On a demand for heat, the room thermostat initiates the purge blower operation for a prepurge cycle of 34 s, followed by energizing of the ignition and opening of the gas valve. As ignition occurs the flame sensor senses proof of ignition and deenergizes the spark igniter and purge blower (see Fig. 2-18). The furnace blower operation is initiated 30 to 45 s after combustion ignition. When the thermostat is satisfied, the gas valve closes and the purge blower is reenergized for a postpurge cycle of 34 s. The furnace blower remains in operation until the preset temperature setting [90°F (32°C)] of the fan control is reached. Should the loss of flame occur before the thermostat is satisfied, flame sensor controls will initiate three to five attempts at reignition before locking out the unit operation. In addition, loss of either combustion intake air or flue exhaust will automatically shut down the system.