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  1. #1
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    Aug 2011
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    Does COP improve when Inverter slows compressor?

    We are about to have a Mitsubishi ZUBA (Canadian) heat pump installed. This unit puts out a steady 38,000 BTUH over much of the temperature range we will see.

    My question is - Does the COP improve when the inverter slows the compressor down to match the required load? The numbers below show how the COP varies with temperature at the maximum output.

    BTW, the part of our home unit will serve has a design heat load of about 42k BTUH at -25C (-13F) design temperature.

    Code:
    Outdoor deg F         BTUH     INPUT KW        COP    BTUH needed
    50                   39710      3.521          3.3     7500
    40                   38000      3.23           3.4     12000
    30                   38000      4.684          2.4     17000
    20                   38000      5.491          2.0     22000
    10                   38000      5.911          1.9     27000
    0                    35720      6.04           1.8     32000

  2. #2
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    4,505
    COP increases due to less temp difference between indoor and outdoor units.

  3. #3
    Join Date
    Aug 2011
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    Kingston, Ontario
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    Quote Originally Posted by 54regcab View Post
    COP increases due to less temp difference between indoor and outdoor units.
    That is true and that is what the figures I posted show.

    But my question was about how COP varies when the inverter driven compressor runs at lower than full rpm in order to only provide the heat required, not the full output.

    I asked Mitsubishi, and they say that COP will increase even at same outdoor temperature. But they so far haven't provided any data.

    It would seem to me that knowing how COP varies with load would be an important factor in determining effectiveness of an inverter drive heat pump.

    At 20F, the COP is 2.0 at full output of 38,000 BTUH. But we only need 20,000 BTUH, so compressor speed is reduced. What is COP then? Mitsubishi says it is higher, but how high?

  4. #4
    Join Date
    Feb 2009
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    Portland OR
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    Quote Originally Posted by Freeagent View Post
    At 20F, the COP is 2.0 at full output of 38,000 BTUH. But we only need 20,000 BTUH, so compressor speed is reduced. What is COP then? Mitsubishi says it is higher, but how high?
    Yes your COP would be higher because you have more coil area with slower moving refrigerant. This means better heat transfer and better efficiency. Keep in mind that as the temperature outside drops so does your capacity unless you have the hyper heat model. So at 47F you may have 38,000 BTU's of heat but at 17F you may only have 24,000 BTU's of heat at full speed/capacity.
    Check out my YouTube channel - http://www.youtube.com/user/skyheating1 We have customer testimonials, product reviews and more!
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  5. #5
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    Aug 2011
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    Quote Originally Posted by SkyHeating View Post
    Yes your COP would be higher because you have more coil area with slower moving refrigerant. This means better heat transfer and better efficiency. Keep in mind that as the temperature outside drops so does your capacity unless you have the hyper heat model. So at 47F you may have 38,000 BTU's of heat but at 17F you may only have 24,000 BTU's of heat at full speed/capacity.
    Thanks for confirming that. It would be interesting to know how much it increases. I guess Mitsubishi will have to tell me that.

    Regarding capacity at lower temperatures, yes these do use teh Hyper-Heat technology. The figures I posted are from manual. At 17F, capacity is still at 38000 BTUH and still there at 5F. Only gets down to 24000 BTUH at -26F! (but by that time COP is down to 1.2)

  6. #6
    Join Date
    Sep 2006
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    1,253
    For one Japanese manufacturer they reported for their VRF system (Variable Refrigerant Flow) with an inverter compressor that the energy required at 50% load was 30% of the energy required at full load. I'm not at all sure how that would apply to your model.

  7. #7
    Join Date
    Jan 2009
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    Keokuk, IA
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    I'm not sure the gain in COP will be that dramatic. You gain extra surface area at the outdoor unit, but not nessesarily at the indoor units and the indoor fans have not ramped down their speed either. If you only gain surface are on one end, you may not have a significant improvement in relative efficiency.

    Sometimes as well, the extra energy needed to increase airflow comparatively offsets any efficiency gains. Otherwise you could simply install a fan twice as powerful on any outdoor ocndenser and see an efficiency imrpovement. Also, if COP could be dramtically imrpoved just by enlarging the surface area of the coil, then don't you think mfg's woudl do just that?

    It's basically deminishing returns.

  8. #8
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    Aug 2011
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    When the units undergo testing for HSPF, would you think that the improved COP at partial loads is accounted for? Seems like it should. At 47F (HSPF test temperature), many units would be operating at partial load once in service.

