This TH147P programmable thermostat can be used to control a gas, fuel oil or electric furnace of 2 or 3 wires, a central air conditioner of 2 or 3 wires, a hot water system with or without pump (2 wires), a millivolt system of 2 wires, or a central heating and cooling system of 4 or 5 wires. This thermostat is not compatible with heat pumps or multi stage systems.