Unit Heater Controls
Two control systems are used with unit heaters. The simpler type provides on-off operation of the fan. The other system provides
modulating control of the heat source and continuous operation of the fan. It is considered the more effective control system.
Modulating control is obtained with either pneumatic or electric equipment. It allows a constant discharge of warm air and eliminates intermittent blasts of hot air. The continuous circulation prevents air stratification, which occurs when the fan is off. A proportional room thermostat governs a valve that controls the heat source or a bypass around the heating limit. Fan operation is governed by an auxiliary switch or a limit thermostat. Either device is designed to stop the fan when the heat is shut off.
In an on-off control system, the fan motor is controlled by a room thermostat. A backup safety device in the form of a limit
thermostat will stop the fan when heat is no longer being supplied to the unit heater.