  9. #9
    Join Date
    Feb 2010
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    Cal
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    Follow this link for the standard on ratings, parameters, and conditions for your topic:

    http://www.ahrinet.org/App_Content/a...dendum%201.pdf

  10. #10
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    Aug 2011
    Location
    Kingston, Ontario
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    Thanks - I had a quick look at get the gist of how they calculate SEER and HSPF. The link is for multi splits, not a single central system. But presumably test methods would be somewhat similar. From my quick look, it seems they don't do any testing at temperatures below 17F. Maybe that is why HSPFs can be misleading when using heat pumps in cold climates like ours (Canada)

  11. #11
    Join Date
    Aug 2011
    Location
    Windsor, Ontario
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    Quote Originally Posted by SkyHeating View Post
    This means better heat transfer and better efficiency. Keep in mind that as the temperature outside drops so does your capacity unless you have the hyper heat model.
    http://www.mitsubishielectric.ca/en/...fications.html

    I'm pretty sure the Zuba doesn't have Hyper-Heat (H2I) available. They might add it in the future.

    I've only heard of one other Zuba Central install - they're not very common around here. The guy worked for the local Mitsubishi distributor.

    I have a bunch of City Multi units without H2I. Neither building (8 Y series and 1 R2 in total) has had a need for supplemental heat even on the coldest days. Pretty impressive technology.

    Hopefully the OP will share his experience if he ends up going with the Zuba.

  12. #12
    Join Date
    Aug 2011
    Location
    Kingston, Ontario
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    Quote Originally Posted by ClaytonZ View Post
    http://www.mitsubishielectric.ca/en/...fications.html

    I'm pretty sure the Zuba doesn't have Hyper-Heat (H2I) available. They might add it in the future.
    .
    I confirmed in the post immediately above yours that it DOES use H2I.

    If you go the bottom of the link you quoted and click on the pdf link, you will find that the Zuba does in fact use the Hyper-Heat technology. To quote from that pdf:
    "The secret behind Zuba-Central
    Zuba-Central provides you high-heating performance, even under
    extremely low outdoor temperatures, thanks to our efficient and
    exclusive Hyper-Heat Inverter (H2i) technology.
    In addition, the unique defrost mechanism provides an extended
    period of continuous heating between the defrost time, and minimizes the
    defrost time required, proving yet again that Zuba-Central is one of the most efficient systems on the market."

    The Zuba uses the same outdoor unit as the commercial City-Multi.

    So far as I know, only Mitsu residential ductless unit that uses this technology is the Mr. Slim MSZ-FD and it is a single split, not multi. That is one of the reasons I didn't pursue multi ductless installation.

    Installation will be complete in Early October, just in time for the heating season.

  13. #13
    Join Date
    Sep 2011
    Posts
    36
    I am also looking at the Zuba Central single central ducted system. I am wondering what your air handler cfm will be set to? The Zuba Central is rated at 38000 btus, 40000max, but for that does it need to run at 1200 cfm?

    Because of my limited ductwork - 8 - 5" ducts and 2 - 6" ducts - I am advised that the air handler would be set to run at 1000 cfm. My understanding is that when cfm is reduced, there is a corresponding reduction in max capacity, ie. the 3 ton unit acts as a 2.5 ton unit but is this correct?

    If that is correct, then my question is what happens to COP when the air handler runs at a reduced CFM and btu capacity level?

    Your question was about how COP varies when the inverter driven compressor runs at lower than full rpm in order to only provide the heat required, not the full output. A partial load. Here, the refrigerant flow is varying / modulated. When that happens, is your CFM staying at the same level on the air handler, eg if is set to the optimum for 3 ton capacity setting of eg 1200 cfm level, does CFM also vary up and down or is it constant? So modulation is varying refrigerant flow/ constant fan speed?

    Or does CFM modulate up and down as well as refrigerant flow? The system modulates to meet demand, but I have seen no description of what modulates, when. So any clarification on this would be appreciated.

    In my case, if the air handler CFM is set down to 1000 cfm (or 900 cfm) due to restricted ductwork, and consequent total btu capacity (if I understand this right?) reduced to a 2.5 ton unit btu level, does the same COP table apply at the lower cfm settings?

    It would be interesting to know what the cop effect is when the zuba system is set to operate at partial btu capacity due to restricted ductwork. If the effect is negative, there would be a case to upgrade ductwork to 6". I would think that improving ductwork capacity would also mean less stress on the system, and less maintenance.


    Thanks for any help.

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