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Community and Q&A

Need to know Mitsubishi heat pump COPs at temps below 17 degrees F

Charles Galgowski | Posted in Energy Efficiency and Durability on

Hi All,

I would like to know what are the COPs (coefficients of performance) are for Mitsubishi hyper-heat variable speed heat pumps at temperatures from + 15 deg F down to -13 deg F..  I am trying to lower my carbon foot print by heating my house with wood and heat pumps.  The electricity to run the house will be generated from wind and sun.  The 1926 vintage house is in Storrs, CT.  This is building climate zone 5.  The heat pump model series and sizes have not been determined or installed, yet.  I have a Jotul 650 wood stove and burn about 4 cords per year.  I am trying to see if this project can be in reach of someone on a middle class income.

At my present level of knowledge, my feeling is the heat pumps should not be the only source of space heat for the following reasons:

  1. If the grid goes down for a few days in the winter during a cold snap, a typical emergency generator of say 6,500 watts will not likely supply enough electricity for the heat pumps.
  2. As efficient as Mitsubishi heat pumps are supposed to be, they still lose performance efficiency as temperatures drop.  At some temperatures I might be better off heating with wood.

    My HVAC Company gave me some data on several Mitsubishi hyper heat compressors shown in the following table.

COMPRESSOR

HSPF

COP AT 47 DEG F

COP AT 17 DEG F

EFFICIENCY AT 5 DEG F

MXZ-3C24NAHZ2

10

4.25

2.53

100 %

MXZ-4C36NAHZ

11.3

3.95

2.85

100 %

MXZ-8C48NAHZ

11.0

3.75

2.70

100 %

 

I am going to define terms and units as I go.  So if I have it wrong, you can correct me.

 

HSPF (heating season performance factor) has units of BTUs/watt hour and is the total heat delivered divided by total electricity consumed by the heat pump  for an entire heating season.

 

There are 3.412 BTU per watt-hr.

 

COP (coefficient of performance is heat energy delivered per unit of electricity consumed with both in the same units of energy).

 

I checked the AHRI (Air Conditioning Heating and Refrigeration Institute) website and found their standardized testing for HSPF (heating season performance factor) is for building climate zone 4 which runs from North Carolina to central New Jersey and New York City.  The HSPFs in the table are for zone 4, so I do not know what my HSPF will be in my colder zone 5.

 

So the average COP for an entire heating season in zone 4 for a HSPF of 11.0 would be:

Average COP  =  11.0 BTU/watt-hr  x  1 watt-hr / 3.412 BTU = 3.22.

 

As the table shows as outdoor temperature drops, COPs drop.  So my colder zone 5 should have a lower HSPF for the heat pumps.

 

The COP coefficient of performance data for 47 and 17 degrees is very useful and can be applied to zone 5.  As the table shows at 17 degrees the heat pump delivers significantly less heat per unit of electrical energy consumed compared to 47 degree temperatures.

 

Apparently the 100 % efficiency at 5 % means the heat pump can deliver as many BTUS as it does at 17 deg.  Unfortunately, Mitsubishi (and probably other manufacturers as well) do not report what the COPs are for temperatures of 10, 5, 0, -10, or -13 degrees F.    So this does not allow one to know how much electricity the heat pump will consume to deliver this heat at these colder temperatures.  Apparently not only are the pumps using electricity to pump heat up to indoor temperature, but also they have a heat unit to melt frozen ice off the compressor.

 

I need to know what the COPs of these heat pumps are at these lower temperatures.  This will help me decide how aggressively I should use the wood stove as temperatures drop.  Also, it will help me decide what size heat pump to install.  If I find that the Mitsubishi’s use too much electricity at cold temperatures, then I might not have the system designed to heat the house by heat pumps alone at very cold temperatures.  This could be a significant savings in capital cost amortized over the expected 15 year life of the heat pump.

 

I would appreciate any information or advice you have on how much electricity these heat pumps will consume at temperatures from 15 deg F and down to -13 deg F.

 

Thank you.

 

Charles Galgowski

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Replies

  1. Steve Grinwis | | #1
  2. David Martin | | #2

    Every month the Northeast Efficiency Coop presents a spreadsheet with COP and other data about the performance of many heat pumps at lower temperatures. It shows that many heat pumps now offer excellent efficiency at low temperatures.

    Regarding backup heat in case you lose power, probably the best plan is to keep your wood stove as it does not need any electricity at all.

    Look here:

    https://neep.org/sites/default/files/ColdClimateAir-SourceHeatPumpSpecificationProductListing-Updated11.30.18.xlsx

  3. GBA Editor
    Martin Holladay | | #3

    Charles,
    GBA articles include some general guidelines.

    My article, "How To Buy a Ductless Minisplit," included this information on Mitsubishi and Fujitsu cold-climate heat pumps:

    "The equipment described in this article should perform at the following COPs:
    • a COP of about 3.2 to 3.7 at 30°F;
    • a COP of about 1.8 to 2.8 at 5°F;
    • a COP of about 1.4 at -10°F;
    • a COP of a little above 1.0 at -15°F."

    In another article ("Bruce Harley’s Minisplit Tips") I reported:
    "[Bruce] Harley summarized recent research into the performance of residential minisplits. Harley said, 'The state of Vermont assumes that minisplits have an average seasonal COP of 2.4. Multi-zone systems have a lower efficiency than single-zone systems.'”

  4. Steve Grinwis | | #4

    I'm going to call you on this Martin...

    The MXZ-4C36NAHZ has a COP of between 2.83 to 2.12 (min to max output) at -13, as per the ratings listed above.

    Much better than you describe... Unless I'm missing something?

