The Switching Relay 303P connects up to three thermostats and operates circulators to provide heating to a zoned hydronic heating system. Zone 1 has optional priority over zones 2 and 3 and can be used to provide heat to an indirect domestic hot water tank. The 303P includes optional exercising to prevent circulator seizure when not in use, and optional post purge operation of the last operating zone to maximize energy savings. The 303P provides a RoomResponse(TM) signal to modulating-condensing boilers to optimize comfort while improving boiler efficiency.