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

Help deciding on Mits Heat Pump design

djmoore14 | Posted in General Questions on

Hello all,

I am trying to lower my electricity requirements by switching from electric resistance to heat pump. I do have a decent size wood fireplace insert but would like to reduce the amount of wood heat I use as well. I was originally thinking about an air to water system given they can be quite versatile (adding solar thermal, DHW, pool heat) but converting to radiant flooring isn’t really in the cards, space for duct work is very limited, and fan coils became cost prohibitive given the number I would need. Air to air seems like a better fit for our house. I have quotes from Mits and Fujitsu but have essentially ruled out Fujitsu because I am not very confident in the contractor. He insisted I needed about 7 heads in my house and I have read enough on here to know that is not a good idea. Mits contractor has been good to work with and I think we might be getting close but was hoping to get some feedback on what is the better choice.

A bit about the house first. Located near Peterborough, Ontario (climate zone 6 I think. -9F or -22C design temp). It was built in 1965 and has main floor and basement that is conditioned. Main floor is ~1200 sq ft and has a large open area (~790 sq ft) with two bedrooms and a bathroom off the one end. Basement is a little choppy but includes a rec room and bedroom (~140 sq ft each) plus storage, utility room, and an insulated crawl space that is ~380sqft. The house is built well and we have improved considerably on the insulation from what it was (R60 in ceilings – there is a section of cathedral we couldn’t do much with but it is quite air tight, R22 walls, all new triple pane windows, basement walls R20, uninsulated slab, crawlspace has R25 on walls and R8 on floor). I used and came up with the following:

Heating BTU – 31186, Cooling BTU – 13719. I’ll attach a floor plan that includes the room by room break down. 

Blower test was done and was 4.5 air changes at 50psa, but we have replaced 4 leaky windows and insulated the crawl space since then so I think it will be a bit better than that now. 

Option 1 – MXZ-5C42NAHZ with three zones. One head in the basement bedroom (MSZ-GL09NA), one in the main living area (MLZ-KP18NA-U1), and a ducted in the crawl space to feed the two bedrooms (SEZ-KD15NA) and put some heat into crawl space/utility room. 

Option 2 – MXZ-3C24NAHZ with two zones. One head in the basement 09NA and the ducted at 15NA. A second unit, MUZ-FH12NAH w MSZ-FH12NA in the main living area. 

Option 2 is what I proposed to him after reading all the articles about oversizing and issues with multi poor modulation leadings to lots of over cycling. I think I still need the multi as we would really like the ducted option for the bedrooms and there doesn’t appear to be a 1:1 ducted option for Mits in the cold climate version. With the single in the main living area, I thought I might be able to use just this in shoulder seasons and fire up the multi in the mid summer and winter periods and then I can better take advantage of the modulation of the 1:1. My contractor is all for option 2, but he doesn’t want to go as small as the 12 being concerned it won’t be enough. Based on my calculations, the 12 would do our living area until -18C, which seems pretty good to me given I have a 60000BTU woodstove in there if needed. If I do go with the 15 1:1 in the living room, the modulation doesn’t look that much better to me that the multi (15 ranges from 5150-24000, the Zuba Multi 36 ranges from 7200-45000 and that would be for the whole house rather than just the single, the 12 1:1 goes down to 3700). I would love to rip the baseboard heaters out if it did cover everything, but am okay with keeping them in if it means I can have a more efficient system. 

Any advice would be appreciated. Are both oversized/undersized? Is one better than the other or will they have similar results. Cost isn’t really a factor, they are both coming in basically the same at 20k. The simplicity of option 1 is nice, but the redundancy and potentially increased efficiency of 2 is good as well. 

Thanks a lot and please let me know if I left out any information. 

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. charlie_sullivan | | #1

    I, too, like option 2. Given that it isn't actually sized to work at the design conditions, I think that leaving the baseboard heaters in place is a good idea. That will help avoid doubts from your contractor, perhaps code officials, and future residents. And it will also allow you to have a backup if you get a mechanical failure in one of the systems in the middle of winter and getting parts takes time. And it will allow you to stay warm if for some reason you don't want to use the wood stove on a day when you are busy, or sick, or whatever.

    If you do want to do a hydronic system, an option you might have skipped over is to use mostly panel radiators, which can be quite inexpensive, and have other nice features: they are silent, they come in small sizes so you can put one in each and every room without oversizing, they don't need electric wiring, and they deliver some of their heat via radiation for improved comfort for people sitting near them. You need only use fan coils where you want to have cooling capability. Still the overall system will be more expensive if you take that approach.

