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  1. #1
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    Heat Pump COP in first stage vs second stage

    Is a 2 stage heat pump more efficient in its first stage compared to operating in its second stage when operating in heat mode? It seems to me that the coefficient of performance would be a little higher in 1st stage but I have no data to back that up. Any thoughts?

    And while we are on that subject, I have read in these forums that while operating in cooling mode 1st stage is more efficient. Is this because the smaller compressor/low speed still uses the entire condensing coil? And I am talking BTU/KW here, not perceived efficiency due to potentially increased dehumidification.

  2. #2
    I am looking at the data for the 18 SEER, 2 stage York that I am getting.

    http://www.yorkupg.com/PDFFiles/345094-YTG-F-0509.pdf

    For the AC they show the 2 stage of compressor and corresponding air flow rates (lower for the the lower stage) it shows the same SEER adn EER for both stages.

    In HP mode they show stage one with lower air flow rate and stage 2 with a air flow rate and stage 2 with a lower air flow rate ( my guess is for higher output temps).

    They only show HSPF for the the stage 2 operations and COP for all 3 modes.

    The COP is the same for stage 1 and stage 2 high air flow and going from stage 2 high air adn stage 2 low air both the HSPF and the COP drop off.

  3. #3
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    Quote Originally Posted by wildbill99 View Post
    I am looking at the data for the 18 SEER, 2 stage York that I am getting.

    http://www.yorkupg.com/PDFFiles/345094-YTG-F-0509.pdf

    For the AC they show the 2 stage of compressor and corresponding air flow rates (lower for the the lower stage) it shows the same SEER adn EER for both stages.

    In HP mode they show stage one with lower air flow rate and stage 2 with a air flow rate and stage 2 with a lower air flow rate ( my guess is for higher output temps).

    They only show HSPF for the the stage 2 operations and COP for all 3 modes.

    The COP is the same for stage 1 and stage 2 high air flow and going from stage 2 high air adn stage 2 low air both the HSPF and the COP drop off.
    Go to the extended performance tables starting on page 30 and calculate the COP (for heating and cooling) by converting the kW used to BTU (kW x 3,413) and then dividing that into the Total Capacity (T.C.). You need to multiply the TC by 1,000. You will find the first stage to have a higher COP (compared at the same outdoor temp for both stages) whether in heat or cool mode.

  4. #4
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    mchild, thank you so much for the link to the technical guide. Very interesting reading.

    wildbill99, interestingly, I show a lower COP in stage 1 heat vs stage 2 from the data tables listed. I was surprised to find this and am a little perplexed. If you look at the table on page 42 and compare it to the table on page 43 it clearly show higher COP in second stage operation.
    Last edited by Illusion; 07-19-2009 at 10:38 PM. Reason: Edited because I stupidly had the COP stages reversed in the first sentence to wildbill99, which was the whole point!

  5. #5
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    Quote Originally Posted by mchild View Post
    Go to the extended performance tables starting on page 30 and calculate the COP (for heating and cooling) by converting the kW used to BTU (kW x 3,413) and then dividing that into the Total Capacity (T.C.). You need to multiply the TC by 1,000. You will find the first stage to have a higher COP (compared at the same outdoor temp for both stages) whether in heat or cool mode.
    I just got a York 4-ton YZH HP/80% modulating NG furnace. So, do I understand that whether in heating or cooling the HP is more efficient at the lower capacity 1st stage (so far it runs a lot in 1st stage but not fine tuned)?

    Guess that would make sense, the 2nd stage is used when the HP isn't meeting the demand and it pulls the stops even though it would be less efficient, correct?

  6. #6
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    jerryd_2008, no, what I am saying from the documents provided in the first part of this thread is that when heating, the second stage has a higher COP. This is the opposite of the greater energy efficiency seen by operating in low speed in cooling mode.

  7. #7
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    I would have lost money on this bet.

    From the Expanded Heating Data for Goodman DSZ18 3 ton heat pump:

    ---------------50F--------40F-------30F--------20F--------10F
    EER (Lo/Hi)---13.8/14.1--12.4/12.7--10.1/11.3--9.1/10.1--7.8/8.7
    COP (Lo/Hi)---4.04/4.12--3.64/3.71--2.96/3.3---2.65/2.95-2.28/2.54

    Hi stage as a higher C.O.P.

    No CFM's are given.

  8. #8
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    Quote Originally Posted by gary_g View Post
    I would have lost money on this bet.

