Here's my situation: I manage an older semi-commercial building, one part of which has hot water heating (radiators) as well as regular a/c with a compressor outside and an air handler in the attic. The a/c system runs on the usual 24 volts, but the hot water thermostat is 120v (the thermostat starts and stops a water pump on the boiler). The thermostats are next to each other on the wall.

The problem is that sometimes the systems get crossed up and people end up turning on the heat and the a/c at the same time. In addition, I'd like things to be configured such that when the boiler circulation pump is turned on, the attic air handler comes on to help circulate the heat more evenly.

Is there any reason I couldn't configure a regular 24 volt thermostat (like a Honeywell FocusPro) to control both systems and hook up a relay such that when the thermostat called for heat, it would turn on the fan and also trigger the 120 volt boiler pump circuit? This would make it impossible for the two systems to be on at the same time.

Thanks for any advice.