Sizing a Minisplit System
I’ve been considering a minisplit system in my 1950s ranch for quite a few years and I’m finally ready to pull the trigger. The primary motivation is to provide AC since there isn’t any currently and only have casement windows, making window units a no-go. However, our heating source is a propane fired boiler with hydronic baseboard. I’d ultimately like to get off of propane entirely so I want to size the system to be adequate as a heating source in the future as well, likely pending additional envelope improvements.
The relevant information. House is a 1950s ranch in Zone 5 (NJ). The first floor walls are hollow CMU with 1/2″ EPS under vinyl siding. Crawl space with no insulation. There is a partially finished attic with a combination of R38 of closed cell foam and dense pack cellulose. Last blower door was on the order of 9.5 ach50, depending on whether or not I include the crawl space in the volume calculation. Quite a few skylights. In short: not great.
I had a manual J done which gave 62k Btu*h heating and 27k Btu* cooling. I did note that the calculation was done based on a R11 wall assembly which is very optimistic for the assembly. I get around R4.5 from what I’ve seen. I’ll have the recalculation done, but also using the R11 results as somewhat representative of what it would be if I were able to do exterior wall insulation in the future.
The first estimate that I got for a system recommended the following:
-Outdoor condenser: AOU36RLXFZH
-One Indoor ASU18RLF
-3 Indoor ASU7RLF1
The 18k unit would be mounted in the living room area which is essentially open to the entirety of the first floor. The 7k units would go in the three bedrooms on the first floor. My thoughts are the following:
-Total output is low to provide full heating supply.
-We have a conditioned attic with no dedicated supply currently. In the future we may turn this into living space, but I’m not looking to add a system to the attic until we decide on that. It stays fairly comfortable currently.
-The multi-split setup is likely giving up some efficiency to multiple single zone systems.
-Because of the low R value of the exterior walls, I don’t know that it’s a bad idea to have individual heads to supply each bedroom. I suspect room to room variation, especially for heat, could be significant.
-I will be attempting to improve the air sealing and at least the crawl space insulation situation in the next couple of years.
My first thought is to put in a single zone unit for the main floor and then, potentially, maintain the multisplit system for the individual bedrooms. I don’t really have a good mounting location for a second, large, single zone unit on the first floor so I don’t know about, say two 18k wall units instead of the current plan. Very interested to hear others thoughts.
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