Fuel gas control

A furnace system comprising:

• a fuel gas control for supplying fuel gas to a furnace, the control comprising an inlet chamber having an inlet for receiving a supply of fuel gas and an outlet chamber having an outlet for providing a supply of fuel gas to the furnace, the inlet and outlet chambers being connected by a valve opening;
• a valve operatively associated with the opening, the valve having a closed position for preventing the flow of fuel gas through the opening and an open position for permitting the flow of fuel gas through the opening;
• and control chamber means for moving the valve between its open and closed positions in response to pressure variations within the control chamber means;
• a combustion air blower for supplying combustion air to the furnace for burning the fuel gas to form combustion gases;
• an exhaust stack for exhausting combustion gases from the furnace;
• a venturi nozzle in the flow of combustion gases;
• and
• means for directly connecting the control chamber means to the venturi nozzle so that the valve is positioned in response to the pressure signal generated in the venturi nozzle by the flow of combustion gases and for moving the valve to its closed position under blocked exhaust stack conditions.