1.  Field of the Invention
This invention relates to a safety circuit in a forced air furnace.  More particularly this invention relates to an improved apparatus for responding to a condition of an inoperative blower in a forced air furnace. 
2.  Description of the Prior Art
Conventional forced air furnaces such as gas-fired furnaces cycle on and off to maintain a desired temperature within a comfort space i.e., within a building interior. 
A thermostat senses the temperature in the comfort zone relative to a predetermined set point temperature.  When the temperature is below the set point, the thermostat closes to supply thermostat ac power to the furnace as a call for heat.  This causes the furnace to come on, initiating an inducer motor to flow combustion air after which a gas valve is actuated to supply gas to the gas burners.  An ignition device is also actuated to light the burners.  A flame sensor then proves burner ignition and sends power to a blower delay timer.  Then after a predetermined blower delay time, which varies with furnace design, the furnace blower is actuated.  The blower moves circulating room air from a return air duct through the furnace heat exchanger to pick up heat from the heated combustion products (carbon dioxide and water vapor) from the gas burners.  The heated circulate air then goes into a hot air plenum and is distributed through hot air ductwork back to the comfort space.  When the comfort space air is warmed sufficient to reach the thermostat set point, the thermostat terminates the call for heat.  When this happens the blower and burners go through a shut off sequence and the furnace awaits the next call for heat. 
In the event the air flow is compromised due to duct restriction, obstruction or similar condition, a main limit circuit, incorporating an air temperature sensor, extinguishes the flame to prevent excessive furnace component temperatures and duct system temperatures.  Upon reactivation of the main limit circuit, the unit initiates a new cycle and re-ignites the flame.  On downflow or horizontal furnace applications where the filters are located above or parallel with the heat exchangers, a second switch is often incorporated to prevent the filter temperatures from rising excessively in the event the blower fails to operate.  The second switch is often a manual reset type switch which prevents reactivation of the safety circuit until the switch is manually reset at which time the underlying fault is corrected. 
While the second manual limit switch works well, it does involve additional hardware and associated wiring that adds to the expense of furnace construction and manufacturing.