow aux to come on.
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