Help with mitsubishi heat pump

[tabner]

I had a mitsubishi hyper heat mini split system installed a couple months ago, and it's acting crazy. Hoping I can get some advice/help.
One outdoor unit, I think it's 36k btu, then three indoor heads. Two are I think 5k, and one is 15k.
Basically they are overshooting the heat set point like crazy. I had one this evening set to 61 all day, the room was 85. That's an extreme example, the other two are usually only 8 degrees off or so, but they'll be 8 degrees up and still dumping heat. It's worst at night and when the temp dips. I got up on a ladder today and sealed all the holes real well, where the units penetrate the walls. Still having issues.
The company is very good and they're going to come out again tomorrow to troubleshoot, but of course there are no error codes flashing, and it behaves much better during the day so they're not fully seeing the problems.
Any thoughts?

 

[stoveliker]

Do you have the wifi + cell app with it? If so, did you make a program/schedule that may be overriding the manual (remote control) setting?

Other than control system parts being broken (thermometer...), I was going to say that cold is leaking in. But you sealed the holes.
How much insulation did they push out of the way in your walls when they installed it? Can you feel with your hand on the drywall (and compare with 3 ft away - or have a IR cam)?

I admit not all likely causes, but ... it's all I can think of.

 

[semipro]

The discussion here may be helpful.
https://forum.heatinghelp.com/discussion/161533/mitsubishi-hyper-heat-minisplit-thermostat-questions
Based on a cursory glance it looks like using a remote thermostat rather than the one built into the wall unit may take care of your issues.

 

[tabner]

Thanks. Yeah, no wifi programming here, so I don't think that's it. They may have pushed around some insulation in the walls, I'm really not sure. The drywall feels a little cool there, but it sorta feels cool everywhere around the house this time of year.
I do appreciate the suggestions tho.
Right now to solve the issue for the night I have two of them on the "smart set" option that lets you drop the temp down to 50. At that setting the dining room is maintaining 65 and my son's room is maintaining 68.
My room is still set to 61 and it's holding around 65.
Another strange thing is that even though all the units are set super low and really shouldn't need any heat at the moment, I can hear the outdoor unit humming as it spools up almost constantly. It's almost like the outdoor unit just really wants to make heat and dump it somewhere and it won't chill out.

 

[tabner]

Update: the installer did some research and they think because the head on my main floor is much bigger than the two upstairs, and it's heating a much bigger space, somehow when it calls for heat the unit is creating excess heat and it's finding it's way into one of the small heads upstairs. Once the small heads upstairs is hot the fan inside it automatically kicks on, overheating the room.
I'm not completely convinced, but I sorta makes sense. The two bedrooms definitely do function better when I turn the living room head off.
They say the best solution is two of the remote thermostats. I guess they have the ability to literally lock out the fan on a unit so I won't get there unnecessary cycling. They're installing those tomorrow.

 

[drewmo]

Hopefully the 1 year labor and 12 year parts warranty is in place. Seems like an overkill with the 36k btu unit outdoors with only 25k being used indoors. Curious to know the fix. Keep us posted and good luck!

 

[lsucet]

I had the same problem with my system that is a different brand and a four zone system. Two of my rooms were getting too hot. Contacted tech support and they sent me a PDF file of the engineer menu that I can access it with the remote control and tell me the settings I have to change to make the fans stop when the setpoint is reached on those two units.
No problems since then.

 

[tabner]

Hopefully the 1 year labor and 12 year parts warranty is in place. Seems like an overkill with the 36k btu unit outdoors with only 25k being used indoors. Curious to know the fix. Keep us posted and good luck!

I got it wrong. I was out there today looking at the unit and it's a 30k. Also the two upstairs heads are 6k. So 27k on a 30k unit.
Something still doesn't seem right though. I just got home, the oil boiler has the room at 66. I turn the big head on at 61 degrees on the remote. And it's dumping heat. So it definitely seems like they're all confused about the temperature. I have 3 years labor and 12 years parts. Altho they do want to charge me for the thermostats, just not for the labor.
If it fixes it I'll pay it, but not if it's still an issue.
The other comment he made is he's going to double check all the connecting electric lines, just to make sure one isn't criss crossed.

 

[stoveliker]

Strange indeed; I have (Mitsubishi) an 18k, two 9k, and a 6k unit on one 36k outside unit (it is common to install more inside than the outside can handle) and I have no such issues.

One would expect that no refrigerant can be pumped to units that are not active...

What if you switch on the big unit (heat) while you have the smaller units in cooling mode (with a setpoint higher than current temps inside so you are not really going to cool)? Do they then still push out heat?

 

[lsucet]

You should have the same .mode on all units. Heat override everything else and the units with different modes will shut down.
If the unit with heat mode runs continuously, other rooms can get real cold. Till the one with heat mode reach the setpoint and goes into standby, the others are off. That how I understand the system works regardless brand.

