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Zoning Mitsubishi Zuba-Multi Systems

_Stephen_ | Posted in Mechanicals on

With my new house, a little over a year old, we built net-zero.  Part of that was a heat pump, and I was told that the specified Mitsubishi heat pump (branded Zuba, but is basically just a P-Series air handler combo) had a zoning solution that worked well.

First the warning:  DON’T DO IT.  Don’t go for the Mitsubishi reccomended zoning solution.  It’s actually an AirZone product, and Mitsubishi tech support doesn’t know about it, and doesn’t know how to help debug it, or support it at all, and there is no feedback to help the techs as to what is going on.  In the last year since the system was commissioned, we’ve had at least two failed dampers,  the zone panel has failed, the one thermostat doesn’t report the correct temperature, the dampers randomly cycle continuously, and the unit fails to provide heat until the zone panel is rebooted,  and the A/C didn’t provide cooling for the first summer.  When the zone panel failed getting parts took 7 weeks.  It’s still not installed correctly, and we were left without heat in our house for two months over December and January.

They’re coming tomorrow to attempt repairs. My expectation at this point, is that the repairs will fail, either on the day of, or shortly thereafter.

Now, the question:  Has anyone used the Mitsubishi Thermostat Interface Module?  It lets you hook up a normal thermostate to the indoor unit, and control it as a staged HVAC.   First stage is ‘Modulating’, and second stage is ‘Max capactiy’.  I want to use this module to integrate a traditional zone panel, in the hopes that I can have reliable heat again, and something that is debuggable.

I welcome any input.

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  1. Expert Member
    BILL WICHERS | | #1

    The problems you’ve had sound to me like they might be related to power transients. You want to make sure your temperature sensor cables especially, but also the other low voltage cables, are not run alongside power cabling.

    It’s possible the brain is sensitive to something coming down the line too. If it’s using a regular power outlet, try a TVSS receptacle, which is a regular power outlet with an integrated surge protector. They aren’t very expensive. If you need a 240v protector, I’d put one on your main panel (which I recommend anyway). These devices are around $50 and protect your entire house (ideally you still want protectors at each sensitive device too). There is a manufacture out of Florida that makes a unit I like. I can dig up the name if you’re interested.


    1. _Stephen_ | | #2

      Hey Bill,

      Definitely interested... But I don't think this is a power problem. I have lots of other sensitive equipment in here, and none of it seems to care. In fact the heat pump itself is sitting there with lots of complicated circuitry, and it's been perfectly fine. It's just the zone system itself.

      And, it's not just that there have been problems with the zone system, it's the fact that they can't seem to diagnose them, and when the guess, it takes them weeks or months to get parts. January without heat has not been fun.

      1. Expert Member
        BILL WICHERS | | #13

        The little unit I use is from ditek:

        Just like in the pic, which is the one I have installed on my panel at home. It’s $52.21 on Amazon.

        This is my “little” protector. I have a “big” one (a polyphaser unit that is much more than $50) ahead of my ATS.

        I spec these devices, known commercially as TVSS units (transient voltage surge suppressor) for critical faculties like datacenters. You basically have two types: large units intended to take BIG hits (direct lightning strikes and power cross) and small units intended for small transients (switching motors on and off, non-direct lightning strikes, etc). You get most of the benefit with the small unit in many cases, and I HAVE seen these little units take lightning strikes to nearby power poles and work — but they usually need to be replaced afterwards.

        The large units are intended to take repeated large hits. My “big” protector was somewhere around $600-800, I forget exactly since I got it a long time ago. Some of the commercial ones I like (I usually spec Liebert, which is Emerson electric, and Eaton units) are thousands of dollars. I will usually design in multiple units in specific places throughout the system. Why? Because you need protection from switching transients generated WITHIN a facility too.

        At home, my “big” protector is for things like direct lightning strikes to nearby feeder cables, and power cross which is when a high voltage line falls on a lower low voltage line. In power terms, “low” voltage might be 4,800 volts while “high” voltage might be 41,600 volts. The high voltage power line voltages, basically. These are events of high energy and relatively long duration. These protectors are placed as close to where the utility service enters the facility as possible.

        My “small” protector is to handle little transients, which include when the ATS switches to the generator and back, and when the power company substation tap changers function (among other things). In large facilities, the small protectors protect against motors turning on and off, and any other inductive transients like that. These are events of relatively low energy and very short duration.

