Tekmar 540 tekmarNet� 4 Thermostat One Stage Heat, One Stage Cool, One Fan The tekmarNet 4 Thermostat 540 is designed to operate a single stage of heating, cooling, and fan within a residential heating and cooling system. It can be used with any terminal unit type, including radiant floor, baseboard, and fan coils. It is capable of controlling the heating device using a zone valve or zone pump through a tekmarNet 4 Wiring Center, Zone Manager or directly from the thermostat. An optional Adaptor Plate 007 is available which allows the thermostat to be installed onto a single gang electrical box.