  5. GBA Editor
    Martin Holladay | | #5

    Steve,
    I'm happy to have you call me on this. Your numbers are more specific than mine, and are more up-to-date. My COP averages refer to a variety of heat-pump models, all of which were manufactured before 2015 (since that's when I wrote my article). Efficiencies have improved since then.

  6. Expert Member
    Dana Dorsett | | #6

    The COP is always going to be dependent upon modulation level as well as outdoor temperature. ( A Fujitsu AAU 12RLF at +5F has a COP of 2.25 at maximum speed, but a COP north of 4 at minimum speed. ) The COPs listed in a submittal document are only at the AHRI rated modulation levels & temperature, but there is no reason to believe YOUR modulation levels will be the same at those temperatures.

    With a multi-split the modulation level is a function of which head or heads are actively heating at any point in time. Even with one zone calling for heat constantly the system COP will rise and fall as other zones turn off & on.

    In Storrs CT the number you really care about is the COP at +25F, which is roughly the binned hourly mean temperature in January at that temperature, and more reflective of what the average seasonal performance will be. The 99% outside design temperature there is between +5F and +7F, which means only 1% of an average year would be colder than that. The efficiency at the extremes don't matter nearly as much as the average efficiency. Nobody in Storrs CT should care about what the efficiency is at -13F at any modulation level, since over the lifecycle of a heat pump it's not likely to see more than an hour or three (if any) at that temperature, and it's certainly not worth designing the heating system to cover that temperature.

    Sizing a cold climate heat pump to cover the 99% load with 10-50% of margin would be reasonable for overall efficiency. Undersizing it doesn't really have any benefit, and oversizing it by more than 50% takes a toll on efficiency due to excessive cycling. Using modulating mini-splits with just a single zone compressor is usually quite a bit more efficient than using multi-splits, but multi-splits can still be good if heads are appropriately sized for the zone loads (which is often not the case.).

    1. Cldlhd | | #9

      That's good to hear. I have Mitsubishi multi splits for our three bedrooms, think they're oversized the way they cycle on and off in the summer but that's another subject, and I have a mini split FH-15NA hyper heat which does our general living area ( living room, kitchen etc) and since I done some remodeling and a lot of insulation including zip system sheathing, , new windows, closed cell spray foam in the bedroom walls, more insulation in the attic etc, I have found that a lot of the time here in the suburbs of Philly I can just run the mini-split unit in the winter and if I leave the bedroom doors open It's warm enough in there that I don't have to run the multi splits. Besides I like it a few degrees cooler in the bedroom for sleeping. The only negative is tomorrow is Thanksgiving and I'm going to have to have another semi-argument with my father-in-law for the 20th time -he keeps trying to convince me that oil heat is just as cheap as my mini split. I guess that's why they invented bourbon......

      1. Robert Silvers | | #10

        "The only negative is tomorrow is Thanksgiving and I'm going to have to have another semi-argument with my father-in-law for the 20th time -he keeps trying to convince me that oil heat is just as cheap as my mini split."

        That depends on the cost of oil vs the cost of electric, and the efficiency of each. In my state of MA, electric is 0.223, and oil would have to be over $3.50 per gallon for heat-pump to save money.

        Oil was about $2 a gallon back before the recent takeover of the US economy, so oil was cheaper. Now even with expensive oil back, it is still cheaper.

        Heat pump saves money if you live in a state not run by the people who run my state.

  7. Charles Galgowski | | #7

    Thank you Dana, Martin, Steve, and David. Dana - you discuss modulation. I appears when the heat pumps are not running closer to minimum capacity, there efficiencies go up. So there seems will be a trade off between buying a larger heat pump (at more cost) to save energy and therefore lower cost. Finding the "sweet spot" might take some judgement and skill. You recommend over-sizing the heat pump from 10 to 50 %. Let say the heat pump is over-sized at 30 %; Would this have the heat pump modulating at the more favorable efficiencies for the bulk of the winter? Charles Galgowski

    1. Expert Member
      Dana Dorsett | | #8

      >"...when the heat pumps are not running closer to minimum capacity, there efficiencies go up."

      Most modulating heat pumps will have higher COP at their minimum capacity than at their maximum capacity, across the entire operating range. Often the peak efficiency is somewhere in the lower third of the modulation range, not the exact dead-minimum, but most are near the max-efficiency when running at minimum speed.

      >"...there seems will be a trade off between buying a larger heat pump (at more cost) to save energy and therefore lower cost. "

      Not really. Bigger is not better.

      Heat pumps are a lot LESS efficient when cycling on/off rather than when modulating with load. The minimum modulated output in most series increases with the heat pump size, and increases the amount of efficiency lost to cycling. Always look at the minimum modulated output @ +47F in the AHRI submittal sheets, and compare that to your heat load at +47F outdoors to get a sense of the temperature at which the cycling begins.

      Oversizing to get a modestly higher COP at your 99% outside design temperature usually gives up a LOT of efficiency to cycling during the shoulder seasons. As a rule of thumb (that will vary with the particulars), oversizing by more than 50% of capacity at the 99% outside design temperature delivers a significant reduction in as-used seasonal efficiency compared to a 1.0x oversize factor. A mere 30% oversize factor isn't usually better or worse than a 15% oversize factor, the benefit (if any) only happening during the 10% coldest hours, which is when there isn't as much difference in COP with modulation level.

  8. Marta Snow | | #11

    We are planning to use the multi-position ducted Mitsubishi SVZ-KP12NA indoor unit with a SUZ-KA12NAHZ outdoor unit for a home in Minneapolis (climate zone 6).
    https://app.box.com/file/930618450998
    Despite all the great information shared so far, we cannot find any data regarding the COP at -15F. Does anyone know where we can get that data?
    Thanks!

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