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Thread: Mitsubishi multi-zone ductless units not respecting set point

  1. #1
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    Confused Mitsubishi multi-zone ductless units not respecting set point

    We have a three zone Mitsubishi ductless system installed by a Diamond Certified installer. The compressor is a MXZ-3C30NAHZ2 and we have two 9btu and one 12btu MZS-GL interior units.

    Here's the problem:

    The downstairs unit is set to 74 degrees, because we like that room warm
    The bedroom units upstairs is set to 64 since we like that room cooler
    The den unit upstairs is set to 69 and is in a very sunny room that get's very hot, so we expect this unit to only come on at night on cooler nights when it gets cold

    What we find is that since the downstairs unit on the first floor is running often to keep the room at 74, the upstairs units are almost always blowing hot air even when the temperature in the room is way over the set point (only one green light on, so not heating it but it is still very much "heating"). The bedroom hovers at about 74 (with a 64 set point), and the den was 85 the other day (set point 69) while the unit was blowing hot air. I've had to run around and turn on/off units to avoid zones getting too hot. I'm worried the inverse will be true during cooling season.

    I thought this was a multi-zone system, but if I have to turn units on/off to manage temperatures in different zones, how does that achieve multi-zone? All the marketing materials for Mitsubishi multi-zone are about "everyone get's to personalize the temperature to what they prefer" etc.

    So, I'm confused.

    I was wondering if anyone understands this problem and can help me understand if:

    1) there is maybe something wrong with my installation or
    2) there is something wrong with my understanding of what mult-zone means?

    Thanks!

  2. #2
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    What does the installing contractor say?

  3. #3
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    He's looking into it

    Quote Originally Posted by pecmsg View Post
    What does the installing contractor say?
    He was surprised by this and is making calls. I was just wondering if anyone on this forum had specific knowledge of this problem to help diagnose what's going on.

  4. #4
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    pecmsg -- I accidentally created a new reply to the thread instead of replying to you: He was surprised by this and is making calls. I was just wondering if anyone on this forum had specific knowledge of this problem to help diagnose what's going on.

  5. #5
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    Improperly wired to the wrong zone, and you have too great of a differential between your zones.

  6. #6
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    Providing that no lines or wiring got crossed it is not likely an equipment malfunction.

    You may have separate zones but you cannot heat and cool at the same time on any residential ductless system, (or ducted) that I am aware of currently. The only way for that to happen is to have separate outdoor units serve separate indoor units.

    Also on the multi head units, not just Mitsubishi, all bleed a small amount of refrigerant through each head in heating. Normally the fans run all the time at a minimum speed unless the head is turned right off. There is likely a way to have the fan not run, but your contractor has to make a modification.

    Your very wide temperature ranges between heads is not helping the situation. You contractor apparently did not do enough investigation on exactly what your expectations were and whether or not his install could meet those expectations.

    The only Mitsubishi units that can truely heat and cool at the same time are the large commercial VRF systems.
    Quote Originally Posted by fgrac View Post
    We have a three zone Mitsubishi ductless system installed by a Diamond Certified installer. The compressor is a MXZ-3C30NAHZ2 and we have two 9btu and one 12btu MZS-GL interior units.

    Here's the problem:

    The downstairs unit is set to 74 degrees, because we like that room warm
    The bedroom units upstairs is set to 64 since we like that room cooler
    The den unit upstairs is set to 69 and is in a very sunny room that get's very hot, so we expect this unit to only come on at night on cooler nights when it gets cold

    What we find is that since the downstairs unit on the first floor is running often to keep the room at 74, the upstairs units are almost always blowing hot air even when the temperature in the room is way over the set point (only one green light on, so not heating it but it is still very much "heating"). The bedroom hovers at about 74 (with a 64 set point), and the den was 85 the other day (set point 69) while the unit was blowing hot air. I've had to run around and turn on/off units to avoid zones getting too hot. I'm worried the inverse will be true during cooling season.

    I thought this was a multi-zone system, but if I have to turn units on/off to manage temperatures in different zones, how does that achieve multi-zone? All the marketing materials for Mitsubishi multi-zone are about "everyone get's to personalize the temperature to what they prefer" etc.

    So, I'm confused.

    I was wondering if anyone understands this problem and can help me understand if:

    1) there is maybe something wrong with my installation or
    2) there is something wrong with my understanding of what mult-zone means?

    Thanks!
    Sent from my SM-G965W using Tapatalk

  7. #7
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    There is a modification your contractor can make to your bedroom unit to prevent overheating. I had to make it to mine for the same reason. The bedrooms way overheated. There is a jumper on the circuit board that needs to be cut. He should be able to get the technical service bulletin from Mitsubishi.
    If God didn't want us to eat animals... He wouldn't have made them out of MEAT.

  8. Likes R600a liked this post.
  9. #8
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    Thanks for this reply -- I think you may have put your finger on it with the bleed.

    I have confirmed that there is no miswiring by independently turning on and off each head and making it start heating using the remote, so I don't think that is the problem.

    I am not trying to heat or cool in different rooms, just not heat when any room it is already up to temp, so a VRF system is not necessary.

    But, your statement that "all bleed a small amount of refrigerant through each head in heating. Normally the fans run all the time at a minimum speed unless the head is turned right off" may be the explanation. Since the downstairs unit is keeping the compresor running, all the heads are getting some of the bleed through and since the fan is running, it pumps out heat.

    I guess the questions I have remaining (and I'll discuss this with my contractor too) 1) is it possible there is more bleed in my system than there should be? Sometimes, even with just the one light on, it is pumping hot air, not just warm air and 2) isn't fan on auto mode only supposed to come on when it is actively heating / cooling?

  10. #9
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    Thanks for the information about the jumper! I'll discuss that with him.

  11. #10
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    As stated earlier, the multizone units send some refrigerant to all heads even if they're not calling for heating, but they don't do it in cooling so you should be fine. You said you turned each unit on in heating to test the wiring, but you need to do the same thing in cooling since it will close the expansion valves of the units not calling.

    The blowers on these indoor units run all the time because the temperature sensor is inside the indoor unit. If you cut a jumper to turn off the blower you'll likely need to add a remote sensor or a wireless thermostat (MHK2) to sense temperature at the wall or you may end up more uncomfortable.

    I know these are advertised as "zoned comfort" but keeping wide temperature variations in the different rooms isn't really the idea. It's more for keeping all the rooms the same (ish) temperature.

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