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Minisplit for a small room

James Howison | Posted in Mechanicals on

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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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    James,
    I think that the simplest solution is to install a window-mounted air conditioner. Instead of mounting it in the window, though, build a sleeve through the exterior wall that is sized for a small air conditioner. The cost is low, and you can buy a right-sized unit.

  2. James Howison | | #2

    Yes, then one could match capacity. But:

    - Walls are CMU block, so the sleeve would be a difficult install.
    - Noise is substantial (although the child does have a white noise generator ...)
    - Aesthetic concerns

    I'm still interested to know what happens when a mini-split is oversized, anyone got experience there?

  3. Sean Cotter | | #3

    Don't they have multi-cassette units with the single condenser? Multizone condenser running 2-3 heads? That just has the refrigerant lines (and condensation evacuation) which can be better insulated through an attic, right?

    Perhaps something like this: http://www.tamtech.com/return-air-pathways if you have adjacent wall space or willing to cut into the door - effectively vented the room air out into the hall or larger house itself?

  4. evantful | | #4

    Generally it will short cycle (turn on and off) instead of modulating once it gets below the minimum Btu output. On the cooling side this lends itself not lowering the humidity effectively and increased energy usage

  5. User avatar
    Dana Dorsett | | #5

    When a mini-split is oversized it does a lot of cycling on/off at minimum speed, operating at below it's rated efficiency. They're still pretty quiet and comfortable- most are quieter than your refrigerator. At some level of oversizing they'll have to be operated in "DRY" or "DEHUMIDIFY" mode most of the time to stay ahead of the latent loads in a TX climate.

    I think MLZ-KP09NA cassettes are designed only for use with multi-split compressors, not it's own dedicated outdoor unit (?). (They only have a submittal sheet for the multi-zone configuration.)

    Better-class half ton PTAC or PTHP units with scroll compressors can be pretty quiet at low speed too, but it sounds like you don't want to deal with the wall-sleeve issue.

  6. James Howison | | #6

    Thanks all. I forgot to mention that this would be part of a multi-zine system, probably 5 or so of these heads.

    But I get the message. Too much oversized means cycling on and off. Bad for efficiency but not terribly noisy. Eventually causing issues with humidity removal.

    The trouble, as has been discussed around here a little, is that mini-splits with closed doors in a cooling climate are problematic. In a heating climate it’s ok, good even , to have th bedrooms a few degrees lower than the rest of the house. But in a hot humid climate you really want the opposite, the bedrooms more conditioned (less hot). But then you run into a small version of the smallest system have too much capacity.

    I’m hoping that the mitsu multi-zine layout software gives us a clear indication of minimum capacity at each head. Then we can decide between ducted mini-split and head in each bedroom.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    James,
    There are two standard solutions to the problem you describe ("the bedroom load is too low for a minisplit head"):

    1. Install a ducted minisplit that serves multiple rooms.

    2. Install a conventional forced-air duct system connected to a split-system heat pump / air conditioner.

  8. James Howison | | #8

    Thanks! Some word from the Mitsubushi people today that the head itself can dial down to under 1,000 BTU, although it all depends on which compressor and what other heads are calling for. e.g. a 4 ton outside compressor can dial all the way down to 6k BTU, so as long as other heads are taking 5K capacity then that room can do ~1k. Might get requirements lower with 2 smaller outside compressors.

    Basically the answer requires running a few designs through the "System Builder" software and making assumptions about what other heads would be doing (which in our case with high thermal mass, a dog in the house at all times, thus no real set back opportunities will be fairly easy). When that's complete I'll try to follow up with the documents.

    I'm trying to avoid ducting in the attic!

  9. User avatar
    Dana Dorsett | | #9

    It's good to know that the cassettes will modulate with load on a multi-split whenever they can, and that the -KP09 can throttle all the way back to 1000 BTU/hr, which is GOOD.

    The whole house cooling load is likely to be above the 6000 BTU/hr minimum much of the time, which means the micro-load of the bedroom would still be reasonably served with most or all of the other zones modulating on their loads.

  10. Akos | | #10

    I had to deal with a similar cooling issues (LG 9000btu head in a small room). It does work just be careful with placing the outlet. I find that even on the lowest setting there is too much airflow across the bed which can get uncomfortable. Never had issues with short cycling (usually have more than one zone running at night time).

  11. Jon R | | #11

    I interpret the Mitsubushi comments to mean that the head/cassette cannot modulate - it's on or off and how many BTU depends on what other heads are on and how big they are. Ie, hard to predict how the refrigerant flow will be distributed. You might sometimes get 1K BTU (or less) - but sometimes 6-9K BTU.

    This agrees with what I would expect from a multi-split (vs a VRF system where each head can modulate).

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