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I have a 1700 sq ft basement on Long Island, NY. It is a less than 1 year old condo with a finished basement. The initial gas heating was supposed to be a separate HVAC zone, but with only one zone control, the builder tried to balance the main floor heat with the basement. It did not work and the builder's second attempt was a 12000BTU electric split unit on a 15 amp circuit hanging on a wall - but that didn't heat more than 2 degrees in winter, upon from 62 degrees. NOW they want to replace it with a 18000BTU unit on that 15 amp circuit. Is that BTU enough andis the 15 amp circuit sufficient?

  • What does the name plate on the unit say about amperage?? What is the model number? – JACK Nov 5 '19 at 22:15
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    I don't think an 18000 BTU unit is even going to come close to heating up that basement, probably twice that amount. The 15 Amp circuit will not be large enough. – JACK Nov 5 '19 at 22:58
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    What brand is the unit? Also the seer rating. Below 19 seer I doubt it could run on a 15 a . I would think 19k is a bit small for that size area and your cold winter location. Several high end units can get Down into the negative temps but not many, most start loosing efficiency at~ 20f so this can be a reason to go bigger. I would contact a local hvac contractor, tell them the size 1700sf basement and you want to heat it with a mini split. They know the area they work there I would bet they recommend at least a 24k and 2 heads (inside units). I agree with jack on this. – Ed Beal Nov 5 '19 at 23:26
  • Have you or your builder run Manual J and S calculations for system sizing, or is everyone going off of "rules of thumb" here? – ThreePhaseEel Nov 5 '19 at 23:56
  • It sounds like a builder and not an hvac contractor, or I would have suggested the calculations. I could tell you that a 24k unit at 23 seer would work in my area but we do not get as cold as N.Y. and we really don’t have enough info yet also. – Ed Beal Nov 6 '19 at 0:09
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18000 BTU = 5200W 5200W @240V= 21.67A

So, no, a 15A circuit is not sufficient. But, I've never heard of or seen a 15A/240V breaker. Seems to me they need would need to install a 30A/240V breaker and 10AWG copper circuit for an 18kBTU system... BUT, it is worse than that!

18kBTU isn't large enough for 1700 sq.ft. This site suggests 24kBTU for 1500 sq ft. (https://www.sogoodtobuy.com/blog/sizing-guide-for-a-mini-split-air-conditioner/). Seems like your builder has, and continues, to grossly under size the system for your needs.

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    I know one of the brands I have installed 19k heating, 18k cooling requires 15 amps minimum but recommended size is 20. It depends on the efficiency of the unit. As far as a 1700 sf I would think that is a bit small even 24k may be pushing it for a basement once the temps drop into the 20’s and lower a larger unit is needed or it will spend a large amount of time in defrost. – Ed Beal Nov 5 '19 at 23:18
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    Eh, 15A 2pole breakers are quite common (baseboard heat circuits, well pumps, and MWBCs, for three examples) – ThreePhaseEel Nov 5 '19 at 23:55
  • Wow. I have never seen one at my local HD or Lowes. You learn something everyday here. Good to know. Thanks! – peinal Nov 6 '19 at 0:00
  • I did not catch that part when I read it+ – Ed Beal Nov 6 '19 at 0:11
  • "18000 BTU = 5200W" -- huh? BTU and W aren't even the same units (one is energy, the other is power). Also, this is a heat pump system, so the heat comes from the environment, not the system itself. The system only needs enough power to move the heat, not to produce it. I don't see how the calculation provided in any way addresses the question. – Peter Duniho Nov 6 '19 at 0:34

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