 

[stoveliker]

You should have the same .mode on all units. Heat override everything else and the units with different modes will shut down.
If the unit with heat mode runs continuously, other rooms can get real cold. Till the one with heat mode reach the setpoint and goes into standby, the others are off. That how I understand the system works regardless brand.

Nope, not for my (Mitsu) system, as the manual clearly states. Whether it's smart to have different modes active at the same time is debatable, but the outdoor unit will cycle/switch to supply to the indoor units running in different mode.

 

[peakbagger]

When I first heard of these units 18 years ago the multihead units were just coming out. I was planning an office renovation and was ready to go with a multihead but our HVAC vendor told me to stay clear due to control issues between the heads.

Some of the new commercial VRFs have multiple refrigerant lines. They pick what the overall demand is, heating or cooling . If heating is dominant and there are areas that need cooling, the system cools those areas with refrigerant cooled from area that was heated. It does the reverse when the system is cooling dominant. For multiuse building its a big efficiency boost. It requires a lot of control to pull it off.

 

[lsucet]

Nope, not for my (Mitsu) system, as the manual clearly states. Whether it's smart to have different modes active at the same time is debatable, but the outdoor unit will cycle/switch to supply to the indoor units running in different mode.

for what i understand the outdoor unit can do just one or the other but not both at the same time. regardless brand the system has to reverse to deliver one or the other. i can be wrong but i think that is hoe it works

 

[stoveliker]

for what i understand the outdoor unit can do just one or the other but not both at the same time. regardless brand the system has to reverse to deliver one or the other. i can be wrong but i think that is hoe it works

As long as the system @peakbagger mentions is not in place, then of course the system can only deliver one or the other. Hence it switches (as I mentioned). But it does not so only after one indoor unit reaches setpoint. Again, I don't know how efficient this is (I did this only once to test after installation), but it is what it is.

 

[lsucet]

As long as the system @peakbagger mentions is not in place, then of course the system can only deliver one or the other. Hence it switches (as I mentioned). But it does not so only after one indoor unit reaches setpoint. Again, I don't know how efficient this is (I did this only once to test after installation), but it is what it is.

yeah i know commercial system can deliver that performance, i didnt know home products can do that.

 

[mustash29]

I've been researching Mitsubishi and Fujitsu units. If you go to HVAC Direct . com you should be able to look up the model # of your heads and compressor, which will allow you to read the design/tech and service manuals. Good learning info there.

As someone else said, maybe commercial (or $$$) setups can do funky things like moving heat from one room into another room, but homeowner multi units can only do heat or A/C, not both at the same time. They switch modes via a 4 way valve that reverses the "freon" flow. Multiple heads hooked up to one compressor can't operate in different modes. All heat or all A/C.

There may be some "calibration" settings that are messed up or need adjusted. For a Fuji 9K single system, go to page 27 of this manual and read. (broken link removed to https://hvacdirect.com/hvac/pdf/9RLS3Y-Design-and-Technical-Manual.pdf)

 

[stoveliker]

I've been researching Mitsubishi and Fujitsu units. If you go to HVAC Direct . com you should be able to look up the model # of your heads and compressor, which will allow you to read the design/tech and service manuals. Good learning info there.

As someone else said, maybe commercial (or $$$) setups can do funky things like moving heat from one room into another room, but homeowner multi units can only do heat or A/C, not both at the same time. They switch modes via a 4 way valve that reverses the "freon" flow. Multiple heads hooked up to one compressor can't operate in different modes. All heat or all A/C.

There may be some "calibration" settings that are messed up or need adjusted. For a Fuji 9K single system, go to page 27 of this manual and read. (broken link removed to https://hvacdirect.com/hvac/pdf/9RLS3Y-Design-and-Technical-Manual.pdf)

That is incorrect. I can run with one unit set to cooling and another one to heating.
Of course they cycle on and off during these conflicting demands of the compressor, but they do not cycle to the next indoor unit to be active only once the previous one has reached set point.


That (cycle before reaching setpoint) may be part a semantic misunderstanding though.

 

[Brian26]

Unless you have a VRF system with a branch box that peakbagger mentioned the outdoor unit can only provide heating or cooling. Refrigerant is still being pumped to every head unit regardless if its in use or not. Another downfall with these multisplit units is the minimum modulation. A 36K Mitsubishi multi split might only be able to turn down to 8k btu as its minimum output. Often times if just one head is being used the minimum output is often way too much. Multiple single zones are superior in efficiency and turn down output. My single zone units can turn down to 1k btus.

These multisplit units need to have a very accurate manual J heat loss done to really get them to work right.

 

[stoveliker]

Unless you have a VRF system with a branch box that peakbagger mentioned the outdoor unit can only provide heating or cooling.

