Sizing/placing ductless mini-splits for whole house with cathedral room
Last Post 30 Oct 2020 08:51 PM by Smart Shop. 2 Replies.
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alexinncUser is Offline
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29 Oct 2020 08:45 PM

We’re wondering where to place two ductless mini-splits in a two-floor farmhouse addition (of a second floor)  that now will have one big double-height cathedral room. Any best practices based on this floorplan?

We are heating-dominated and want to place these with that in mind. House stays relatively cool here with two mountains on either side, and a creek running past, despite full sun from 11-4:30 in the summer.

This house used to be only one level. At that time, we ran a 15K BTU Fujitsu unit and a woodstove and were fine. Fujitsu struggled below freezing to do the whole house, but was perfect for bringing 40˚F up to 68˚F without the woodstove. All the new walls, including in the great room, will be double 2×4 stud walls with 1/4″ air gap, rockwool insulation. 18″ of rockwool to go in attic.

In the roof plan you can see that the “false dormer” part of the house will have 18″ of blown rockwool attic insulation, and the steep cathedral portion of the roof will be 18″ of spray foam.

L1 floor in the crawlspace will be flash and batted with 6″ rockwool batts. Thinking a vented crawl that’s not part of the thermal envelope.

We have not determined if a solar attic fan would be helpful or not.

Thanks for insights or best practices! Wondering if this is a paid job for a Manual J contractor. In which case, I’m open to suggestions.


Attachment: 2020-03-10_Floor_Plans.pdf

newbostonconstUser is Offline
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30 Oct 2020 10:37 AM
Nice house. I think the consensus is solar attic fans to get the heat out aren't worth the money.

Though we love our Tamarack house ventilating attic fan in the spring and the fall. We have it automated to a thermostat that looks at outside and inside temp to control it based on those conditions. Our inferred camera shows no heat loose in the winter....

As far as the heat load goes, I hope others will chime in. There is software out there to help. I would just say don't mount them to high on the wall.

I would definitely get a IR camera to help you insulate especially on a rebuild.....We reached .79 ACH with little effort. Just used the camera to find cold spots before final drywall coat, drilled a small hole and sprayed foam in.....rechecked the next day.

"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
Smart ShopUser is Offline
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30 Oct 2020 08:51 PM
18" of spray foam is a bit excessive.

Any heat you generate is going to rise so if heating is the main use of your AC, it might be better to keep the AC downstairs and put the thermostat in/near the master.

I'm going to get a multi-zone mini split for my new building with one large zone downstairs, an exterior grade door separating the downstairs zone from the upstairs and upstairs one zone for the living room, one zone for the master and possibly a third zone for the "office". The zones are mainly so that people anywhere in the building can have some control over the temperature and zones not in use can be set to higher temps in the summer and lower temps in the winter to possibly reduce energy usage.

With your open space between levels and many more rooms, it seems like controlling zones is not your purpose, just increasing capacity. If that is the case, I would just try to place it where I'm not likely to hear it.

It sounds like it gets cold there and I have never lived in a place that got snow except a light dusting every decade or so. The daytime highs are rarely below the 40's so there is a lot about building for cold climates that I just don't know.

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