Mini Split question.

[Dave Zellers]

A year ago we had a Fujitsu mini split installed. One outside unit that drives four indoor units (heads?)- 3 bedrooms upstairs and a larger one that does the downstairs. Couldn't be happier, it's fantastic.

My question is regarding the daily operation. The kids are gone so 2 of the bedrooms are off except when they visit. This question is about the daily operation of the one upstairs unit (our bedroom) and the one downstairs unit (the entire first floor) Our installer (who is fantastic and I trust completely) says it is more efficient to leave them both on all the time even if you were to turn up the temp when you are not using the room as would be the case when you leave the bedroom in the morning or leave the downstairs to go to bed in the evening.

For me, the whole point of separate zones, is to turn them off until you need them. I'm trying to wrap my head around any idea that leaving it on, even set to a higher temp (we are only talking about summer A/C operation here) is somehow more efficient. He also mentioned that moisture builds up in the unit and letting it run helps by drying that moisture out.

I ask because I know many here have mini splits installed and I'm simply looking for the logic involved re letting it run vs only turning it on when you are in that room and need it. Don't forget- we will crank up the temp when we are not in the rooms- specifically, the bedroom during the day, and the downstairs at night.

Thanks for reading, numerous edits were made to try to make this simple yet specific.

[Bill Dufour]

My thoughts must mention our climate. AC time of year here has no rain, ever, and humidity is not an issue. We turn ours off when no one is in the room. It is a sealed system how can moisture get into it?
In your humid climate you may get condensation and mold from natural humidity in the unit and in the furnishings I suppose. Do non ac house get mold in summer?
Bill D

[Michael Weber]

If the rooms get really hot, wouldn’t that heat radiate to the rest of the space through uninsulated interior walls even if closed off? It’s going to add load to the remaining units still running. A wall or ceiling just a few degrees warmer than the interior radiates a massive amount of BTU’s because of the large area. Personally I would leave them set a few degrees higher but leave them running. A happy medium. Those mini splits are remarkably efficient. As mentioned, humidity may be an issue especially if the unused rooms are closed off.

[Jim Becker]

These things are so efficient, leaving the units "on" but with a higher (or lower, depending on the season) temperature setting keeps things more even and as Michael mentions, helps insure that radiation is lessened between the conditioned space being used and the conditioned space not being used. They are not going to actually "run" all that much to maintain temperature under normal operating conditions. This is a personal opinion, however...

[Aaron Rosenthal]

I'm involved in purchasing 2 mini-split systems for my religious institution. The installer tells me that they are even more efficient in the cooling and heating cycles than even the boilers @20+% efficiencies.
I assume the installer is looking at the huge heat build-up when the system is off, and all he refrigeration work load when you try and get it down to a comfortable level when you decide it's needed.
What does your local energy supplier say? They have a whole department which analyzes energy usages of various appliances.

[glenn bradley]

As mentioned it’s more efficient to maintain a temperature range than to push things from warm to cool or vice versa.

[roger wiegand]

Heat transfer is a function of deltaT, the difference in temperatures between the warmer and cooler object, pretty directly for conduction and radiation, in a more complicated way for convection. (See Newton's Law of Cooling which reasonably approximates what's going on, it gets much more complicated from there.) This means if you keep some or all of your house cooler it will absorb more heat from the outside, in proportion to the temperature difference. This will, in turn take more energy in cooling to run the AC. Any way you reduce the temperature difference will lower the cooling load. So turning the AC down or off in part of the building will result in greater overall efficiency.

I've often hear the argument that setbacks don't save energy because it somehow takes more energy to cool from a higher temperature than it does to maintain a low one. I think this reflects a fundamental lack of understanding of thermodynamics. While the building (or room) is at a high temperature it will have absorbed less heat overall from the outside, and therefore take less energy overall to move that heat back out again.

That saving may well be minute, depending on the extent and timing of the setback. A couple degrees for a couple hours doesn't do much, and there may be a comfort factor in the amount it times to bring a hot room down from a high temperature.

As a practical matter, we can easily see the difference in energy usage when our upstairs rooms are occupied and the AC is turned on up there, vs the typical situation where the AC is turned off and the upstairs of the house is 15-20 degrees warmer than our downstairs living space.

[Adam Herman]

we have a 3 head Mitsu system and leave the unoccupied rooms off. if the shades are closed and you are not getting too much solar gain, I would think leaving them off would be more efficient.

[Dave Zellers]

Appreciate all the replies. Conflicting opinions, yet they all make sense!

I think when it is very hot outside (high 80's today) it would make sense to leave the bedroom unit on during the day and turn the setting up to 80º. That's what I did today and I could see that the drain tube outside was dripping so it is at least 80º up there and also good that it is controlling the humidity.

[Adam Herman]

Appreciate all the replies. Conflicting opinions, yet they all make sense!

I think when it is very hot outside (high 80's today) it would make sense to leave the bedroom unit on during the day and turn the setting up to 80º. That's what I did today and I could see that the drain tube outside was dripping so it is at least 80º up there and also good that it is controlling the humidity.

fair. we run a humidifier here year round. haha. only time it does not run is when its raining. and i have it set to 35%.

[Bill Dufour]

80 is very hot to you? I wish summer here was only 80. All last summer from mid june onward it was about 95-100 everyday. Looks to be similar this summer. It was 115 last week.
Bill D

[Dave Zellers]

80 is very hot to you? I wish summer here was only 80. All last summer from mid june onward it was about 95-100 everyday. Looks to be similar this summer. It was 115 last week.
Bill D

Well, first, I did say high 80's by the way.

Second, no one is forcing you to live there.

Third, I already know you live there because you love it and are just having fun. As am I. Party on.

But we do have something you don't have- Humidity! UGH. No one is forcing me to live here either and you would have to drag me away kicking and screaming.

I actually love working outside in the summer, building up a good sweat, and then showering and relaxing either inside with the A/C, or on the deck with the sun setting and the air cooling rapidly.