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Community and Q&A

Mitsubishi ASHP – Questions about hitting the +5 & -13 limits

Scott K | Posted in General Questions on

We are in Minnesota (Zone 6a). 2200 sq/ft single level, FPSF slab on grade.  We have a pretty good house with R60 in ceiling (2″ CS sprayfoam & blown cellulose), R34 10″ densepack double stud walls, 2″  under slab, 4″ rigid on perimeter slab.  .79 ACH.  9.6kW solar PV  Built in 2018

Mitsubishi claims 100% heating capacity down to +5.  And will provide heat down to -13.  But what does that really mean?  

So this is the first winter living in our new house and we are about to hit the -13 claimed heating limit.  I am curious, what happens when we drop below that temperature.  Will it shut off?  Should I shut it off before we hit that mark?  Will the unit draw increasing amounts of electricity with no heat produced?  

Mitsubishi says 100% heating capacity down to +5.  This extreme cold got me thinking about the claim…is it actually less efficient between +5 and -13?  Or is it just unable to provide as much heat in that range and would it make sense to supplement the ASHP with the electric baseboard heaters in that +5 to -13 range?

Heating System specs…
Mitsubishi
(1) MSZ-FH15NA 1.25 ton indoor head
(3) MSZ-FH06NA .5 ton heads
(1) MXZ-4c36NAHZ-U1 3 ton outdoor compressor
(1) PAC-MKA50BC 5-port branch box (installed in unconditioned attic space)

 We have electric baseboard heat for backup and a 27,500 btu gas fireplace.

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Replies

  1. User avatar
    Dana Dorsett | | #1

    The "rated" heating capacity is the AHRI nominal modulated output that gets efficiency tested at both +17 and +47F. Mitsubishi guarantees the units will be able to deliver 100% of that "rated" capacity at +5F (not -5F).

    At -13F the total capacity will be lower, but still a large fraction thereof, and the actual capacity is specifed in their extended temperature capacity tables. With the FH series the capacity at -13F is typically 75-80% of the AHRI "rated" capacity.

    For instance, the "rated" capacity of the FH15NA is 18,000 BTU/hr, which is also it's maximum output at+5F when on a single zone system. At -13F that falls to 14,700 BTU/hr, which is about 82% of it's "rated" capacity. On an oversized multi-zone system it might still deliver the nominal 18,000 BTU/hr @ -13F if the compressor can keep up, but not if the compressor is running out of capacity.

    The FH06 are good for 8700 BTU/hr , which x 3= 26,100 BTU/hr. Add that to the nominal 18K and you're at 44,100 BTU/hr. The MXZ-4c36NAHZ-U1 is good for 45,000BTU/hr @ +5F outdoors, 70F indoors, so it will be able to keep up with the 44,100 BTU/hr with all heads active down to a few degrees cooler than +5F, but not at -13F.

    The capacity of the 4c36NAHZ at -13F is 36,000 BTU/hr, which is 80% of it's "rated" output, per NEEP's spreadsheet data (I didn't confirm that with the extended temperature capacity tables, but sounds about right.)

    Efficiency falls with temperature, but it varies with load too. The COP of the 4c36NAHZ at maximum speed at -13F is still a healthy 2.2. At +5F it's COP at maximum output (all heads actively running) is 2.4, and at minimum output (one FH06 running ) is 3.7. At +17F (which is probably your approximate average temperature in January) at max (all heads active) it's COP is 2.78, at min it's 4.74, and at it's "rated" output of 45,000 BTU/hr is 2.85.

    It will keep running and delivering heat at temperatures colder than -13F, but at an unspecified capacity. At some temperature cooler than -18F some Mitsubishi compressors will turn off when their outdoor temperature sensor is too far out of range, but I'm not sure if that is true for the 4C36NAHZ. Those Mitsubishis that shut off to self protect automatically re-start when the temperature sensor measures something close to -13F or warmer.

    So, have you calculated your actual heat load? If yes, at what outdoor temperature?

  2. Scott K | | #2

    Thank you Dana for the detailed response. That's fantastic and will really help me understand my system better. And thank you for the -5 correction. I edited my original post to reflect the correct rating.

