My Mitsubishi system was installed (Diamond certified) last year in the Bay Area:

* MXZ-SM48NAM
* MFZ-KJ09NA
* SVZ-KP36NA
* Daikin One Premium Air Cleaner MERV 15

The mini split is in an office that’s separate from the main building, and only conditioned during some weekdays and only during the day. 48K was also meant to have room for a future mini split in a furnished basement.

Like https://www.greenbuildingadvisor.com/question/mitsubishi-ducted-heat-pump-short-cycling the behavior I’m seeing zero modulation. The system always ramps to a flat 2000w (measured with Emporia Vue) with zero modulation. I’m assuming this is from the MXZ-SM48NAM enormous minimum capacity of 27k btu (2000W seems to match 27k BTU in: https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006%5CDB_M-P0855_MXZ-SM_NAM(HZ)-U1_JA22.pdf, page 70) 🙁

Besides inefficiency, two main issues:

The lack of modulation and relatively long run times (I’m assuming this is to prevent extreme short cycling) results in indoor temperatures that swing at least 2 degrees, and are inconsistent based on outdoor temperature (as in if it’s cold outside it will swing less which means a lower average indoor temperate), which is bad for comfort. If I configure the fan to be always on it generally feels more comfortable, but right now it’s too loud, especially around the return duct, to be workable for me.

But also on a rare relatively cold morning of 35 degrees the system struggles to go above an indoor temperature of 65 degrees (still pulling a flat 2000W). Why is my 36K air handler not able to draw more heat? Is it because airflow is too low preventing it from taking on more heat without overheating? My return duct grill is 30X10 and I have the MERV 15 filter. Could it also be I don’t have enough supply?

I understand short cycling is bad for efficiency, but right now I’m prioritizing comfort.

Since the system was installed I’ve learned about issues with oversizing and multi-zones in particular, but looking to make the most out of the situation.

My conclusions:

* Multi zone is a horrible fit given that I’m mostly not running the mini split, which results in a severely oversized outdoor unit that can’t modulate down at all.
* The air handler is also somewhat oversized given that 2000W (27k?) is almost enough to comfortably heat the space. Should be 30K? Or even 24K  with heat strips (so far I’ve only seen 2-3 days this year where it struggled)?
* The air handler is undersupplied causing more noise and inability to heat on coldest days.

Does this sound plausible? Besides replacing the multi zone with dedicated outdoor units (even a smaller 36k multi-zone has minimum capacity 21K?), my only course of action then is to try to increase the supply and see if that makes the system less noisy which allows me to run it continuously, and also allows it to heat during the coldest days. Any other adjustments that wouldn’t amount to starting from scratch?

Appreciate any pointers that would help me make a case to my installer. As mentioned the installer is supposed to be Diamond certified…