Hunter Fan 40050A is a mechanical heat-only thermostat designed to operate with gas, oil, electric, or forced hot water heating and air conditioning systems using 24-volt or millivolt controls. It features a heat anticipator to increase or decrease heating cycle length and is compatible with systems with one wire marked "R" or "RH" (4-wire system) or separate "RH" and "RC" wires (5-wire system). The thermostat has a temperature selector to set the desired temperature and a heat/off/cool system selector to switch between heating and cooling modes.