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Most efficient way to run Mini Split ACs

kjmass1 | Posted in General Questions on

I’m currently cooling ~1900 sq feet home, 2 floor Colonial zone 5, with an AOU24RLXFZ hooked up to a 12K wallmount @ top of open stairway, and a 7K in the master BR which is 1/2 of the second floor. I’m very impressed with the cooling abilities so far as Boston has seen some days close to 90. I keep the stairway unit at 71 (1 degree offset in function settings), and BR is at 72 in economy mode, but on low fan so it is always running quietly (fan makes annoying ticking noise in auto fan mode). Entire home +/- 1.5 degrees of 71.

With days in the 80s, the condenser has been cycling about 6.5 minutes on, 1.5 min off. I also have another 12K attached to a second AOU24RLXFZ that can serve the entire open first floor if needed (used more for heating), as well as a 12k in a finished attic room.

My question is are there any efficiency gains to be had by running the second condenser for the first floor, for either humidity control or for keeping the second floor unit more in the sweet spot of modulation? These units aren’t the most efficient and I would certainly change some things if I did it over again.

Any reason to keep the second floor unit on low fan vs auto? I will say it doesn’t seem like the unit has to try very hard, it’s not like it’s blasting on full fan trying to cool the house down, although we’ll see how this set up does for a week in the 90s.

House has updated windows, sprayfoam in attic and large shady oak trees. I had come up with 30k BTU load on coolcalc but I’ve read that can be up to 30% high.

At roughly 1200 sqft per ton that seems pretty ambitious for this to continue on throughout the summer, but was curious if anyone had any insights.


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  1. Expert Member
    Dana Dorsett | | #1

    In general running it at low speed rather than auto will deliver higher latent load control (humidity) but a somewhat lower sensible load efficiency (temperature). But if it's running at low speed even when the compressor isn't running it puts some of that moisture back into the room. Auto mode would be more efficient overall, but if it's too annoying to sleep...

    I'm not sure what you mean by "second condenser". The term "condenser" in the air conditioning world refers to the outdoor unit that has the "condensing coil" and compressor. With a heat pump that's only a condensing coil when in air conditioning mode- when it's in heating mode it's an evaporator coil. So you are saying that you have two AOU24RLXFZ's? (That's how it reads to me.)

    Is there a third 7K head on the compressor running the MBR and head at the top of the stairs? (If so, where?)

    Just one 24RLXFZ is keeping up with the cooling load (including humidity), keep the other completely off (as in "turn off the breaker panel or switch", not just "OFF" mode using the remote) . It's more efficient to have just one compressor cycling on/off at high duty cycle than having two cycling at a low duty cycle.

    The AOU24RLXFZ units are not fully modulating with load, but will step up or down in response to how many zones are calling for coolant. With the upstairs head providing the bulk of the cooling, the duty cycle on the downstairs heads will be very very low, and if they are on a second compressor, that compressor will have a correspondingly low duty cycle (= very low average efficiency).

    If you need more humidity control when it's sticky outside, operate the 12K head at the top of the stairs in "DRY" mode.

    A ratio of a ton per 1200' isn't very ambitious at all for a 1900' house, in fact it's in the middle range:

  2. kjmass1 | | #2

    Thanks Dana, I appreciate the insight. Yes, there is a second AOU24RLXFZ compressor on the other side of the house. There is a 12k on each floor (3), plus the 7k in the master. Locations of heads were on complete opposite sides of the house so 2 condensers were proposed instead of running linesets all over. In hindsight I could have easily stepped down to 2x AOU18's instead as the attic could get by with a 7k. Or, 1 high efficiency unit for first floor heating, and put the other 3 on one unit really just for AC...lessons learned.

    I'll keep the first floor unit off until I need it, and try to just run the 12k by itself and see how that works. The swing mode has worked great as it drops it right down the stairwell, and it's impressive how everything just dissipates around even with a back room addition.

    Side note- how do I turn back on email notification for posts?


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