        I also use surge protectors on my computer and TV stuff, one protector at each device. These are for whatever might be left, but also induced energy from nearby lightning (wiring acts like an antenna for these events).

        A system designed this way is pretty well protected. You can get most of the benefit with just the “small” protector, just know that it can probably only handle a BIG hit ONCE without needing to be replaced. To get full protection, you MUST make sure ALL wiring coming into your house is bonded (grounded) together at one spot, known as a “single point ground”. All your grounds (ground rod, water pipe, etc) MUST be connected together too. You actually get more protection from doing this than you do from the protector itself, but they aren’t for exactly the same issues so you really need both working together.

        BTW, I design systems commercially to be able to handle 100,000 amp lightning strikes. Some 96% of lightning strikes are less than this, so I can state that I provide a particular level of protection to my customer.


        1. _Stephen_ | | #15

          Thanks for this. Will be ordering one.

          1. Expert Member
            BILL WICHERS | | #17

            No problem. For best performance, you want to keep the wires to the unit short. Not crazy pulled-tight short, but you don’t want excess loops of wire. The protector itself needs circuit breaker protection, so tap it off of a double pole breaker (you have to be sure both legs of your service are protected, and using a double pole breaker is the easiest way to be sure). I have mine tapped off of the 15A double pole breaker that feeds my well pump.

            I recommend periodically eyeballing the protection LED too, especially after any big thunderstorms or suspected power issues. If that LED goes out, you need to replace the protector.


    2. Trevor_Lambert | | #3

      Around $50 seems optimistic. I recently shopped for one of these pretty extensively. The price range I saw was $80 to $250. I ended up with an Eaton CHSPT2ULTRA, which was on the lower end of the range but had pretty good specs.

      1. _Stephen_ | | #4

        What were your thoughts on their overall effectiveness?

        1. joshdurston | | #7

          I went with the CHSPT2ULTRA based on the spec and price. It was easy to self install if you're comfortable working in a panel safely.

          I'd say whole home protection is a good first line of defense, but I would still put point of use protection on important assets. The whole home protector will help the downstream ones last longer. The point of use ones typically are more sensitive to responding to smaller surges, but don't have the same raw capacity to dump voltage like the whole home ones do. So it makes sense to do both.
          I've been meaning to add one to my mini split disconnect as well.

          Here is a decent write up:

  2. jwasilko | | #5

    We have two SVZ air handlers with the PAC-US444CN-1 thermostat interfaces. We're using Honeywell Prestige IAQ thermostats. They were installed in October, so we've got a few months with it in heating mode. has a tech note about zoning with it using Honeywell panels. I'd suggest Honeywell or EWC panels. has a flow chart of how it operates.

    Basically, the thermostat interface starts off on low, and every 5 minutes it ramps up the capacity a bit. It never seems to ramp down in any significant amount.

    One issue we ran into was in trying to use the CNF/CN25 connectors with a steam humidifier. We weren't able to get the air handler and humidifier to run without a demand for heating. The installer seemed to think the humidifier contacts only work in heating mode, but the thermostat interface doesn't leave the air handler in heating mode when it's idle.

    We ended up adding an air proofing switch to the humidifier circuit and doing it the old fashioned way.

    1. _Stephen_ | | #8

      Looks like it tries to hit a target heating / cooling rate of 1f / 5 minutes, which is kind of quick IMHO. I'd be fine with 1F / 15 mins or 1F / 20 mins...

      It's still better than what my current zoning system does which is all over the place.

      EDIT: Also, this is exactly what I was looking for, thanks so much.

      1. jwasilko | | #9

        Happy to help. I couldn't find much written about the Thermostat Interface when we were trying to figure out controls.

  3. joshdurston | | #6

    EDIT. someone already posted this.

  4. MattJF | | #10

    Have you tried contacting AirZone USA directly to see if they have any techs in your area that are particularly well versed in the system?

    How complicated is the zoning and house layout? Have you tried having the system balanced and running it without zoning?

    1. _Stephen_ | | #11

      AirZone is apparently coming by on Friday. I'm not super enthusiastic about it, but they might be able to sort things out.

      It would be exceptionally difficult to balance the system as implemented. It was designed from the ground up to be zoned, so there are three main trunks coming right off the system, and there are just wildly different lengths for each of those trunks. Like, I'm sure you could do it, but it would be challenging, and would require a lot of fiddling between seasons.