    I had Vanguard run a Manual J and gave it to my local HVAC installer. The outside db design temp used is -6 F. Inside is db is 70 F. Heating equipment load is 23234 Btuh.

    1. User avatar
      Dana Dorsett | | #4

      Without knowing the individual zone loads and the ratio of capacity/load on each zone it seems possible that the 4C36NAHZ is sub-optimally oversized for your house, since it has over 50% capacity margin at your design temperature. But if you need 4 zones and can't combine a couple of them using mini-duct cassettes it's about as good as it gets.

      If the FH15 head is right-sized for the load an the FH06 are all crazy-oversized for their loads you may experience overheating/overcooling the FH06 zones even when they're nominally "off", from just the bypass refrigerant when just the FH15 head is running. This condition is somewhat rare, but it happens.

      1. Scott K | | #5

        That's interesting Dana. I think you may be right about over-cooling We weren't living in the house last summer, but the system was installed and running. And it was crazy cold in the house even when I had each head in "dehumidify" mode. I was having a hard time keeping the humidity down if I didn't have the units running, but it was getting down into the low to mid 60's inside the house, I believe. And we weren't even cooking or showering in the place yet. I was in the middle of finishing the house, so I didn't have time to think about or figure out what was going on. I've attached my Manual J documents, if you care to have a look at them. Cooling load is only 13632 btuh.

        We have not had any overheating issues. My only complaint with the heating system is the whooshing and boiling noises the heads make in the middle of the night.

        On another note, my compressor is still running at -30 this morning. Or at least the fans are turning on the compressor. It's certainly not keeping up with heating the house, baseboards and fireplace are heating fine though.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Scott,
    Dana Dorsett has answered your question thoroughly. (Thanks, Dana.)

    Other GBA readers who have similar questions may want to read this overview article on selecting ductless minisplit models for a cold-climate house: "How To Buy a Ductless Minisplit."

  4. Doug McEvers | | #6

    How many lineal feet of standard electric baseboard (for backup) would it take to produce 23,234 Btuh as needed for Scott's house at a - 6 F design temp. It was -28 F in Minneapolis this morning putting a lot of heating systems to the test.

    There were at least 2 power outages reported in MN this morning, a frightening thought with temps in the minus 30's. Heating redundancy is hard to quantify without grid power, it would take a large portable generator to heat a typical MN home with electricity. All the more reason to make new and existing building envelopes more efficient.

  5. WPmichael | | #7

    Hi Dana

    if you look at the engineering manual for H2i heat pump there is some data that has stumped me. Here is a link to the engineering manual: http://meus1.mylinkdrive.com/files/13_5_MSZ-FH_High-Performance_Wall-Mount_Heat_Pump_Systems.pdf

    Let's take the FH15NA that you mentioned above, on page 7 of the manual the FH15NA is rated for 11,000 BTU/hr at 17 degrees F. But if you look further on in the manual on page 27 there is graph that show basically 100% capacity until 5 degrees F.

    Would you know how many BTU's you can expect from these unit at these low temps?

    I have this particular unit and I like them a lot. Even at single digit temperatures ( expected tonight) the home remain warm.

  6. User avatar
    Dana Dorsett | | #8

    Again, the "Rated Capacity" level at +17F is the modulated output level at which it was tested for efficiency, not it's maximum capacity at +17F. When married to a multi-split compressor like the 4C36NAHZ it doesn't really modulate with load (unlike single zone mini-splits) and delivers something close to it's maximum 18,000 BTU/hr at any temperature as long as the compressor can keep up.

    The capacity curves on p26 across temperature are the capacities at maximum speed of the MUZ-FH series single zone compressors married to their corresponding MSZ-FH-heads. The performance of the MSZ-FH head is not identical to those curves when tied to a bigger multi-zone compressor. The sharp knee in the curve at +5F is an artifact of how they are controlling the compressors and blowers. With different control algorithms they could get more heat out of them at warmer temps than shown on the curves, at the cost of lower efficiency.

  7. WPmichael | | #9

    Thanks for the explanation Dana.

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