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Heating/Cooling solution for top floor of 1940s home – Mitsubishi heat pump?

Nicvan | Posted in General Questions on

Hi all,

New here and i’ve been reading up on the FAQs. I have been working with HVAC professionals to find a solution for a heating/cooling problem in my home and i’m looking for some additional advice to see if i’m on the right track.


Zone 5a small 1940s 1200 sq/f home (with finished basement) with a second (1/2) storey that does not cool or heat adequately. The 1/2 storey has our two bedrooms and is approx 250 sq/f plus a small landing, both rooms have knee walls and very small attic space, the ceilings are low. The attic is insulated but the knee walls and sloped ceiling are not, we will eventually spray foam the knee walls but the not the sloped part of the ceiling, we have 1 large good quality window in each room. We live in Ontario so it gets very cold and very hot/humid. We currently have a ducted gas furnace and electrical AC unit supplying the whole house. The size of these are adequate but but cool/heat does not reach upstairs very well. We are energy conscious and are not looking for a super cold or hot space, in summer with AC our main floor thermostat is usually at 25c (77 fahrenheit) and in winter the main floor is at 20c (68 fahrenheit) it would be great if we could make the upstairs the same temp without have to crank the AC or heat unnecessarily. We currently have to make the main floor and basement too cold/hot to accommodate upstairs. Our home is pretty open plan, the finished basement is one room, main floor is one room, and we have two doors on our bedrooms upstairs.

We aren’t looking to change our entire heating/cooling system at the moment as the furnace and AC still have a lot of life left. But we do want a solution for the upstairs that could eventually become part of a bigger electrical efficient system.


We are looking at the Mitsubishi heat pump units. We do not want wall or ceiling mounted units as we have very minimal space. We have room in the knee walls for a hidden unit. We have struggled to find HVAC folks who can advise on the ducted heat pumps. But we finally got a quote for this set up SVZ-KP12NA and SUZ-KA12NAHZ-TH, which is a 12,000 btu multi position air handler heat pump with outdoor condenser. This will be hidden in the knee wall with 2 short ducts to our rooms and 2 air returns.

Is there anyone who could answer these questions:

– Is this an appropriate size/solution for this issue?
– Would any of the other units be more appropriate?

We do not want to end up with over cooling/heating our home, we just require some added comfort so that the main furnace and AC don’t have to overwork. In other words we want to be comfortable in every space of our small home (currently, one of us works from home in the basement and the other one on the second floor, so it’s a real struggle to find a happy medium). Very likely with a unit upstairs we wouldn’t even have to use the main AC that much anymore as it is really only the upper floor that gets much too hot. Similar for the heating system as we also have a new efficient wood stove on the main floor.

Thanks for the your help it is very much appreciated! I’m definitely a beginner in this process so if I have left anything out, please let me know.

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  1. user-2310254 | | #1

    I'll give your question a bump.

    Have you had a Manual J done? The specified unit is 1 ton, which sounds like a lot for 250 square feet. Maybe the SEZ-KD09NA4 would be better (or maybe even that unit would be too large). Let's see if one of the HVAC experts will weigh in.

    1. Nicvan | | #4

      I appreciate the response. I have not been offered a Manual J assessment. I don't think any of the HVAC folks that i've talked to do this. Would this involve another professional? I have had a pre-energy audit conducted but that is it.

  2. paul_wiedefeld | | #2

    Maybe consider this one too: SEZ-KD09NA4 & SUZ-KA09NAHZ. Its slightly smaller. Both turn down lowish, so it’ll be oversized but may be better than putting a ductless head in both rooms. It might be worth pricing out a ducted system for upstairs with a ducted system for downstairs at the same time. A multi-split to be precise, like Mitsubishi’s MXZ.

    A different direction would be window units for upstairs and electric baseboard. Not glamorous, but extremely cheap upfront. While more expensive to operate, for a space that size total cost will be low.

    1. Nicvan | | #5

      Interesting. I'll take a look into the smaller option. I was told that the 12,000 btu unit was the smallest it could go for what we want. Yes, I did ask about a system for the whole house but the conclusion from the HVAC professional was that it would require two air condensers outside, not really sure why? Perhaps it has to do with wanting separate zones? It looks like I should keep searching for some HVAC experts who know a bit more about this stuff.

      1. paul_wiedefeld | | #6

        Yup that’s nonsense, but sometimes having two separate units is preferred. Adds redundancy and they usually modulate better. 12k is the smallest high static pressure ducted option but for small rooms close to one another, you might not need the high static option. Several brands make options like this.

  3. Deleted | | #3


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