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Mitsubishi branch box vs. port-type service: Do the indoor units always flow refrigerant?

monkeyman9 | Posted in Mechanicals on

I have a question I’ve been trying to figure out between the Mitsubishi MXZ-C multi zone units and if there is flow to the heads while they are off. The Branch Boxes have Linear Expansion Valves.  From Mitsubishi documentation: • Linear expansion valve open/close through stepping motor after receiving the pulse signal from the branch box controller board. • Valve position can be changed in proportion to the number of pulse signal.” EDITED my original questions after finding more info: After looking through the Mitsubishi service manuals both the Port Type and Branch Box units have Linear expansion valves for the liquid size of the flow lines to each indoor unit. I read several times on forums that coolant always flows to indoor units even when they are off with the MXZ-C units.  I’m not sure if that is true or not and hoping someone may have looked into this deeper.  What I can’t tell is the logic in the programming. If they each have an LEV on the liquid line I would think the logic would be to shut off flow unless the compressors min flow is higher than the single or multiple indoors units calling for refrigerant.  From what I pasted from the Branch box service manual above and the diagrams, it appears the LEV’s are controlled by stepper motors and can vary refrigerant flow. Anyone have any input here?

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  1. FluxCapacitor | | #1

    While I don’t have the specific technical answer you are looking for I can tell you that with my MXZ branch box some refrigerant seems to get through to all indoor units even when only a few indoor units are calling heat. Regardless if it’s above or below maximum turndown of the outdoor unit.

    I have a 5-Zone system. If I shut off one or two indoor units while other zones are heating then the shut down units will not blow but will get luke warm to the touch and their internal thermistor will rise to about 79-80F.

    More importantly, if you leave the wall units in “auto” or “heat” mode they will almost always blow lukewarm air in “extra low” fan mode even when they have reached the set temperature...and this is where an oversize wall unit (or undersized room) can get a substantial temperature overshoot, particularly during moderately cold days days when very little heat is needed in a room.

    Why doesn’t doesn’t the fan turn off Completely?... I suspect it’s because Mitsubishi wants to use this branch box bypass to maximize efficiency AND because the indoor unit temperature sensor would not read the room correctly if no intake air was passing over it.

    I have found a workaround that seems to works for me. Mitsubishi indoor units have a “setting” that allows to completely turn of the indoor unit fan when the desired temperature is reached. This will only work if using an external temperature sensor, such as a MHK-1 thermostat or the Kumo Cloud wireless sensor.

    The “fan off” setting actually involves permanently cutting off a resistor on the indoor unit control board. As crazy at that sounds this cutting appears to be a legit Mitsubishi approved “technician only” modification that doesn’t void the warranty....but you should check with your own installer yourself.

    This modification was done to one of my zones and it has worked perfectly so far. (only implemented 2 days ago). That zone was overshooting but now it’s right on (+-2)


    1. monkeyman9 | | #2

      Which outdoor unit do you have? I'm curious if it has a branch box or port type service (all the lines come right off the outdoor unit).

      Noticed you have the new MLZ cassette's and the SVZ. I was debating SVZ, PEAD or oversized 6k wall units for bedrooms. Downstairs I was looking at an FH head or MLZ in 2 rooms. Do you like the MLZ units?

  2. FluxCapacitor | | #3

    I have a MXZ-8C48NAHZ with a 5 port branch box in the basement (see picture).

    MLZ Positives:

    To me the MLZ looks better than wall units but probably the biggest positive of the MLZ’s to me is the built in condensate pump which, combined with running the refrigerate lines above the unit, makes it easier to install away from exterior walls and minimize outdoor refrigerate piping and condensate drains, especially in retrofit installations like my old ranch style.

    I planned carefully to keep outdoor clutter minimal. My 5 zone system has only one line set running from the outdoor unit into the basement. All other line sets and drains run through closets or wall cavities. All five drains terminating in a basement sump pit.

    MLZ Negatives:

    By nature the MLZ tends to give up efficiency because the install involves punching a huge hole in you ceiling into potentially unconditioned space. I insulated my MLZ’s with 8 inches of batt insulation to minimize losses.

    Fitting a MLZ between 16” joists can be a very tight fit if your joist are not perfectly square. Hand tool planing of joists is sometimes necessary to get the MLZ in.
    -Uses condensate lift pump (another thing to break)
    -Noise from lift pump? (I’ve only used it in heating mode)

    I did a small review of my system earlier this week here:

    1. monkeyman9 | | #5

      Appreciate the info. The MLZ gets me away from a wall unit that blows right across the family room couch, since I could place it halfway down over it. Unfortunately the connections are on the wrong side. So I'd have to come trough the next joist and U the connections around. Never easy :P

      Appreciate the tip on the loud branch box. It was going to be right under the couch in the basement originally. I'll rethink that one.

      1. FluxCapacitor | | #6

        I would not be worried about noise from a branch box mounted in basement under a living room. I doubt you would notice.

        The branch box makes some on and off hissing noises that could be disruptive if you installed in a bedroom closet and wanted a dead quiet bedroom.

        The MLZ fan is basically unnoticeable.

  3. FluxCapacitor | | #4

    One more thing.

    The branch box is far from silent. Quite a bit of various hissing noises. Don’t install in a bedroom closet.

  4. jmidway22 | | #7

    Wow, I am surprised no one has answered this yet. I would love to know how the variable refrigerant flow works past the branch box. Great the exterior condenser is variable but if its paired with interior heads with drastically different conditioning environments, its effectiveness is nerfed a bit it seems?

    Doing a remodel of a cape cod and the upstairs has 2x6 cathedral roof/ceiling on the front roof plane. Hard to get enough insulation in that area and when you pair it with zone 3 and additional cooling load due to it being upstairs, it would be nice for those heads to be as variable as possible decoupling it from the main floor and basement.

    1. joshdurston | | #8

      Sounds like you might want to consider a mix of 1:1 units, or maybe separate smaller outdoor units that are tied to units with similar load profiles. So if there is heating or cooling bleeding between zones it's likely necessary.
      Also, with small rooms dedicated wall heads on multisplits are often a big compromise that has two big problems. All the zones force you into an oversized outdoor unit (only the big units offer 8 zones), and the head itself is oversized for the space leading to overheating or short cycling.
      I would rather slightly undersized equipment with a secondary means of topping up on the coldest days, especially with multisplits and their poor turn down.

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