Weirdness with new Honeywell Thermostat Install

scagger

I recently purchased 3 Honeywell T6 smart thermostats to replace my old stats. The first floor one I installed correctly. I had to connect the 3rd unused wire to the COM terminal in my Taco SR501 relay. No issue, got it up and running fine.

2nd floor thermostat runs off of a 2/3 zone Taco SR502. That relay handles the 2nd floor, Great Room, and Aquastat. Okay so I go ahead and run the extra unused wire from the 2nd floor stat to the COM terminal in the SR502. I connect all wires just as they were from the old stat to the new stat including the new c-wire connection. Thermostat powers up, configured, heat works, AC fan works....I'm good, nope. Once I had connected the new thermostat, I noticed a few minutes later that the heat in the Great Room had turned on. This is a completely separate zone in the SR502. The thermostat in the Great Room is turned off completely. I notice once I pull the new thermostat off the wall, the heat turns off in the Great Room. I'm baffled why this is happening. I've checked everything. Thoughts??

scagger

I think I may have figured it out. According to Honeywell diagram, the common wire has to be connected to the air handler and not the boiler relay. I guess I know what I'll be doing tomorrow.

HVACNUT

If using separate Rh and Rc, then yes, 24v should come from the air handler. I don't think that explains why the 2nd floor thermostat turned on the Great room zone. Maybe it's always been hooked up wrong in the 502.