Do thermostats coordinate with each other in a multi-zone system?

Question

Say I set my thermostat to 72 F. (Yes, I'm in the crazy U.S. and use their strange measurement system). I am under the impression that an HVAC system will wait for the temperature to be off by a certain threshold before turning on and then it might even overheat/overcool by a degree or so to prevent short cycling. For example, if it's summer, my AC might kick on at 73/74 and cool to 71.

First question - is that accurate?

Next, I am wanting to know about dual zone. I see two possibilities here:

1. Two thermostats trigger a cooling cycle or heating cycle when needed. The furnace/AC is turned on to handle the requested zone(s) at the time. This seems very inefficient as Zone 1 might get to temp and shut down while Zone 2 could trigger a cycle again 5 minutes later.

2. Two thermostats communicate between the furnace/AC and themselves. In my first example, let's say that Zone 1 hits 74 and turns on. Meanwhile Zone 2 is at 72. A 'smart' system might open the dampers in Zone 2 to cool it down to 71 while it's also working to get Zone 1 down to 71. Even though the thermostat at Zone 2 would not have triggered this itself. These dampers would only open for a short time but would prevent Zone 2 from requesting a cooling cycle shortly after Zone 1's cycle turns off.

Which of these is it? Or is it something else?

Last question - does it require special thermostats?

Answer 1

Conventional multi-zone arrangements work independently. They don't know or care about each other. They're tuned individually to achieve the desired outcome. Sure, there's some minor inefficiency, but not usually enough to warrant extra equipment costs and complex control algorithms.

Also, such systems would be difficult for homeowners to manage and adjust on a seasonal basis. Heck, many don't even know to change their filters or how to manage humidity properly. How are they going to work sophisticated thermostatic equipment?

Answer 2

Recently on this site there was a discussion of the fact that some thermostats installed in the US allow setting a temperature difference between when the a/c turns on and when it turns off. The purpose of this is to reduce wear on the compressor by reducing the number of on-off cycles. A disproportionate amount of compressor wear occurs when the compressor starts.

The idea is that if the thermostat is set to 22 C, then the thermostat waits until the temp is 23.5 C before it calls for compressor on and it stays on until the temperature is 20.5 C.

My 26-year-old Honeywell programmable thermostat does not do that as far as I know. It has a negligible gap.

Answer 3

The answer really depends on the technology and design of the HVAC system. I have a residential multi-zone forced air system. The individual zones are controlled by motor dampers in the supply ducts. These dampers are controlled by an intermediate piece of logic, which the thermostats talk to. This controller talks to the furnace & A/C compressors, and blower.

When I bought the house, this was a piece of relay logic. The thermostats where simple mercury on/off switches, and the mode of the system (heat,cool,fan) was controlled by a rotary switch on the furnace next to the controller.
When one thermostat calls, the system turns on, opens that damper, and supplies. There was an option (slide switch) allow either or both zones. (eg: either 'stat opens for itself or both) There were problems with that controller in that it closed the dampers while the blower was still in cool down. Modern dampers actually open at the end of the cycle and the off zone closes at the beginning.

I replaced that with a Honeywell TZ-4 controller and new dampers. This system allows modern programmable electronic thermostats (I use RTH6300Bs). It listens to both, and after a delay decides what to do. If both are in the same mode, it opens the appropriate dampers and proceeds. If they disagree on mode, there are rules for what it does. Basically first call wins, and it will change modes to satisfy the other call (after first satisfied or 20 minutes)