Mitsubishi Hyper Heat overshooting temp
Hi. Home owner here in IECC zone 4C.
We have a garage conversion that we recently “upgraded.” Room is 26×25 with a 9′ ceiling. It was originally tied into the main house HVAC but the performance was poor, so we had that disconnected and in its place:
– Insulated the attic space (added 6″ rigid foam with 1″ air gap for vented roof assembly).
– Installed a Mitsubishi SUZ-KA15NAHZ Hyper Heat *ducted* minisplit.
– Installed a Broan HRV
– Installed an Aprilaire air filter
The ducts, HRV, and Aprilaire are all within the envelope. We have the wired remote controller: PAR-40MAA
Our problem is, the room is often too hot (we’re in the PNW, so temps are cool outside now). The room temp is often 3-4 degree F higher than the thermostat setting and seems erratic — it will be stable for a while and then suddenly begin climbing way above the set point.
That wouldn’t be too bad, if we could compensate for the difference… but apparently these minisplits cannot be set below 63 degree F.
What we have tried:
– The wired thermostat is on an external wall (insulated with fiberglass batts, but likely not well). We removed the thermostat and discovered a couple of holes made during installation that were not patched. Patched them but no difference seen.
– We have actually tried *two* different mitsubishi minisplits, since the company installed the wrong one the first time (it was not the hyper heat model). Both units have exhibited this same behavior.
– Apparently there is a 4-degree offset built into these units! I found this PDF for removing the offset:
Discovering that was very encouraging and could explain everything. But I’ve made the above changes and we’re not seeming any difference.
So, my question is: any ideas on how to troubleshoot this further? Is there something else I need to change on the minisplit itself to honor the removal of that 4-degree offset?
Is there a different thermostat remote we should try? I saw reference to an MHK1 remote… would it make sense to try using that rather than the wall control? Is that possible?
Any other ideas? The company that installed everything seems stumped.
Worst case, is there another minisplit brand that would likely not exhibit this issue?
Also of note, in *cooling* mode, we are not seeing this problem. Set to 67 and it cools and maintains right around 67.
We have an AcuRite sensor in the space to track the temp. Attaching some photos so you can see the temp spikes. There’s no obvious correlation between outside temp changes and when we see these spikes. The wall controller is set to 63 degree F in all cases.
I did find some earlier threads that sounded related, but not clear on resolution:
Any pointers would be greatly appreciated.
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