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A split conventional AC needs to be replaced due to age. The current AC is 48K BTU (4-ton).

If replaced with an inverter heat pump, should it be sized with a 48K BTU unit? Or should it be sized with a smaller BTU unit given its ability to throttle the compressor speed?

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    I am not sure, but I think the BTUs stay the same while the efficiently of the unit increases. This is if the BTUs were size right the first time for the old unit, too big or too small will change things.
    – crip659
    Commented Mar 1, 2023 at 13:28
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    did you do any insulating since the heatload for the old unit was determined? if so you may wish to redo a heatload calculation and see if you can downsize the unit.
    – ratchet freak
    Commented Mar 1, 2023 at 13:57
  • Are you replacing with a split unit or mini-splits? There's a bit of nuance I can add to Ecnerwal's answer depending on which it is.
    – KMJ
    Commented Mar 1, 2023 at 17:46
  • Heat-load calculations are a black art, because they don't (can't) do a good job accounting for your #1 source of heat - solar gain. it's not the hot air that gets you, it's the sun. Build 2 identical houses, one shaded by tall trees the other in open sun, and they'll have dramatically different A/C needs. Experience is your best advisor: what duty cycle does your A/C currently run on the hottest day of the year? Does it run rarely or almost continuously?
    – Harper - Reinstate Monica
    Commented Mar 1, 2023 at 20:47
  • @KMJ There is very old split unit: not a mini-splits.
    – gatorback
    Commented Mar 2, 2023 at 22:17

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"The ability to throttle the compressor speed" typically (in a good unit) means it can run from rated capacity to about 1/4 rated capacity (so 48K-12K in this case) and below 12K it will turn off and on (which is less efficient than just running slower.) It does not mean that a 36K BTU unit can do 48K when needed. But the 36K unit can throttle down to 9K, typically.

IF the 48K BTU unit is right-sized, then 48K to replace it is right. That's a big if, given that oversizing of units is both common and typical (more money for the guy selling it, no complaints about it not doing the job, but more running cost, which you pay and they don't. Also has a negative impact on dehumidification.)

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  • Throttling the compressor speed is mainly relevant for a NON-"bang-bang" thermostatic control. The idea is that instead of running 48K BTU 33% of the time, they'll run 16K BTU continuously - quieter, more efficient and easier on the equipment.
    – Harper - Reinstate Monica
    Commented Mar 1, 2023 at 20:48
  • The area is 1832 sq. feet. I read that the rule of thumb is 20 BTU per square foot or 36000 BTU should do the job. This section of the building is for storage, so keeping the humidity down efficiently is the priority. Maybe once a day someone enters the area to add / remove items.
    – gatorback
    Commented Mar 2, 2023 at 22:23
  • Given that comment and your other one, go with the smallest unit your contractor is comfortable installing. You might end up with extra runtime (no big deal) and worse temperature control (i.e. it getting too hot on the hottest days). You will always have great dehumidification though.
    – KMJ
    Commented Mar 3, 2023 at 0:01

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