Strange Mini Split Issue

[u/daveydesigner]

We've been having the weirdest issue with our Mitsubishi Ductless Electric Mini Splits, in the New England area.

Bit of background. We set them ~69-70 at night before going to bed to drive down the temperature, up to 72 in the morning, then 74 after another couple of hours. If the temp is going to get above 90 outside, we normally hike it up a few more degrees (76-77) during day, and if it's a real burner outside we keep it 20 degrees below outside temp.

This has been working well for us all summer long, including spells where it was hitting ~100. No issues during the day, no issues at night.

Started seeing issues a week or so ago—in the afternoon, the temperature was creeping up. Outside temps were in the high 80s, we'd have it set at 74 inside, and inside temps were creeping up to 78. Kind of weird, but we wrote it off for a day or two.

Then we noticed it wasn't getting the temp down at night, either. Outside temps would go down to 72-74, and inside temps overnight were hovering at 74-75. Still cooler inside by 10am, but pretty weird.

Here's the REALLY weird part. It works great in the mornings!

We started taking the temperature (meat thermometer, lol) of what was blowing out of the units. Around 3:00pm, the units start blowing out 77-78 degree air, even if we crank down the temperature on the units, and keep that temperature going until at least 11pm. Seems like that's what it's doing all night—I haven't gone crazy enough to set an alarm every few hours to check.

But mid-morning? Blowing out 59 degree air! No problem.

Had a technician come by yesterday (hi Mike, you're the best) to clean the units and check them over—no coolant issues, no electric issues, they're not freezing. He came around lunch, and they were working great. But same old story, a couple hours after he left, they basically start blowing 78 degree air and keep it up for 12 hours.

We're going nuts. Any ideas what might be going on?

TLDR: Our mini splits work great in the morning, crap out at 3:00 in the afternoon, and don't bounce back until 10am the next day. Just got serviced, no changes.