These furnaces are designed to be used with a single stage room thermostat. They are similar to the Two-Stage Variable Speed (GMV) models. These furnaces are capable of the following heating modes, two-stage burner- low stage to high stage based on an adjustable time set on the board (not controlled by the thermostat) ). The blower also has two speeds in the heating mode, low and high, and it will change speeds according to the high or low stages. Single Stage where the furnace will run as a standard furnace with one flame level and a single speed blower (Factory setting).