Mitsubishi Head Overheating Way Past Set Temp
Is it normal operation for a wall mount Mitsubishi head (MSZ-GL15NA) set to AUTO fan in heat mode to vastly overshoot the temp on remote when the house heats up on a sunny day? Recent example: remote set at 72, temp of air next to head unit 84, temp of air blowing from unit 102.
Presumably, the unit temp sensor would be reading something close to 84, which is way higher than set temp of 72. Why isn’t the head shutting off or at least reducing to minimum fan speed? Fan speed remains constant even though set to auto fan.
Also noted: The head fan stops completely for 1-2 seconds every 15 minutes, then resumes. No apparent ramping up or down of fan speed. Seems to always be on a medium speed.
This head unit is the only one I’m running off a 3ton MXZ-4C36NA3 (3 heads) while trying to learn how to live with a $20K whole house Mitsubishi install last November. Diamond contractors not able to answer my questions. kWh usage for heating are much higher this winter in comparison to old poorly ducted heat pump. 20 year old house, Zone 3/borderline 5. Single level. 2k sq foot, average insulation.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I don't have any specific recommendation but a thought that may point in the right direction. Compressors have a minimum output level, if they're running at all they're running at their slowest speed and that's what they produce. They usually also have a minimum run time, because starting and stopping is hard on the compressor. So the compressor can override the thermostat in the heat and send heat even when it's not being called for.
Usually when you have multiple heads on one compressor that heat will be sent to all the heads. It sounds like what's happening is all of the heat is going to that one head. Where I would look is not at the settings for that head, but at the settings for the other two which aren't picking up any heat.
Step one- Have the installer cut the jumper for the constant fan running. Step two if step one is not sufficient- have a remote (sensing) thermostat or temp sensor installed - do not rely on the head temp sensor as it will act exactly as you describe.
The min output of your 15K BTU head is 3,100 BTU. But the min output of your compressor is 11,300-11,700 BTU. What's happening is your head will call for heat (let's say the min 3,100 BTU), so the compressor fires up, but it generates an extra 8,000 BTU that gets put into the refrigerant lines. Once that one head is satisfied, the extra BTU need to go somewhere--some of it is dumped not by that head, but some of it does get dumped into the room. As others point out, the blower on that head will exacerbate the issue since it will be blowing air over the hot lines, and the way to solve that apparently is to cut the wire for the constant blower fan. That's either an oversight in design by Mitsubishi or an intentional design to mitigate heat buildup of excess heat elsewhere (I haven't read definitively one way or another).
Btw, I'm just a guy, not an HVAC professional, but I've done extensive reading and have discussed this issue with installers.
Sounds like this a shoulder season heating issue I’ll have to deal with by babysitting the system, as in turning all head units off on sunny days. I was used to my old whole house heat pump shutting off when heat was not needed, hence the same expectation for this multi-head unit. Beginning to understand that efficiency designed in to turn the condenser on and off minimally produces this issue.
Anyone care to comment on why the head unit fan cycles off for about 1-2 seconds several times an hour? The diamond contractor has already replaced one of the multi’s circuit boards to try to address this but not solved. Still trying to figure out why my energy usage to heat just the main great room (with all bedroom doors closed and those heads turned off) is so much higher than the old poorly designed floor ducted heat pump that used to heat the whole house. And I’m still using a propane fireplace to take the chill off on 30-degree mornings.
Lastly, will the air conditioning performance be better?