Single Ducted Minisplit in Attic vs Heads in Each Room
A question for the community: Is it better to have 4 mini-splits in bedrooms, or a single ducted mini split serving all the bedrooms from an unconditioned attic? The main goal is cooling.
We’re in California, Zone 3C, in a 1961 house with a ventilated, unconditioned attic. The second floor is 1200 sq ft with electric baseboard heaters and no AC. Based on a Manual J calculation from CoolCalc, 15000 btu is plenty of heating, and 18000 for cooling. However, there’s an open staircase to the first floor, and a lot of the cooling will flow downstairs.
Here are the pros and cons that I can think of:
* No duct loss
* Mini splits are in-room, not in unconditioned space
* Each room is its own zone
* Oversized. Even 9000 btu is more than needed for a 150 ft2 enclosed bedroom. 4 rooms * 9000 btu = 36000 btu, which is about double what we need.
* No way to heat or cool the bathroom or tiny office if the mini splits are in the bedrooms
* Expense: 4 mini splits vs 1 ducted unit.
* A single ducted mini split with registers can easily handle all of the rooms, including the bathroom and tiny office
* Less visible, since a register is much smaller than a wall or ceiling unit
* Need to run ducts
* Duct loss and unconditioned space likely means a big loss of efficiency
Some ideas we had:
* Turn the attic into conditioned space. Unfortunately, there’s not really enough space to get in there and add more insulation, except maybe by blowing it in. The roof was recently replaced and we added solar panels, so we don’t want to add a layer of foam on top and redo it all.
* Put the indoor unit inside the conditioned space, like in a soffit or a closet, and run just the ducts through the unconditioned attic. The ducts would be R-6, most likely.
* Build soffits and run ducts inside the conditioned space. In practice, the only place we could do this would be down the length of the hallway. We’d have to move the lighting and smoke detectors. We’d end up with a long hallway with a low ceiling. Might be ok?
* Add a window AC in a couple rooms. The windows are all horizontal sliders, so not a great match for a window AC.
Any other ideas? What solution would you choose?
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