The present invention generally relates to fuel-fired heating appliances and, in a preferred embodiment thereof, more particularly provides a fuel-fired water heater having A specially designed combustion cutoff system operative to prevent (1) the creation of an unacceptably high level of carbon monoxide in the combustion chamber of the water heater and (2) thermal damage to a PVC vent pipe to which the flue portion of the water heater is connected. 
It is now a common practice in the water heater industry to install a flame arrestor structure at or near the combustion chamber portion of a fuel-fired water heater, the flame arrestor being operative to permit combustion air to enter the combustion chamber but inhibit combustion chamber flame outflow through the arrestor in the event that extraneous flammable vapors are ingested into and burned within the combustion chamber.  Flame arrestors are potentially susceptible to blockage caused by external contaminants such as lint, dirt and oil.  If significant enough, such blockage can create undesirable xe2x80x9csourxe2x80x9d combustion within the combustion chamberxe2x80x94a phenomenon caused by an increased level of carbon monoxide within the combustion chamber.  To shut down the water heater prior to the point at which such sour combustion occurs, various designs have been previously utilized in which a sensor is positioned within the combustion chamber, the sensor being operative to sense burner flame temperature increase or instability and responsively terminate firing of the water heater prior to the creation in its combustion chamber of an unacceptably high level of carbon monoxide. 
In fuel-fired water heaters having associated draft inducer fans coupled to PVC vent pipes, another design criteria that needs to be satisfied is the prevention of thermal damage to these meltable plastic vent pipes caused by excessive temperature in water heater combustion products internally traversing them.  This has previously been accomplished by installing in the draft inducer fan a thermal device which is operative to shut down the water heater prior to the temperature of the combustion products traversing the vent pipe reaching an unacceptably high level. 
Thus, at least two separate sensorsxe2x80x94one in the draft structure of the water heater and one in its combustion chamberxe2x80x94have previously been necessary to protect the PVC vent pipe from thermal damage and prevent excess carbon monoxide levels from being created in the combustion chamber.  This undesirably increases both the complexity and manufacturing cost of the water heater.  It would thus be desirable to provide these protective features in a simpler, more cost effective manner.  It is to this design objective that the present invention is primarily directed. 
In carrying out principles of the present invention, in accordance with illustrated embodiments thereof, a fuel-fired heating appliance, representatively a water heater, is provided with a specially designed combustion shutoff system which substantially prevents thermal damage to a draft structure portion of the appliance, and also substantially prevents the creation of an undesirably high concentration of carbon monoxide within the combustion chamber of the appliance during firing thereof. 
In illustrated embodiments thereof, the fuel-fired water heater, which may be either a natural draft or power vented water heater, basically comprises a tank adapted to store a quantity of water to be heated, a combustion chamber positioned beneath the tank, and a fuel burner disposed within the tank and being operative to burn received fuel and combustion air and responsively create hot combustion products in the combustion chamber.  A valve is coupled to the burner and is operative to permit and preclude the supply of fuel thereto, and a passage is provided through which combustion air may be supplied to the burner. 
A flue extends from the combustion chamber, and through the interior of the tank, and has an outlet.  Coupled to the flue outlet is a draft structure which includes a vent pipe, the draft structure being operative to create a draft through the flue to facilitate discharge of combustion products from the flue outlet and through the vent pipe.  The water heater further comprises a draft structure which is coupled to the flue outlet and includes a vent pipe, the draft structure being operative to create a draft through the flue to facilitate discharge of combustion products from the flue outlet and through the vent pipe. 
In accordance with a key aspect of the invention, cutoff apparatus is provided which is operative to sense a parameter, preferably temperature, of combustion products traversing the draft structure and, in response to a predetermined magnitude of the parameter, prevent the creation of an unacceptably high level of carbon monoxide in the combustion chamber, and thermal damage to the vent pipe caused by an unacceptably high temperature of combustion products traversing the draft structure, by terminating further firing of the water heater, the magnitude of the parameter being correlated in a predetermined manner to both the level of carbon monoxide in the combustion chamber and the unacceptably high temperature of combustion products traversing the draft structure. 
From a draft structure standpoint the water heater may be either a power vented water heater or a natural draft water heater.  In illustrated embodiments of a power vented water heater the draft structure includes a draft inducer fan coupled between the water heater flue and a PVC plastic vent pipe, and the cutoff structure is a thermal switch disposed within the draft inducer fan.  In response to the combustion product temperature within the draft inducer fan reaching the set point temperature of the thermal switch, the switch outputs a control signal which, in one embodiment of the water heater, is used to terminate water heater firing by preventing further fuel supply to the burner, and in another embodiment of the water heater by preventing further combustion air supply to the burner.  In a natural draft embodiment of the water heater, the thermal switch is positioned within the natural draft structure of the water heater adjacent the PVC vent pipe and may be similarly utilized to terminate firing of the water heater by shutting off either fuel or combustion air supply thereto.