GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Best way to balance COP vs short cycling?

hvacuser | Posted in Energy Efficiency and Durability on

I’m switching from gas furnace to a ducted heat pump. Temperature is mild, 50th percentile is 50F, 99th is 28F, and 100th is 17F.

Doing a load calculation:
– at 100th percentile, 17F: 21.5 kBTU/h
– at 99th percentile, 28F: 17 kBTU/h
– at 50th percentile, 50F: 8.8 kBTU/h

Currently these are my heat pump options:

– Fujitsu AOU24RGLX/AMUG24LMAX, capacity range at 47F: 5,400-32,400 (!/product/33397)
– Mitsubishi PUZ-HA24NHA/PVA-A24AA7, capacity range at 47F: 10,000-28,000 (!/product/34518)

Since Mitsubishi has a low of 10,000 BTU/h I’m assuming it will short cycle for half of the heating season given that my 50th design load is 8.8k. Is this a good assumption? It’s not obvious to me since usually during a given day nights are colder than the day, so typically heating is needed until 8AM, and then no/very small amount of heating is needed until the evening. Which means on a daily average I’ll need 8.8k, but the distribution throughout the day is not uniform.

Fujitsu goes down to 5,400 BTU/h which is obviously better for the short cycling problem. However, it uses the same amount of power (0.55kw/h) at 5,400 that Mitsubishi does at 10,000 BTU/h. So even though it goes lower it’s much less efficient at that range. I’m surprised that Fujitsu has a much lower COP at 47F.

Mitsubishi seems to be generally much more efficient at 47F, but it doesn’t go below 10k BTU/h, which means it will short cycle for half of the heating season. How does one balance this out? Is it better to go with the more efficient unit that short cycles, or the less efficient option that has a lower minimum capacity?

In general I’m leaning more towards the Mitsubishi option due to some other reasons as well, but I don’t know how big of a problem this short cycling is going to be.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. armuNkEGatEp | | #1

    Bumping this as I am in a similar design dilemma. My peak heat is 14,000 BTUs/Hr. With a Daikin RXL18 unit, it'll generate 21,600 rated, 9000 min @ 47F. In 4A we are mostly seeing 47F days for the heating season. Daikin RX18 generates min 5,700, 21,600 rated @ 47F. I have to choose an RX18 or RXL18 due to other design constraints, even though RX12/RXL12 may be a little better sized.

    I suspect 9000 min will lead to short-cycling in the heating season, but @ 4.88 COP. Meanwhile, RX18 @ 5.07 COP, but the capacity drops sharply under 47F, to 13,600 at 17, and max 12,470 at 5.

  2. agooch | | #2

    If you go with Mitsubishi, make sure you splurge for the external thermostat. I love our Mits unit, but it short cycles like a som’ bitch without the external thermostat.

    1. hvacuser | | #3

      Thanks for the tip. I'm going the ducted route, the air handler comes with a thermostat that will be installed somewhere in the house.

    2. shrayd | | #8

      Hi Agooch! I'm seeing terrible short cycling with my Mitsubishi heat pump ( Have only had it for 9 months. First winter with it. Boston area.

      Installer recommends getting MHK2s for each head unit to help with overheating/overcooling. Did you install the MHK2s as well? I would be delighted if that ends up solving my short cycling in addition to overheating/overcooling.

  3. Jon_R | | #4

    > peak heat is 14,000 BTUs/Hr. .. RXL18 unit, it'll generate ... 9000 min

    Which, in terms of cycling, is still better than a single speed unit. And in my experience, reasonably sized single speed heat sources provide good comfort.

    1. twoodson | | #5

      I have both the 24k BTU Fujitsu mid static and the multi-position (what you are looking at). The have the same outdoor unit and they have the same min output.

      The mid static has been running on an upstairs apartment that is under construction for 2 years and was sized just right for summer, but slightly oversized for winter. It runs almost constantly at a low level in the summer and makes the apt very comfortable. I couldn't be happier with that unit. If you can, get that one, it's definitely a higher quality indoor unit than the multi position and you could probably go with the smaller size 18kbtu.

      The multi position is in the unit I currently live in and was installed this summer. It is undersized for my climate in the winter with current airsealing/insulation/windows. (Prob to the 90-95 percentile). During November, it has run at between 550 and 800 watts most hours of the day between 8pm - 8am, but cycles during the day. We hit 22 last night and it bumped to 2kw after a defrost. I think it maxes out at 3.5kw input. I expect December-February will really put it to the test. My hope was to hit the sweet spot for most of the hours of those months and I have the electric backup aux add-on for extreme cold snaps and malfunctions.

      I'll try to keep the forum posted, probably more if I start getting cold :)

      1. hvacuser | | #7

        Thanks for sharing your experience and your suggestions. For various reasons I've decided to go with the Mitsubishi option, hopefully it won't cycle too much in the early heating season.

  4. twoodson | | #6

    Another option you may consider is the ARU12RGLX. It easily meets your 99% design, is more efficient, min output is 3100 and is probably cheaper and definitely smaller. Another option you may consider is the ARU12RGLX. It easily meets your 99% design with 17.6k but & 17f , is more efficient, min output is 3100 and is probably cheaper and definitely smaller. You prob won’t have a problem with short cycling on this one.

    Amazing how oversized gas furnaces usually are…

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |