Mitsubishi ducted/ductless system operation

Rickoo

Need opinions on limitations of a Mitsubishi ducted/ductless system. I’ll try and make a looong story as short as possible.

Back in the spring of 2021, I contracted a Mitsubishi elite diamond dealer to install A/C in my 23 year old colonial house about 2300sf. First floor has a family room, living and dining rooms, kitchen and a half bath. Two story open foyer leading to the second floor that has three bedrooms, two full baths and a laundry room. I also have about 4-450sf of unfinished space above our garage that will be a 4th bedroom when it is completed. I wanted the whole second floor ducted (including the unfinished area in preparation for completion.) The first floor was to be cooled with a wall unit. It’s a pretty open floor plan.

First try:
Outdoor unit: MXZ-4C36NA2
First floor wall unit: MSZ-GL09NA
Second floor air handler (located in an unconditioned attic.): SVZ-KP24NA

This system just did not work. The outdoor unit was connected to the first floor wall unit and attic air handler with separate line sets direct from the outdoor unit. As soon as you turned the first floor wall unit on, the temperature would quickly rise on the second floor. And the section of ducting for the 450sf unfinished space was not even yet connected/operational. After many MONTHS of screwing with this, the contractor brought a Mitsubishi rep in. It was determined that the system was too small and was not configured properly. They did another heat loss and reconfigured things.

This year…..

Second try:
Outdoor unit: MXZ-SM48NAMHZ-U1
First floor wall unit: MSZ-FS18NA
Second floor air handler stayed and was reused. SVZ-KP24NA
A separate PEAD-A15AA air handler was also added in the attic: (to condition the future unfinished space over the garage with its own zone.) This unit is installed but not yet operational.

A branch box was also installed in the attic. The outdoor unit connects to it and from there the box connects to all three evaporators.

This setup has been operating since around May of this year. I have MHK2 thermostats on both first and second floors with wifi Kumo cloud integration. On a day in the mid to upper 80s, with both the first floor wall unit and the second floor main air handler operating, and the thermostats set on 70, by 5pm or so, the temperatures at both floors will rise to 74 or 75. The first floor unit seems to discharge air in the 50s all the time. I’m more concerned with the second floor unit.

I’ve checked supply and return temps at the second floor and they seem to vary like crazy.

At times, I’ll have 63 degree supply temp with an 8 degree delta-t between supply and return. At other times, I’ll have supply temps in the mid 50s with a 15 degree delta-t.
My contractor saw this happening, was concerned and brought the Mitsubishi rep back in. When he came, the thermostats were at 72 and it was 73 in the house. He saw this, thought the system was operating properly and offered no help. I’m not sure he believed me when I told him what was happening.

Does this seem proper to you folks? I mean the house is relatively comfortable, but when I walk upstairs on a hot, late afternoon, you can feel the difference. Both in temperature and humidity. How can a 4 or 5 degree difference between the MHK2 set point and the room temperature with 63 degree air and an 8 degree delta-t be normal?

Let me just say that the contractor I’m dealing with has been terrific. They have been super responsive to these issues and have done everything they can think of to make this right. As much as I’ve grown to dislike this Mitsubishi equipment, I would recommend these folks to anyone. If I had to do it again, I would definitely NOT go with Mitsubishi. Or at least not more than one indoor unit connected to a single outdoor unit.

Things I’m happy with: This outdoor unit is super quiet. We have a deck and a patio area that is right next to the unit and we are not bothered by it in the least. The fact that we can use this system to supplement our oil heat in the winter is also huge.

Things I’m unhappy with: This system, as it stands, just cannot cool my house to my liking. I was hoping to cool this area to 70 or so when I wanted to. I cannot even cool it to 72 when it is hot.

If you’ve made it this long, thanks! Any opinions on if this system is working properly and if not, what could be done to make it better? Would there be any reason to maybe let a different contractor with a fresh set of eyes have a look at it?

Hot_water_fan

I have that air handler and a 9kbtu ductless head, but attached to separate outdoor units. One thing about the air handler is that it’s default setting I believe is constant circulation - so the fan should always be on. That could explain why sometimes the air leaving it is close to room temp. Have they checked that the thermostats are operating correctly? It could be those are malfunctioning and what you’re seeing is the constant circulation and thinking that it is trying (and failing) to cool. 

Rickoo

Thanks. They set the air handler fan to NOT run constantly. The wall unit does run all the time unless turned off.

Rickoo

Another maybe noteworthy condition. Right now, it's 83 degrees out. Pretty humid. Dew Point is 70. The RH on the second floor is 68%. 65% on the first floor. Seems high to me, but what do I know.

pecmsg

Minis are not good at dehumidification when oversized!

try Dry Mode.

Hot_water_fan

Another maybe noteworthy condition. Right now, it's 83 degrees out. Pretty humid. Dew Point is 70. The RH on the second floor is 68%. 65% on the first floor. Seems high to me, but what do I know.

That's high, I'm hotter with a 65 degree DP but substantially lower RH (40's) with the same air handler.

Rickoo

pecmsg said:

Minis are not good at dehumidification when oversized!

try Dry Mode.

Don't think this is oversized. Runs constantly and can't even maintain 72 degrees! Pretty discouraged with this system. Would be happy to pay someone for their time if anyone knows of a highly skilled guy in Central MA that could look at this system and see if they can figure out what the problem is.

pecmsg

Minis ramp up and down. 

At low RPM poor dehumidification