The HTC-4DE temperature controller is primarily designed for the control of 2 Heat and 2 Cool air-conditioning units, and also incorporates a Modulating Economy Cycle Output with a Comparitor Override. This output is produced by comparing the outside air temperature to either the return air or room temperatures. If the HTC-4DE is in the cooling mode and the outside air is lower than the room/return air temperature the Economy output will start modulating.