THE HEATING CYCLE
When the thermostat is in the heating position, a decrease of temperature will cause the thermostat to make connection between terminals R and W. This permits a circuit to be completed through the automatic gas valve (AGV). When the AGV is energized, gas is per- mitted to flow to the main burner where it is ignited by the pilot light. Two high-limit contacts are connected in series with the automatic gas valve. One is labeled auxiliary limit, and the other is labeled main limit. The wiring diagram shows the main limit to be located in the fan-limit switch. The auxiliary limit switch is in a separate location. Both of these switches are normally closed and are shown to be temperature activated. The schematic also shows that an increase in temperature will cause them to open. Because both are connected in series with the AGV, the circuit will be broken to the valve if either one opens.
In the heating cycle, the indoor blower fan is con- trolled by the fan switch. The fan switch is temperature activated. After the gas burner has been turned on, the temperature of the furnace increases. When the temperature has risen to a high enough level, the fan switch will close and connect the low-speed terminal of the blower motor to the power line. Notice that the fan switch is connected in series with the normally closed BLR contact. When BLR relay is deenergized, the fan switch is permitted to control the operation of the blower fan. The blower fan relay
permits the fan to operate in low speed when the unit is in the heating cycle, and in high speed when the unit is in the cooling cycle.
THE DOOR INTERLOCK SWITCH
The door interlock is shown on the schematic as a normally open push button labeled (DIS). The function of this switch is to permit the unit to oper- ate only when the furnace door is closed. When the door is opened, the 120-volt power supply is broken to the unit. Most door interlock switches are so designed that they are actually a two-position switch. When the door is open, the switch can be pulled out. This causes the switch to make connec- tion so the unit can be serviced.