Yes sequentially. This is not different from switching between cooling in summer and heating in winter: it switches over. Just faster than twice a year. And it does so before a setpoint is reached on one indoor unit. This may not be efficient (and I will never use it like this), but it is possible.

This is not theory, the experiment has been done...

 

[tabner]

They came today and checked the electric lines and said everything is good. Then they opened up the units inside and stuffed some insulation in from the front. In particular the large head in the dining room they said the wall felt very cold behind the unit and they stuff a lot of insulation in there. Then they installed the touch screen thermostat controls in both bedrooms. They are the MHK2 controllers, cool looking touch screens. That moves the thermostat point from inside the head, to inside the controller on the other side of the room (wherever it is mounted). Then they called mitsubishi support, opened up the two upstairs units, and snipped a jumper wire to stop the fans from cycling (only in heat mode) when the unit is not calling for heat. My upstairs bedrooms are quite small, and 6" walls, so it doesn't take much heat leakage to overheat the room. Then they did a hard reset on the whole system.
Not sure which one of these changes made the difference, but so far (it's only been a few hours) it's a night and day switch.
The rooms are rock solid on temp, they're not cycling anywhere near as much, they're all stable at the set point, and they aren't working anywhere near as hard.
Trying not to get my hopes too high, really I should wait a day or two, but so far I'm very pleased.
Tempted to add another MHK2 on the downstairs unit. I guess you have to buy their special router, but at that point you can have the whole system on wifi.
Thanks again for everyone's help.

 

[tabner]

Just to elaborate a little bit, and I could be wrong cause my knowledge is minimal, but the way it was explained to me, anytime there is a little residual heat in the head it is designed to run the fan to leak out that heat, in order to be maximally efficient. So in my case snipping that jumper wire was designed to stop this feature. Thereby making sure that any extra heat which found its way up those lines would not leak out. Apparently the downstairs unit being much larger and heating way more space, with way more windows, was just creating excess heat in the refrigerant system.
They made sure to not snip jumper lines for the coolant, so in the summer I should still see that benefit of fans blowing every last bit of cool air out of the coils before closing up.

 

[tabner]

Sorry, follow up question - does anyone know roughly how many watts one of these systems should draw?
When I lose power I have a 5500 watt generator that plugs into the whole house panel. Obviously we don't run the microwave or hair dryers etc, but it runs the well pump and the oil boiler and the fridges, etc. Wondering if I were to lose power could I run this system? Or would that be a crazy load?

 

[stoveliker]

I think 2-3 kW on average - but I am not an expert and don't have the data on your system. The data for mine is in the documentation that I got with the install.

 

[Brian26]

What is the model number on the outside unit? I can look up your performance data and give you some more information.

One thing you need to know is the minimum heat output on your 30k outdoor unit. It may have a range of 7k-30k btu for example. Let's say you turn on just the big 15k unit and it only needs 4k btu to maintain heat in the space. Since your outdoor unit can only drop to 7k btu the other 3k btu was probably being bleed off by the 6k units cycling the fan. Hot refrigerant is being pumped through those 6ks units even if you aren't using them. My guess is since you stopped the 6k units from bleeding the extra heat the 15k unit might overshoot the set point now. You would be surprised how little btus a space often needs. My downstairs 12k split will often run at minimum speed only delivering like 2k btu to maintain the space.

I'm in CT as well and use 2 splits for my main source of heat. Have you looked into solar? CT has one of the best incentives for panels. As you know CT has like the 3rd highest electricity rates in the US. Heat pumps and solar are a great combination here. I assume you got the state rebates on the mini splits? My electric bill is $9.62 every month and I will reach my ROI on my panels after 5 years this April.

 

[tabner]

What is the model number on the outside unit? I can look up your performance data and give you some more information.

One thing you need to know is the minimum heat output on your 30k outdoor unit. It may have a range of 7k-30k btu for example. Let's say you turn on just the big 15k unit and it only needs 4k btu to maintain heat in the space. Since your outdoor unit can only drop to 7k btu the other 3k btu was probably being bleed off by the 6k units cycling the fan. Hot refrigerant is being pumped through those 6ks units even if you aren't using them. My guess is since you stopped the 6k units from bleeding the extra heat the 15k unit might overshoot the set point now. You would be surprised how little btus a space often needs. My downstairs 12k split will often run at minimum speed only delivering like 2k btu to maintain the space.

I'm in CT as well and use 2 splits for my main source of heat. Have you looked into solar? CT has one of the best incentives for panels. As you know CT has like the 3rd highest electricity rates in the US. Heat pumps and solar are a great combination here. I assume you got the state rebates on the mini splits? My electric bill is $9.62 every month and I will reach my ROI on my panels after 5 years this April.

MXZ-3C30NAHZ2​