      I paid for a zoned system, I have a contract that says it should be a zoned system, and it's under warranty, so if they can't fix it, I expect them to replace it with something that is zoned.

  5. MattJF | | #12

    I looked around for a technical or engineering manual to see the logic these systems use and the design guidance and didn’t find one.

    Do you know if the dampers are open/close or modulating? Is the zone controller able to control the fan speed?

    Any zoned system should be able to be balanced. This becomes a bit easier with a modulating system in a highly insulated house. The problem comes when the load proportions change significantly between different zones. The biggest variation is solar gain.

    Hopefully a good tech can sort things out. I don’t have a ton of guidance. It seems like we are an equipment generation or two away from having being able to support full zoning on a mini split system and maintain efficiency.

    1. _Stephen_ | | #14

      The system I have isn't quite as good as a mini split but it has been performing very well, when the zone system works.

  6. _Stephen_ | | #16

    AIrZone guy came out and diagnosed a failed damper actuator, but the system is still behaving badly.

    I've got thermostats freezing, I've got thermostats calling for heat, but won't engage the system. I've got one case where it ran the system with all the dampers closed all night, and wouldn't heat the house.

    I honestly don't feel like this system is a viable long term solution to heating my house. We're to the point where we're rebooting the zone panel *daily*. Technically all this is under warranty, so we'll see.

    I've sent stern emails to both the builder and the HVAC company in an attempt to get them to replace the zoning system. The actual mits heat pump seems great. Just the zone panel is terrible.

    1. Expert Member
      BILL WICHERS | | #18

      Sure sounds like flaky power supply problems to me. The units own internal power supply might be bad. Really noisy power can cause all kinds of weird seemingly random problems.

      I’d push for replacement. If the system is so flaky it needs daily rebooting, it’s essentially useless. Be the squeaky wheel on this one and you can probably get it replaced. Maybe tell them you’re thinking about writing an article about the problems to submit to GBA :-)


      1. _Stephen_ | | #19

        Hey Bill,

        Power supply was replaced today and problems persist.

        1. Expert Member
          BILL WICHERS | | #22

          Well that’s a bummer :-(

          Wonky system problems are always a pain. I hope you’re able to get them to replace the unit.


  7. MattJF | | #20

    Sorry to hear it isn’t working.

    The dying zone actuators are surprising, those should be reasonably tough. Are they a standard actuator or something custom?

    Do you have any engineering documentation that describes the logic? I am interested what it communicates to the indoor unit. Is it just feeding a temp setting? Is it able to demand fan speed changes based on the zones that are open? I know they advertise the ability to communicate with the unit.

    I would love for someone to publish the full digital communication scheme used by the remotes on minisplits so it could be utilized by other devices.

    1. _Stephen_ | | #21

      They communicate with the minisplit using the CN105 connector, which speaks 2400 baud even parity at 5V TTL.

      The full specification isn't published, but there is a python library that can control it. If it wasn't all under warranty, I'd go and design and implement my own zone system for it.

      I don't want to spend the $2k for dampers and thermostats only to have to sink piles of time into writing and debugging it.

  8. MattJF | | #23

    Thanks for the information that someone had worked out the serial communication on the Mitsubishi. I am interested in doing this for a Fujitsu slim duct. I haven't found any info from anyone else yet. The fujisu units can have two wired remotes, so my thought is to work out the protocol used there.

    I would be temped to spin my own zoning, but I am firm believer that warranties are often not worth their cost. It sounds like labor might be included in your current warranty, which is worthwhile.

    See if you can swap standard Belimo actuators onto the dampers, which would simplify the update. Although, I guess make sure the dampers are not mechanically deficient resulting in actuator burnout. Ebay is full of commercial thermostats that speak BACnet and similar that would work well for a spin your own application.

    1. Expert Member
      BILL WICHERS | | #24

      I used to always think “Belimo=actuator” too. Then one of my contractors talked me into trying the Honeywell actuators. The Honeywell actuators are quite a bit nicer! Both are pretty reliable, but Honeywell appears to have listened to the users and has some nice design differences.


  9. Expert Member
    Akos | | #25

    Maybe I'm missing something.

    These modern mini splits are pretty robust, on mine I've chocked the flow down by 50% and it still works well (less BTU but no freeze up issues), not like a standard furnace.

    Instead of fancy zone controls, why not just go with a standard thermostat driving the zone damper?