    From the Expanded Heating Data for Goodman DSZ18 3 ton heat pump:

    ---------------50F--------40F-------30F--------20F--------10F
    EER (Lo/Hi)---13.8/14.1--12.4/12.7--10.1/11.3--9.1/10.1--7.8/8.7
    COP (Lo/Hi)---4.04/4.12--3.64/3.71--2.96/3.3---2.65/2.95-2.28/2.54

    Hi stage as a higher C.O.P.

    No CFM's are given.
    Gary, these numbers surprise me. Isn't the balance point usually around the high 30's and that is the point where COP is about 1? According to your numbers, the HP is still getting 3 for 1 at 30 degrees, right?

    I was thinking that I should tell my contractor to set the balance point on my York Affinity 18 SEER DFHP at close to 40. Don't want defrost and have great modulating furnace. Bad idea?

  9. #9
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    Quote Originally Posted by jerryd_2008 View Post
    Gary, these numbers surprise me. Isn't the balance point usually around the high 30's and that is the point where COP is about 1? According to your numbers, the HP is still getting 3 for 1 at 30 degrees, right?
    Economic balance point and heating balance point are 2 different things.

    Heating balance point is the ambient temp at which the heat pump cannot provide enough btu's to maintain setpoint, so auxillary heat is needed. My balance point is mid 20's.

    Economic balance point is the ambient temp at which the aux heat is cheaper to run than the heat pump.

    In the DSX heat pump example, COP = 2.07 at 0F on hi stage, but btu output is down to 14,700 from the initial 36,000 at 47F. So it is still 2.07 x more efficient than straight electric, but its putting out less than half of the btu's. This is OK for me because I have electric strips for my aux heat, so I would let the heat pump run and run and run (but my output temps at 0F would only be 81F).

    Take care.

  10. #10
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    Quote Originally Posted by jerryd_2008 View Post
    I was thinking that I should tell my contractor to set the balance point on my York Affinity 18 SEER DFHP at close to 40. Don't want defrost and have great modulating furnace. Bad idea?
    Economic balance point depends on your delivered rates for natural gas and electricity, as well as furnace efficiency and heat pump C.O.P. at various ambient temps.

    You can have the installer set the switchover point, but make sure you know how to change it. You can experiment and see how your comfort changes by dropping the switchover temp. This set-up is usually in the installion manual and not the operating manual. You want to have both manuals.

    Good luck.

  11. #11
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    Quote Originally Posted by gary_g View Post
    ...

    In the DSX heat pump example, COP = 2.07 at 0F on hi stage, but btu output is down to 14,700 from the initial 36,000 at 47F. So it is still 2.07 x more efficient than straight electric, but its putting out less than half of the btu's. This is OK for me because I have electric strips for my aux heat, so I would let the heat pump run and run and run (but my output temps at 0F would only be 81F).

    ...
    Thanks, Gary. I think you nailed the reasoning for me to switch to NG by pointing out that the Btu output also drops off big time.

    To me making the HP defrost is a waste. And we want to make sure the temp out of the registers is warm (our new York DFHP with an HSPF of about 9 has some fancy-dancy means of raising the register temp). I know that electric rates affect the choice - ours is about $.10/kWh. But is the very high 30's or 40 a good trade off for comfort/economics?

  12. #12
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    Quote Originally Posted by jerryd_2008 View Post
    I know that electric rates affect the choice - ours is about $.10/kWh. But is the very high 30's or 40 a good trade off for comfort/economics?
    Jerry:

    If your delivered price of electricity is 10 cents per kw-hr (that should include generation + transmission + distribution), what is your gas rate (in $ per therm, or $ per ccf)?

  13. #13
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    newer heat pumps with on-demand defrost, don't defrost very often in most cases 2-3 times per day, unless it's very damp weather, and when it does, the furnace will provide soem heat, and hte defrost cycle only lasts for 1-2 minutes.

    This is at least based on my observations.

    During ice storms however, I did do to emergency heat. Ice was accumulating on the fan balde and making it unbalanced and very noisy... and likely reducing airflow. So I shut it down for the evening. The next morning I turned it back on and it was a heck of a display watching it defrost with all the ice.

    A unit sized properly for cooling, may have a balance point as high as 35-40F... expecially the further north you get. But it depends on your climate and construction. Also, if you're heating a basement, you may have a slightly more disproportionate heat loss compared to heat gain.

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