I have 3 identical Rheem furnaces in my new home (built 1964) each equipped with a 3-ton cooling coil --- yet I have only 2 outdoor fan units. One each of the furnaces, cooling coils and outdoor fans power the 2nd floor --- so we can forget about those for this discussion.
The remaining two furnaces/cooling units are connected each to their own thermostat --- one in the master bedroom (for the MBR suite) and one in the dining room (DR, for the main part of the first floor). The ductwork for each system is independent of the other.
I only briefly occupied the house over the summer and moved in full-time earlier this month --- December. During the summer, I noticed that in order for me to cool the MBR, I'd have to turn on the Dining Room thermostat and set it to a temperature low enough that would insure that the outdoor fan ran continuously, and then set the MBR thermostat to the temperature for sleeping.
In other words, I'd have to run the entire downstairs all night, even though I only needed cooling in the MBR.
I recently brought in highly regarded experts from an HVAC company who will be servicing my equipment annually. Their explanation was that the cooling part of the system in the MBR is only a slave to the DR one --- the single coolant line from the outdoor fan has a T-connection inside the house that splits the coolant to each of the coils. They pointed out that the 5-ton unit cannot feed only one of the two 3-ton coils because it would freeze it --- it must always feed both units simultaneously. So, when the DR unit is powered on, it automatically turns on the MBR cooling unit. So, in effect, the MBR thermostat, for cooling at least, is doing very little. (The heating part of the system works independently). Since it's 30-40F outside, we can't run the system to test their theory.
My theory --- and I'm no pro --- is based on: "who would zone a system into 3 parts, knowing one zone would never work?" My belief is that the MBR should be able to turn on independently, and that some relay from the thermostat in that room should turn on the outdoor fan, much like the DR one does. (And that that relay is broken somehow) Of course, that would suggest that the 5-ton fan could power each of the cooling coils independently without freezing them.
Anyone ever heard of a similar master/slave situation?