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Community and Q&A

Mismatched Heat Pump Units

landg1 | Posted in Energy Efficiency and Durability on

In Oct of 2020 we’d had 3 minisplits installed in bedrooms that the central ducted furnace never quite kept warm, and we’d been very pleased.  That set included Mitsubishi‘s MXZ-3C24NAHZ2-U1 outdoors, and one MSZ-FH06NA and two MSZ-FH09NA indoors.

Then, in early Feb of this year, the heat exchanger on our gas furnace broke.  It was cold (we live near Boston), but our bedrooms were warm, and we’d been planning to replace the furnace with a ducted heat pump. We bought a couple space heaters and solicited bids.

First quote we received was from the same company that installed our original minisplits and it was for a second outdoor unit, SUZ-KA36NAH, and the 3 ton air handler to match:  SVZ-KP36NA.  Unfortunately, then we were told their supplier couldn’t get the outdoor unit quickly due to supply chain problems, so they proposed replacing our two ton outdoor unit with a four ton unit (MXZ-8C48NAHZ2-U1).  They gave us credit for the two ton unit they hauled away, and since we wanted heat ASAP, and we trust them, we accepted the proposal.   Our new equipment was installed and fully operational by the end of February.

Since we had the space heaters, we did not have them install any kind of resistance heat in the air handler.

My question is, is there a problem with having the outdoor unit be 4 tons, but all the indoor units add up to 5 tons?  If so, any advice?

I tried to do the “15 minute calculation”described in Dana Dorsett’s “Replacing a Furnace or Boiler” using our natural gas bill for Dec – Jan of this year (although our 3 minisplits were contributing to some of the load, too), and I got about 3 and a half ton.  (The natural gas furnace that was replaced was 7 tons!)

Also, I want to thank Jon Herrod for the excellent 5 part series on Planning a Furnace to Ducted Heat Pump Retrofit – I learned a lot!

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  1. Deleted | | #1


  2. Expert Member
    Akos | | #2

    Mitsubishi usually allows up to 130% capacity difference between indoor/outdoor units, so your 4 ton unit with 5 tons of heads should be not a problem. You can check the actual limits here:

    1. landg1 | | #4

      Thank you! That is reassuring. I also appreciate the link - I have been studying them; I have much to learn!

  3. joshdurston | | #3

    Ideally the outdoor unit matches your load, sounds like a 4ton unit is perfect if your actual calc was 3.5ton.
    You're better off with a 4ton than you would be with a 5ton especially with a multi-split. It will be less likely to short cycle, and should still have the capacity to match your loads. The minimum capacity is almost half of the 5ton (16kbtu/4ton versus 31kbtu/5ton).

    How did it perform for the last bit of winter?

    1. landg1 | | #6

      My calc didn’t include the electricity that powered the 3 ductless minisplits (2 ton total) though, so I’m not sure.

      In March, we were comfortable, but I noticed the ducts often seemed to be blowing cool air. I’m beginning to learn about ducts and figuring out the pros and cons of insulating our basement.

      1. Expert Member
        Akos | | #7

        "ducts often seemed to be blowing cool air"

        Unfortunately this is the limitation of multi splits. The turndown on them is not great, so when the weather turns warmer, the unit spends a lot of time cycling instead of running at low output. If this creates comfort issues, the unit can be programmed to turn the fan off once setpoint is reached. This is somewhere in the installer menu on the thermostat of the SVZ.

        Since it looks like the system is well sized, so once you need real heat, it should be blowing warm air all the time.

  4. Deleted | | #5


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