Hi everyone,

Based on recommendations I have read here, I installed a VisionPRO TH8321 thermostat (with dehumidification control) this past weekend. Everything is working well.

However, I did have a question during installation. Before I ask my installation question, the following is important: This thermostat is operating two separate systems -- a furnace in the basement and an air handler in the attic, each with its own transformer.

My question: The thermostat I replaced was powered via a common "C" wire, which is connected to the heating transformer. The VisionPRO installation manual states that "when installing the thermostat in a two-transformer system, use the common from the cooling transformer to connect to the C screw".

When installing the thermostat, I used batteries and did not connect the common wire as I was afraid of damaging the thermostat. Is there a reason why the thermostat has to be connected to the common from the cooling tranformer? I assume that it would not work if connected to the heating transformer?

Thanks in advance!

Tony