My two-story house has one 5-ton unit that is zoned (using automatic dampers) to provide an upstairs zone and a downstairs zone.

So, the question is:

What happens if you accidentally set one zone's T-stat to AC and the other zone's T-stat to Heat?

Does the system just say "I can't do that, so I won't do anything"? Or, does one zone's setting win out over the other?

Have always wondered.

It depends on the zone controls and the equipment. Some give priority to heating and satisfy that before responding to a cooling call. Others just fail and don't do anything until the t'stats are corrected.
This is precisely why it is preferable to have indivual systems for each zone. Plus if one system breaks, there is a second system in the house that can still operate.

 

Honeywell zone systems have a purge cycle between stages.
So for instance, if the upstairs was set to cool, and down was set to heat, whichever one was calling first would activate until the thermostat was satisfied, but then a 5 minute purge cycle keeps everything off, and then the other system would activate until it satisfied, and it that pushed the other thermostat back on, it would again go to purge 5 minutes, then switch to the now calling zone.

 

Thanks very much for that explanation. Makes sense.

I have a Honeywell HZ322 TrueZONE controller on my system. I happen to have the installation manual for it, but I could not find the answer to my question in that manual.

Thanks again.

 

Your Honeywell is one of the best for simple operation. Some other systems also have a priority call and/or a dealt T differential to set the priority operation. Some of these systems can get ridiculously complicated with some choosing the largest differential between zones and number of calls. It's mainly all up to the factory programmers and/or the programming manual switches located within the boards. Typically, the closer sets points to each zone is the better the systems will perform.

 

Yes, I saw all those switch settings in the installation manual. Rest assured that I won't be messing with any of them!

Thanks again.

 

Why not just try it out and see what happens if you're curious? It won't damage anything - all zone panels have logic for this situation programmed in. Handling these situations is the whole purpose of a zone panels - that's why we can't just wire thermostats straight to the zone dampers and equipment.