I have a two zone HVAC system with two compressors (A and B). When I run the heat on Zone 1, it uses compressor A and when I run the heat on Zone 2, it also uses compressor A. When I run the A/C, it uses compressor B.

Is this an installation mistake?

How much of a problem/concern is it to have both zones running on the single compressor simultaneously?

Or, is this something I should have corrected immediately?

  • Is the system giving you problems. Why do you think it is not set up correctly? Was it not installed by a professional? Have you asked the person who installed it why it is set up that way?
    – Alaska Man
    Dec 8, 2020 at 21:34
  • 1
    This is entirely dependent on the system design. If the system is set up to use different compressors for different purposes, then it would appear to be working as designed. You've provided no detailed information about the system, so there's no way to tell.
    – Ecnerwal
    Dec 8, 2020 at 21:37
  • No problems... I'm a first time homeowner and have no experience with dual zone HVAC systems. I want to be sure 1) two zones running simultaneously on a single compressor is not a problem and 2) there won't be negative consequences in terms of efficiency.
    – mikeLdub
    Dec 8, 2020 at 21:37
  • What does a compressor do in a heat cycle?
    – isherwood
    Dec 8, 2020 at 21:51
  • 1
    A compressor does the same thing in heat pump mode as it does in cooling mode.... Compress the refrigerant.
    – Gunner
    Dec 8, 2020 at 22:06

2 Answers 2


Prior to variable frequency drives (most people call inverters) having 2 compressors was the most efficient system out there but the cost was quite high so many just oversized the compressor these systems 2 pump have longer life if well sealed because the compressor job is split up.

All heat pumps that I have seen have a reversing valve this reverses the flow of the “Freon” or refrigerant so instead of removing heat from the house it is removing heat from outside and using that heat on the inside. This is why heat pump efficiency drops off depending on the type and age efficiency roll off starts at ~24f but some newer high efficiency models can run all the way down to -5F.

The only problems I have found on 2 compressor multi zone systems is the control boards die and can be quite expensive. Some of these units are 20+ years old and only needed a recharge when they were having problems (other than the controller)

So your system appears to be a high efficiency model of its day and was quite expensive so I would not mess with it unless something is broken. Running a big compressor all the time will cost more. The small compressor costs less to run but is not large enough to push 2 heads especially in heat mode.


Your installation is unique , because modern day air conditioners have a feature called reverse cycle , which the compressor can reverse the stages of operation between the high and low side , which allows either cooling or heating stages depending on the expansion or compression of refrigerant.

Normally when you have multi zones , you have a single compressor (large) outside that does the work compressing gas and sends cold/hot gas to an evaporator unit (big coil inside your roof with the fan) .Air runs over this coil and you get a cooling/heating effect, from the refrigerant expanding/cooling.This air is spread amongst your ducts installed in your ceiling. When you add zones this is basically a partition for the air travelling to your outlets(a valve opens and closes and that feeds the central set of ducts .

Think of it as a splitting of a tree , the main feed of air is the central trunk , the group of sub branches from the trunk is the zones , and your outlets are the smaller leaves attached to the sub branch.

Now it is normally not a concern for running two zones off a single compressor , if the unit is undersized it just means it will not cool as effectively(thermal capacity of compressor).

What is more strange is that you have two compressors for a different stage of operation, given what i have mentioned above about the reverse cycle.

Have you check the model number of your unit and seen the specifications for it? for more clarification on its operation.

  • The reversing valve is and always has been part of a heat pump nothing unique about that at all. 2 compressor systems are for efficiency a 1 ton pump will be most efficient on a 1 to 1-1/2 ton head (inside unit) as the load increases a larger pump 2-1/2-3 ton is used. Large pumps that are on or off waste a large amount of energy with each cycle. Today we vary the pump speed to match the load to get the best efficiency with the pump rarely shutting down because start up is where the big inefficiency’s are.
    – Ed Beal
    Dec 9, 2020 at 15:07

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