    1. djmoore14 | | #4

      Thanks for the feedback. I did look into the panel radiators a bit but maybe not as much as I should have. For the main floor we would like cooling, but for the basement they could work. I found a few options that would work at 120F water but it seems like there are less options and the ones I found were starting to get a bit pricey and large for the BTUs I needed. Is there a type that you think work well?

      1. charlie_sullivan | | #8

        A lot of them aren't spec'ed for lower temperatures, but the heat output vs. temperature curve doesn't change much for different types, so you can pretty much use the same derating factor. I think the output at 110 F is about 1/4 the 180 F output, and that's a good design point to use--you can go up to 120 if you need to.

        I used Myson radiators. Here's dealer that lists them with decent prices:

        They go up to 12,000 BTU at 180, meaning about 3000 BTU at 110.

        As for the main floor, I would think in terms of one or two small fan coils that are plenty for cooling, but insufficient for heating, and then a few radiators in addition to spread the heat around better.

        1. djmoore14 | | #9

          Good thoughts, thanks!

  2. Expert Member
    Akos | | #2

    There are plenty of ducted single zone hyper heat units:!/product/34558!/product/34553

    The heat load feels still a bit on the high side based on the description of the place.

    With that much insulation in the basement, I doubt a single wall mount will be small enough for the actual heat loss. Either duct to it or keep the baseboard heat.

    I would just go with two ducted one-to-one units. One in the basement/crawlspace feeding the main floor+basement and one upstairs for the bedrooms. Wall mounts are maintenance item, I know it seems simpler to install one in the living space but better off to bite the bullet and run the ducts now. Much simpler in the long run.

    1. charlie_sullivan | | #3

      Yes, the heat load numbers are likely high. I meant to suggest monitoring the electric consumption of the baseboard for a week without using the wood stove as a way to get a more accurate estimate. This isn't the best time of year for that, given that there can be a lot of solar gain, but if you get some cloudy days in the mix it should be easy to discern. An IotaWatt system can monitor individual circuits and log data.

      1. djmoore14 | | #6

        I have thought about doing that but the electricity bill scares me! I'll look into the lotaWatt system. Thanks

    2. djmoore14 | | #5

      I must have missed those, thanks for that information.

      Re heat load. I've struggled with that. I have been quite careful with my measurements so I think it is pretty accurate. It sounds like that website often overestimates by 10-20% as well. My contractor thinks it is too low (based on his experience). He thinks it is more likely in the 35000-40000 area. Based on everything I have read in this group, I want to trust the lower numbers but would be kicking myself if it turns out to be undersized to a significant degree.

      Good thoughts on design. The middle of my basement is finished so I was trying to stay away from too much ducting (and why I stopped considering Zuba Central or Trane options). The two ends are unfinished and could each hold a ducted system but wasn't sure if a few supplies on either end of a 790 sq ft room would be enough? My wife isn't big on the look of the wall mount so she wouldn't be upset to see that go.

      Interesting point on maintenance. Not having used either, I was thinking the ducted would be more of a maintenance concern for some reason. I'll look into that more.

      1. Expert Member
        Akos | | #7

        We can do a back of envelope calc based on your data. Assuming 8' celling and 1200sqft, you have about that much wall area with 20% of say R6 glazing.

        With that much insulation, your basement heat loss is going to squat, so say 3000BTU.

        A rough ballpark, your 4.5ACH at 50PA translates to around 50CFM of natural air leaks. It is wrong, but at least a number to start from

        With a -9F outdoor and 70F (79F delta) indoor you loose.
        -walls: 1200*0.8*79/18=4200BTU
        -roof: 1200*79/60= 1600BTU
        -glazing: 1200*0.2*79/6=3200BTU
        -air leaks 50*1.08*79=4200BTU
        -basement ~3000BTU
        -HRV/ERV at 75CFM ~1500BTU

        Total ~17700BTU. You want to be safe you can call it 20k. Definitely nowhere near your contractors estimate and bellow the on-line calc. If it was my house I would have no issues with installing a single 1.5 ton unit (or say one 3/4 ton + 1 ton) for the whole place especially if you leave some of the electric baseboards in there "just in case".

        With the basement chopped up, might be easiest to install one ducted unit on one end and another ducted unit on the other end. This would give you a bit of zoning and temperature control of the bedrooms.

        1. djmoore14 | | #10

          That is great, thank you!

  3. jwasilko | | #11

    FYI Mitsubishi recently announced 2/2.5/3T single zone Hyper heat units for the US (no idea about Canada):

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |