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I have two small rooms (about 15 feet apart) - one 8 by 10 (8 ft ceiling) and the other 11 by 15 with an 8 ft ceiling that I want to cool with a mini-split system. Both are reasonable well insulated (R 14+ or more). The smaller room is for a small hot tub, the larger is for my shop office in which I plan to home brew beer and wine. So 80 sq.ft. and 165 sq. ft. respectively. I can not consider how even a 9000 BTU exterior unit would be overloaded by two inside units. I have a automotive hvac license - will take the household if I need to. I ask your opinions please on adding a second interior unit to cool both rooms!

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    How much air circulation is there between the two rooms? I ask because while multi-split systems exist, the indoor units for them only go down to 7k BTU or so at the smallest... – ThreePhaseEel Aug 24 '18 at 2:07
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    Also, where are you on this planet? (Need to know for climate reasons) – ThreePhaseEel Aug 24 '18 at 2:07
  • I am in Arkansas (lots of humidity and heat in the summer). The rooms are about 20 feet apart and in separate buildings, so there is no circulation between the rooms. – Clyde in TSAR Aug 24 '18 at 12:40
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Multi-zone outdoor units are a thing...

There is such a thing as a multi-zone mini-split heat-pump system -- most of the major mini-split makers make them, with options ranging from 2 to 4 or even 8 zones.

however, they may not come in a small enough size, so you may be better off with two separate systems for the separate buildings

The downside is that the smallest multi-split outdoor units are generally in the 1.5ton (aka 18kBTU) range, with the matching indoor units going down to 0.5ton (aka 6kBTU), and the need to have no capacity "left over" in the system that's not able to be used by an indoor unit somewhere as a design constraint.

This, along with the distances involved, mean that you may be better off with two small through-the-wall or packaged terminal heat pumps (6-7kBTU each -- Amana has some 7kBTU, 230V units for instance) instead of trying to make a single multi-split do all the work for both buildings.

  • Don't some of these systems have the ability to modulate their power down quite a bit? Like 2-stage cooling systems, only more advanced because they use fancy proprietary controls between the outdoor unit and the indoor ones. If that's the case, maybe they're stuck buying more capacity than they need but it wouldn't necessarily hurt efficiency or longevity of the system. – Shimon Rura Aug 25 '18 at 1:30
  • @ShimonRura -- they do have modulation capability, but it's limited to about a 2.5 to 1 turndown AIUI. I suspect the real trouble would come during defrost though, where the compressor would be forced to full speed... – ThreePhaseEel Aug 25 '18 at 1:46
  • could I combine 2 6000 BTU interior units with a 9000 or 12000 BTU exterior unit? please remember that this is for a 80 Sq ft and a 165 Sq ft room - total of 245 Sq ft. Heating will not be a problem a small hot tub and a 2 door counter height cooler will provide all the heat I will need. I know that a couple of small window units would be easier, but I have challenges related to an 11 in thick wall and a cedar paneled room to consider. – Clyde in TSAR Aug 30 '18 at 16:20
  • @ClydeinTSAR -- have you looked at PTAC or TTW gear? (wall sleeve vs a true window unit) as I have not yet seen a multi-zone capable 9k or 12k BTU exterior unit and I suspect PTACs would be more likely to be set up for thick walls like yours – ThreePhaseEel Aug 30 '18 at 23:02
  • To ThreePhaseEel (and yes, I am familiar with Three Phase - My father worked for a major electric utility for many years ;unfortunately, he died about 20 years ago, so I am missing my mentor.]) I blanked out on the abbreviation PTAC. I have considered TTW for one area - but am unsure about the one with the 11 inch thick wall. Would the window unit be extended back so the front of the unit would be recessed – Clyde in TSAR Sep 1 '18 at 2:20

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