Sizing a Minisplit System for Wide-Ranging Heating-and-Cooling Loads
Hi, I’ve spent the last few days reading pretty much all the different posts regarding the multi-zone vs multiple single mini splits trying to figure out what would be the best scenario for my project, without much success.
I actually had two contractors come at the house, but they both proposed only wall units using the rule-of-thumb technique…Being a mechanical engineer myself, that’s not enough for me and I’m having a hard time finding anyone willing to do proper calculation in my area.
My house is a 1250 sq ft single story (excluding basement), and I’m not looking to install a heatpump for the basement. I’m focusing on the ground floor for now and I want to build a setup to heat the during winter and cool during summer. I want to be able to close the doors of the bedrooms, so a wall mounted unit in a hallway is most likely not going to cut it.
As you can see on the attached house layout, the bedrooms are completely on the opposite site of the living/kitchen/dining, and are separated by a very closed off hallway so a single head for the whole house isn’t going to work either.
I decided to take some time to do a manual J calculation on coolcalc to figure out the different loads per room. Assumptions are the following for my house (in Quebec, Canada):
– Air infiltration = Tight
– Wall insulation = R21
– Ceiling insulation = R44
– All windows properly sized in coolcalc, double pane low-e (R4)
– Summer temp = 80F
– Winter temp = -10F
I’ve ended up with the following values:
Full house heating load = 24 000 btu, including basement room loads
Full house cooling load = 17000 btu, no cooling for the basement
Living/kitchen/dining: cooling = 11 800 btu, heating = 8171 btu
Bedroom 1: cooling = 1415 btu, heating = 1833 btu
Bedroom 2: cooling = 764 btu, heating = 1516 btu
The rooms not listed (entry, bathroom) will be heated through radiant floors and do not need cooling as much as the living area and bedrooms.
At first, I was looking at a 2-zone 18k unit with a 7-9k ducted cassette for the bedrooms and a 12k wall mount for the living room.
However, after reading on here, I’m questioning if I’m going to have issues with cycling on/off due to the very low loads in the bedrooms compared to the size of the compressor.
Separate outdoor units:
– One 9k ducted unit for the bedrooms such as fujitsu 9RLFCD which can modulate down to 3100 btu (still oversized, but couldn’t find single ducted units below 9k…)
– One 12k wall mount unit for the living room, which can also modulate quite low for the in-between seasons.
I’m currently leaning toward option 2, but I’d really love some inputs from more experienced folks on here, as maybe I’m overthinking this and option 1 would do the trick just fine.
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