Time-delay circuit for automatic shutdown of furnace systems

Abstract

Time-delay circuit for automatic shutdown of furnace systems has an induced draft blower motor and a first circuit for providing energy to the blower motor from an energy source. A second circuit for energizing the primary control system of the furnace, includes a thermostat and an electronic switching means. When the thermostat is 'closed', the second circuit is completed to energize the electronic switching element which starts the blower. An airflow switch is provided to sense the start of the blower and to thereby energize a primary control which provides gas to a burner of the furnace. A timer set to a predetermined time interval has an output and two input terminal connections, one connected to the output of the thermostat and another to the output of the primary control system. When the thermostat is 'closed' and the burner is operating, the timer is disabled. However, when the thermostat is 'closed' and the burner is not operating such as occurs during 'lockout' conditions, upon elapse of the predetermined time interval, the timer output interrupts power to the electronic switching means, thereby shutting 'off' power to the primary control and to the blower motor.