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Mitsubishi Mini-split question re 2 zone condenser operation

A question for the braintrust here.

I have a Mitsubishi mini-split system for my house.  It's about 3 years old.

The system is: 
1. Mitsubishi MXZ3C30NAHZ2, 30,000 BTU, 3 zone outdoor condenser with hyper-heat.
2. A first floor Mitsubishi MSZFH15NA, 15,000 BTU, indoor wall unit.
3. A second floor Mitsubishi MSZFH09NA, 9,000 BTU, indoor wall unit. 

While I have a pretty good knowledge of residential heating and DHW systems, my knowledge of mini-split condensers is not as deep.

Here is my situation.
I am using 2 zones of the 3 available.  Zone 1 first floor, zone 2 upstairs. 

When I have the air handler for zone 1 in heat mode with the fan blowing warm, and the air handler for zone 2 is powered off, the coil of the zone 2 air handler is warm.  Just as warm as the zone 1 coil.

Is this the proper operation for this system?

My understanding is that each zone on the condenser has a supply line and a return line.  Very much like a residential hydronic system with individual circulators for each heating or DHW zone. Is this accurate? 
If that's the case, in the interest of energy conservation, I would think that the condenser would be smart enough to only "energize" the zone calling for heat.

In my case my zone 1 first floor air handler is about 15 feet one way from the condenser unit.  A short, easy run. Not a lot of heat/cooling loss. The zone 2 second floor air handler is about 75 feet one way from the condenser.  It runs outside the house for a bit, then through the cold unfinished attic space, then outside down the other side of the house and into the bedroom.

Even though the lines are insulated lines, I would think there is still some significant heat loss in the zone 2 lines. And then some more in the zone 2 air handler coil, even though it's off. (We keep the bedroom much cooler than the first floor.)

My intuitive guess:  The main condenser supply/output is manifolded to the 3 zone outputs but because there are no individual controls on the zone outputs, it's pretty much a "free for all" for the refrigerant to circulate to both zones. 

How'd I do?

Thanks for any input on this..


  • pecmsg
    pecmsg Member Posts: 4,893
    If it feels it needs to release more heat it will operate additional loads. 
    Was this sized for heating or cooling?
    where are you located?
  • ratio
    ratio Member Posts: 3,654
    The refrigerant flow to each head is never stopped completely (for oil return), so the other head will always produce some heat/cooling. You may be able to fiddle with some advanced settings to get the fan to stop completely if it doesn't already.
  • Paul Wolf
    Paul Wolf Member Posts: 38
    Pecmsg, It was sized for cooling. I am in Southeast NH

    ratio, Re oil flow. This most likely explains what is happening. It just seems to be an inefficient use of energy. Of course I haven't measured what the energy loss is so I really have no idea of the magnitude. Regardless, we LOVE the system and it has saved us a LOT of energy money both in heating and cooling.


    In my case above, the zone 2 unit was powered off (by the remote) fan in zone 2 is was not on when I felt the zone 2 coil. They were just warm. The downstairs (zone 1) fan was deliberately on.

    Thanks to both of you for the replies. I really appreciate them.
  • pecmsg
    pecmsg Member Posts: 4,893
    That’s the extra refrigerant flow. 
    HVACNUT Member Posts: 5,865
    The system charge is for 30K BTU. When say only the 15K unit is running, that extra refrigerant isn't waiting in a storage closet. It's still in the system and must be distributed.
    There is no energy loss. Whatever heat is lost at the unit not being used is only going to the heated space.