My heating system is kind of strange. It's in a condo unit that was built in the 1970's. I have hydronic baseboard heat that is throughout the building, and an A/C unit in the ceiling. One thermostat controls it all. I want to put a new stat in. The old stat has wires going to G R W1 W2 the new stat has the normal wire slots B O G Y W R Rc. How do I wire the new stat in?

What model and brand is your new stat? It may not be correct for your unit.

Sound's like you bought a heat pump stat.

Quite likely, here is a chart Thermostat Wire Colors Code | HVAC Control (http://highperformancehvac.com/thermostat-wiring-colors-code/) The orange wire is only used for a heat pump.

With no 'Y' terminal, I don't understand how the A/C operated. Any numbers on the old stat?