    What I mean is setup up the system with the zone over supplied by say an extra 20% flow. Then set up the damper for the zone so at min opening it supplies 80% of the flow. If the zone thermostat now needs extra heat/cooling it just opens the damper. No matter what, there is always some flow through the zone just the rate is modulated a bit.

    With the damper closed there is a slight bit of extra restriction but most of the extra flow would just simply be sent to the rest of the house. With a mid static unit, I doubt this would even change the flow through the head.

    The main unit runs on its own factory thermostat, no other interconnection or control.

    1. _Stephen_ | | #27

      The system you describe breaks down in my house, as the "main" zone has a lot of glazing and faces south. So, in the winter you freeze out the rest of the house, as all that sunlight floods the main floor, keeping it toasty, so that stat would never call for heat on sunny days in the winter, so the other floors never get any attention.

      In the summer, the main floor and upstairs need AC, but the basement needs literally zero, so you'd want that set to be like, nothing.

      I think writing my own zoning system, or using the thermostat interface module from Mitsubishi would be a better fit for what I'm trying to do.

      1. Expert Member
        Akos | | #28

        I think it would be pretty easy to set it up with the relay module. If you want fancy, you can even set it up so that the fan speed increases as more zones call for heat/cool. I don't think it would even need programming, you should be able to do most of it with a slight bit of relay logic.

        I wouldn't worry too much about the basement, the heat load there is low enough that you can have it on a simple thermostat+zone valve, with a damper that fully closes. This would let you keep it off in the summer.

        You might be able to this without the relay module if you put your main thermostat upstairs.

        I guess the biggest issue is that you payed for a zoned setup, this should not be your problem.

        1. _Stephen_ | | #29

          The thermostat interface (which is what I think you meant? What's a relay module?) Will work but be suboptimal.

          The heat pump basically only needs to run at minimum modulation until it's below -18C, which happens really around here.

          If I wrote my own, I could get it to do that.

          1. Expert Member
            Akos | | #30

            Yup, some manufacturers call the thermostat interface module a relay module.

            From what I understand the first stage heat call on the Mitsu unit is still modulating.

            If you can figure out a better central location for the factory thermostat and still get heating and cooling as needed into the zones, than it can run on low without issues. I guess in that case the zone dampers would have to restrict the flow more than with better control, don't know how much difference that makes on efficiency.

  10. MattJF | | #26

    Akos, the approach you describe is one I’ve designed into my slim duct system covering my second and third floors. The main thermostat on the second floor controls the system. A thermostat on the third floor controls a damper. This damper never closes, but would set to vary the cfm +/- 30%. The 3rd floor doesn’t see much use and would be setback most the time. The only challenge would be shoulder seasons and and wanting to warm/cool the 3rd floor from a setback.

    I should clarify that this is not implemented yet. I've been hoping to get to it for about 6 months now, but that is the way life goes sometimes.

    My main interest in knowing the Fujitsu remote communication protocol possibly implement a fan speed vs humidity scheme in cooling mode. The current system does not even allow setting a different fan speed for cooling vs heating.

  11. _Stephen_ | | #31

    As an update, in case someone else finds this in the future, we figured out the AC cutting out and the no heat case.

    The AirZone system works by setting a setpoint on the mits unit, opening the appropriate dampener, turning on the fan to circulate air, and then ... nothing. That's all it does. So, when my system was setting a setpoint of 22C upstairs, but the return air temp is 21C because it's cooler downstairs, the unit doesn't come on at all. Why would it? The return air sensor is telling it that it's below setpoint already.

    The solution to this is to use 'comfort mode' on the airzone in the summer and shoulder seasons, that introduces a 2C offset, and causes the unit to substantially ramp up, more than it would normally need to. This definitely wastes energy, but at least my HVAC works. In the winter, I can use eco mode, and it'll modulate nicely, to hold the setpoint within a fraction of a degree. This is the bulk of where my HVAC energy gets spent, and in this case it works flawlessly, and lets the mits unit operate as expected.

    1. Isaac_Savage | | #32

      Seems like a good case for installing corresponding zone dampers on the return side of the system.

  12. user-3055627 | | #33

    Steve, I stumbled upon your thread. Would love to touch base and pick your brain on this. I have what sounds like the exact same system (installed Dec 2018). Has never worked well, can't find anyone who can help me although have paid for a few guys to come look at it for no solution. I'm in Toronto. Thank you.

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