The HTC-4D temperature controller is primarily designed for the control of 2 Heat and 2 Cool air-conditioning units, and also incorporates an ON/OFF Two position 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-4D is in the cooling mode and the outside air is lower than the room/return air temperature the relay will energise.