Struggling with MHK2 thermostat install on Mitsubishi Minisplits

papbct

I have Mitsubishi Minisplits. There are three indoor units (2x MSZ-FH09NA and 1x MSZ-FH12NA) connected to my outdoor unit (MXZ-3C30NA2).

I have been trying to get the MHK2 thermostat installed so I can disable the fan when room hits set point ("Fan speed thermo off"). As far as I can tell, my hardware is compatible and I should have access to these advanced menu options.

I power off the system and connect the Redlink Receiver to one of the head units inside. I then link the MHK2 thermostat and it operates as expected. However, I cannot access the Function Codes menu. All I get are the basic install codes menu — even after 5s press of menu and entering the stat's PIN.

Everything seems to suggest there is a communication issue between the stat and the indoor unit, preventing the loading of the advanced function codes. ive tried multiple wires and power cycles without success

Any ideas? Thanks.

GGross

I'm not a mitsubishi guy but their website doesn't include the MSZ-FH models as compatible units?

https://www.mitsubishicomfort.com/products/c-series/mhk2

Kaos

Looks through the app note here. On some units you have to snip parts on the circuit board inside the head.

https://www.mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006%5CApplication%20Note%203048%20ME%20-%20How%20to%20turn%20off%20indoor%20unit%20fan%20when%20set%20point%20is%20met.pdf

papbct

Thanks guys. I definitely misread some of the compatibility charts and also wrongly trusted AI about how my system operates with the MHK2 and the Kumo wifi adapter (Ive also added the wifi adapters on my systems). It doesn't help that this also came at a time not to long after they rolled out the comfort app, which appears to have less functionality than the old Kumo app (and all guides/resources still reference the Kumo app). The short of it is, I wasted a lot of time hunting around for advanced settings that my system is not meant to display in the comfort app nor on the MHK2 thermostat. ugh!

Shortly after my post, I ended up getting a response to an email I sent Mitsubishi support. They said my unit has a physical jumper, not function codes/advanced settings in the app or on the MHK2 (which is what you both suggested). I then found Application Note 3048 that Kaos linked above, which confirms it — for my unit you have to cut the jumper to stop the fan.

Thanks for the help all.

GW

the jumper is teensy. Do you know which one to cut?

pecmsg

That fan should run 24/7. When not calling for heat, you should barely feel the air flow. When it gets colder out it will ramp down and still supply heat.

How were these units sized?

GW

small rooms, they are oversized 10 times out of 10. Mini units/ multi zone systems can only turn down so much. The add in the hot gas (multi zone is always moving ref), you have overheating

papbct

Its oversized. Its a brand new condo that was finished before I bought it. The issue is actually the residual moisture and increase in RH on the AC side.

Its ~1000 sqft. Two bedrooms, each with a MSZ-FH06NA in them, and then a MSZ-FH12NA unit covering the kitchen/livingroom. They all connect back to the same outdoor unit, which I think modulates down to 8,000 btu at its lowest (if i remember correctly). The smaller bedroom is ~90sqft. So oversized.

The issue is — when the small bedroom gets to temp when cooling, the compressor stops but the fan keeps going. Moisture collected on the evaporator coil just blows back into the room — RH goes up to ~70% (essentially no moisture is removed and, possibly some additional moisture is collected and release into the room from the adjacent space). Assuming my theory here is correct, I figured I can run a dehumidifier (which probably makes the most sense but I'd prefer not to) or I can enable thermo fan stop on the head unit and use a remote temp probe or the MHK2 thermostat to report temperature for the unit.

I've identified the jumper from the application note and service drawings and will probably take closer look this weekend at the board.

Thanks for the help — I am open to advice or other theories/recommendations if mine above don't make sense!

pecmsg

that’s oversized. Not much you can do at this point.

Kaos

The equipment is what it is, enabling fan off is your best option. You can also look at restricting the airflow which will reduce cooling and improve SHR, these units are smart enough not to freeze the coil. I use a couple of magazines on top to partially block the intake, essentially run it like it had a very clogged filter.

I have to deal with a similar oversize issue with a multisplit and I rarely run the small heads in the cooling season. Crank the larger living room head at night to overcool and dehumidify the place.