Some thermostats are designed to provide proportional control for valve and damper motors in heating or cooling systems. This type of controller is generally referred to as a proportional thermostat.
The Honeywell T92 proportional thermostat, shown in Figure 4-39, contains a bellows that adjusts one or two potentiometers in proportion to temperature changes. These potentiometer adjustments regulate the power supplied to the controlled device. This particular thermostat is designed to provide 24- to 30-volt proportional control for valve and damper motors in the system. Those models equipped with two potentiometers are capable of unison or sequence control.