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Location: US, NYC. I want to improve the air conditioning setup for an apartment in a 2-family house. Current situation:

  • 200 sq ft master bedroom, horrible old through-the-wall AC on a dedicated 20A 120V circuit
  • two 80 sq ft bedrooms, currently without air conditioning, each have dedicated 20A 120V circuits for AC, but the window shape is unfriendly to normal window ACs
  • sitting/eating area connected to kitchen and hallway, around 350 sq ft total, horrible old through-the-wall AC on a dedicated 20A 120V circuit
  • basement containing washer/dryer on a 240 V circuit.

My idea was for a multizone mini-split AC with a single outside condenser and 4 indoor units - one in each bedroom, and one in the sitting area. I realized I would need 240 V for the compressor, but thought that could be gotten from the basement. However, after looking at the specs for models on the market, I see what looks like two problems:

  • the multizone indoor units for which I was able to find specs seem overpowered for the two small bedrooms: for example, Mitsubishi doesn't advertise any indoor units under 6000 BTU, while various online calculators suggest 4000 BTU for such small rooms
  • the multizone mini-split systems for which I have been able to find specs seem to require 240 V for indoor units, which I think implies a very expensive and very disruptive wiring job.

Is my assessment of the situation correct? What is the right way to set up a mini-split AC for a home with small bedrooms? Is a separate AC per bedroom the better way to go? (Alas, four external compressors lined up on one exterior wall would look quite ugly.)

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  • The wiring might not be as expensive as you might think. If you are in a conduit area (which applies to much of NYC) then adding/replacing wires where there are already circuits is not necessarily so hard/expensive to do. If you are in a cable area (most places outside NYC...) then if you have a 240V unit that does not require a neutral, you can reuse and existing 120V cable by changing the receptacle and changing the breaker (provided you have room in your panel for a double breaker to replace the single breaker). Commented Mar 28, 2022 at 3:04
  • The 240 is not hard to install but with an outside unit you will also need a 120v service receptacle. The nice thing about multi head mini splits is that if you put a little larger head in the room than it requires it will get to temp and is not as wasteful as a single units would be. So having a 240 service to the outside with a disconnect and a service receptacle is not that big of a job. Having the multi head compressor is only 1 disconnect, having 4 compressors is going to be more wiring and 3 more disconnects.
    – Ed Beal
    Commented Mar 28, 2022 at 4:45
  • It strikes me that running the line sets containing the hot & cold refrigerant will be much more disruptive than dealing with the wiring.
    – FreeMan
    Commented Mar 28, 2022 at 12:32
  • 240v from basement? Are you suggesting tapping into the circuit that is provided for the dryer, which would be an violate UL and NEC if the dryer instructions call out a dedicated circuit? Commented Mar 28, 2022 at 15:19
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    The units I have (which are Mitsubishi) the indoor units are entirely powered from the outdoor unit, via a cable that parallels the refrigerant lines, so the 240V I supply is only going to the outdoor unit. They may well use 240V, but it's supplied on the cable from the outdoor unit that's part of the install process, NOT from within the house. I think this is common/normal, but I KNOW it's how mine work.
    – Ecnerwal
    Commented Mar 29, 2022 at 0:24

2 Answers 2

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There are several types of indoor units that can be attached to the outdoor unit: Cassette, wall mounted, ceiling mounted, duct ready. If the two small bedrooms are next to each other, an indoor unit that is duct friendly could be used to supply both rooms from one unit. Unless the layout precludes running refrigerant lines, you could easily get by with one outdoor unit.

Running a 240v line is no more expensive than running a 120v line. Since the existing A/C circuits are dedicated, you may be able to re-purpose them at 240v (with some wiring changes in the main panel), if the outdoor unit amperage requirements are under 20 amps. (Ed: does this need to be derated as a continuous load?)

Anyway, if an electrician says running a 240v line is more expensive than a 120v line, get a different electrician.

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    it's more expensive if it's a 240 run with a neutral, but that's only material cost, the labor cost increase is negligible. Commented Mar 28, 2022 at 8:19
  • Not continuous. Maximum overcurrent or fuse size needs to be provided per A/C label, so a 20A for branch circuit breaker and wire might require lower specific type protection at the disconnect. The conductors are not even subject to small conductor derating of 240.6, but ultimately the limiting factor is probably the 60°C limitation for NM cable. Commented Mar 28, 2022 at 15:04
  • Economic reality MAY suggest more than just the two bedrooms sharing a ducted interior unit. I was a bit taken aback at the fact that the difference in installed cost per unit was utterly negligible on capacity, at least from ~6000-24,000 BTU or so, but putting in 4 6000 heads cost a huge amount more than one 24,000 head would, at least when/where I had mine done. I ended up with 2 12,000 units (and two outside units, because they are more efficient that way, and also for 1-unit failure protection) rather than the fleet of 6000's I expected but found too expensive.
    – Ecnerwal
    Commented Mar 29, 2022 at 0:32
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12K BTU might be enough, 24K BTU might be too much. The answer depends on how well your exterior walls are insulated and how much sun exposure those walls have. The primary heat gain will be through windows. You probably have a humidity issue as well. Oversizing the ductless units will reduce their ability to control/remove humidity.

Given that cost is always an issue and based upon the comments to date, here is what I would install.

One ductless unit in the main living area and a high-quality portable dehumidifier. I would then install through-wall fans in the two small bedrooms. Make sure the doors are undercut enough to allow sufficient air to flow back out of those rooms or add jumper ducts. Provided that there is sufficient airflow between rooms and minimal sun exposure, that should be enough. Remember, heat moves and that is hard to prevent which is to your advantage. Interior walls have a low R-value so heat will flow through the walls easily.

If your budget allows, add another ductless unit in the master bedroom, but split the BTU load so that you are not oversizing. Another concept is that two correctly sized units can be a backup for each other. If one requires service the other can continue to maintain comfort. Two units can also provide what I call party load. This is when you have guests over or there is an extreme heat wave.

Order the portable dehumidifier now and monitor how much that changes room comfort while waiting for the ductless installation.

I would not worry about the electrical installation. The power requirements for ductless mini splits are low. The electrician can easily handle the installation provided there is a route to install the wiring to the outside.

Make sure that the units you are installing provide heat as well as cooling. The efficiency of these units will reduce your heating bills during the winter. Look for units that have a COP of 3 or higher.

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  • What makes this superior to using a low-static "pancake" ducted indoor unit? Commented Mar 29, 2022 at 23:20
  • @ThreePhaseEel - Low-static is better. However, the OP stated ceiling drywall are joists which run perpendicular to any potential route between bedrooms. Given that he cannot, or does not want to cut the joists, this is a reasonable workaround. Commented Mar 29, 2022 at 23:33

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