Minisplit for a small room
I’m considering a mini-split install in our house. I’m in Austin TX, so it’s a climate that requires significant cooling. I have calculated loads and I have questions about what happens if one has a mini-split running in a low load room. It’s a child’s bedroom and the doors are closed all night.
The room has internal walls on 3 sides and just one 3×5 window. Currently the load calc shows it at about 3300 BTU (of course that is at design temperatures of nearly 100°F and 72°F inside).
We’re considering using the new Mitsubishi ceiling mounted cassettes (that fit between 16 inch on center ceiling joists), the smallest of which are rated at 9,000 BTU. MLZ-KP09NA https://nonul.mylinkdrive.com/item/MLZ-KP09NA.html I can’t find the minimum capacity, but the contractor expects to be able to report that when the system is modeled set up in Mitsu software. Lets assume that it has a turn-down ratio to 30% of capacity?
What should I expect from such a set up? My thoughts are that that means that at design temp it will basically turn-down to around 3000 BTU of cooling and thus run continually with low fan speeds.
What what about in shoulder seasons, the majority of the year when let’s say 1,000 BTU of cooling is needed? It would cycle on and off frequently, right? I’m concerned about noise (on/off, although I’m assured that it’s barely audible), swings in temperature and inconsistent fan speeds.
Another concern would be humidity removal, but I do plan to have a dedicated dehumidifier in the house (UltraAire 90H probably), so I’m not really relying on the AC for dehu.
The alternative would be a ducted mini-split to service two bedrooms and a bathroom, combined loads of about 9,000 BTU at design conditions. The downside of that is:
– unit and ducting would have to be in the unconditioned attic (whereas ceiling mounted cassettes can be covered with blown-in insulation)
– noise transfer from ducting
– lack of zoning.
I could ameliorate the hot attic unity/ducting by doing a fur-up (“reverse bulkhead”) in the attic, which is possible, but a fair amount of framing, air-sealing, and insulation work. I’d do that, though, if I thought that a very oversized mini-split unit was going to cause big comfort problems (not so much concerned about efficiency here, sadly).
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