TRVs consist of two parts, a valve that opens or closes to control the hot water flow, and an actuator that controls the opening of the valve. The actuator adjusts the valve opening based on the temperature in the room via a mechanical linkage or pin connected to the valve. The actuator is usually a plastic unit containing a wax plug, the wax plug expands or contracts as the temperature of the room rises or falls. The plastic unit is preset by a screw mechanism that positions the wax plug a set distance from the connecting pin.
TRV temperature control is suitable for large rooms or buildings that are not constantly occupied, (such as warehouses and garages), where precise control of the room temperature is not critical. They are subject to fairly wide swings in temperature inside the heated space due to external conditions, e.g. ambient temperature and/or wind velocity and direction, and since their setting is not readily adjusted, once the initial setting is made it is rarely changed.
It should be noted that TRVs should not be used in the same room as one with an electronic thermostat. The operation of the two devices are not compatible and would interfere with each other so that effective temperature control would be lost.