A thermostat is provided comprising a first connector for connecting to an air conditioner, a second connector for connecting to either a conventional heater or a heat pump reversing valve, and a selection means for selecting from at least a first mode of operation and a second mode of operation. The first mode of operation is configured to establish cooling by providing an on signal to the first connector and to establish heating by providing an on signal to the second connector. The second mode of operation is configured to establish cooling by providing an on signal to the first and second connectors, and heating by providing an on signal to the first connector but not the second connector. When the second mode is selected and the second connector is connected to conventional heating, the thermostat detects when both the conventional heating and air conditioning are operating simultaneously during cooling.