Sizing Minisplit Mitsubishi Heatpump
I live in a townhouse in the Northeast, that is 1100 sq feet. 550 down, 550 up. Kitchen, living room downstairs, 1 window kitchen, 1 slider livingroom,; a passthrough between kitchen and living room. A 12 step staircase, up 3 bedrooms, one very small, 1 medium and the third a SMALL master, maybe 12 x 16. Bathroom upstairs and small bathroom downstairs on an outside wall. MassSave put extra insulation in the crawlspace under the first floor and a zippered enclosure over the drop down stairway to the attic crawl space. They said the insulation was adequate.
I have 2 separate estimates with opinions about sizing . 1 estimated 12,000 up, 12,000 down, 20,000 compressor. His rationale was that this was appropriate for the square footage, he didn’t ask about my usage, number of people in the house, etc and didn’t address heat rise. The other estimated 9000 up, 18,000 down and a 30,000 compressor. His rationale was loss of heat to the upstairs and the lower BTU for the upstairs was adequate and that 12,000 was too big for the upstairs. 3 unit owners put in 12, up, 12, down and 20,000 outside and they said it is effective. The guy who estimated the 9000 up and the 18,000 down said it would be more efficient and more cost effective to run than a 12,,000 up and and a 12,000 down. There is about a $500.00 difference between the two estimates.
My reading is that you have to be careful about oversizing. I asked the other unit owners how they are doing, they said that their upstairs and their downstairs has been sufficient on the 12 , 12 and 20 although one of them said their third bedroom is slightly less warm .
I liked the fact that the second person asked me specific questions and honestly explained why he was recommending 9000 up and 18,000 down. My question is is the 18,000 to oversized for the downstairs and will the 9000 upstairs be enough for the bedrooms ( with doors open understanding the air may. not get around corners well).
Need to get this done and need input.
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What is your existing heat? Instead of guessing, I would run through the math here and figure out what you need:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Typical townhouse with reasonable insulation is very low load. The only exception is if you have large west or south facing windows, you might need extra cooling capacity.
My feel is that the 20k outdoor unit is already oversized, so definitely don't go up from there.
Existing heat is electric baseboard, zoned in every room with thermostats. Will keep them connected as a back up system. Do not have large windows, no bay windows. A standard slider in the backyard, a standard window in the front. I think the back where the slider is North East. I get VERY LITTLE light in the house. I get some sun it in the morning up through about 12 pm through the slider and it goes over the house to the kitchen , parking lot side. The upstairs bedroom where the minisplit would also go is above the slider in livingroom with similar exposure except a little more light than downstairs. There is little to no sunlight in the kitchen. I The downstairs is not "open concept" . THere is a kitchen as you walk in on the right, with a passthrough in the wall to livingroom . You walk in the front door, down the little hall and your are in the living room facing the slider. The stairs are to the left of the slider.
Electric heat is not problem for the heat loss calculation. Subtract your lowest shoulder season electric bill from a winter time bill. Take the kWh and multiply by 3414 to get BTU total which you can use for the math for the link above.
Doesn't sound like cooling will be much of a problem in your case, start with the heat loss estimate and see where that ends up.
Shoulder season, you mean March to April or September to October or both? or Neither. What I do know is that when I went into a unit owner's identical unit , same sq footage in the summer while she had the AC on, it was really cold. I can't imagine that I would need 18000 BTU downstairs to compensate for loss of heat to the upstairs. I have standard height ceilings, no vaults, just the high area over the stair way that is typical and I did put a door at the top of the stairs for animal reasons.
I am not so great with the math, but will try to do what you are suggesting. Your input is sound. I just don't see why I need the power potential of a 30,000 compressor and why the 12000 isn't enough for the bottom.
Now the top, he tried to tell me that 12,000 was too much for the upstairs and that there would be too much cycling and I just didn't understand it. Granted the master bedroom where the minisplit would go is right across from the second bedroom and the bathroom and the third bedroom are across from each other around the corner,, I understand that the heat may not easily go around a corner. Then he said I may have to put the baseboard on in the bathroom. It seems to me that the 12,000 should be able to handle it and I was not sure that dropping it to 9000 was reasonable.
He said that I do't need the 12,000 because much of it is lost going through the roof.
The roofs are new, there is insulation on the floors of the attic crawl space, and insulation down to the soffits, with P vents to stand off the insulation. It is a trussed roof. IT gets hot as hell in the attic. And I can tell you that the top floor has always been horribly hot in the summer , suffocatingly so to the degree that I used to sleep downstairs. What is your thought about that in terms of 12000 being a waste because of loss of heat through the roof,
Let me try to do this calculation, Please clarify shoulder season wintertime bill.?
Use your lowest electricity use number. You want to pick a month that does not have AC or heat use. Either early fall or late spring both work.
Run the calculations first and see where you end, from there can look at what the right sized unit is.
I just renovated a barn to conditioned space, 800 square feet per floor. We're in Western Massachusetts, zone 5a.
We decided on a 12k for upstairs and a 15k for downstairs. The idea was we would primarily heat the place with the downstairs unit and then the upstairs when we want to add heat upstairs. And the upstairs unit was sized to effectively cool the entire place. When I was doing research I did read many times about the importance of not having an oversized unit for cooling because the cooling runs more effectively at full capacity and will dehumidify properly.
I can't speak to efficiency in terms of cost to run it all. I can say that the systems work well in the ways I described above. Could probably have dropped the size of each unit. We were pretty detailed with our airsealing, walls r-25, cathedral ceiling r-60