Control techniques for shut-off sensors in fuel-fired heating appliances

In a flammable vapor sensor-based shut-off system of a fuel-fired water heater the sensor resistance output signal degradation caused by aging of the sensor is automatically compensated for using an operational timer having an output signal indicative of the total life of the sensor subsequent to its installation on the water heater. In one embodiment of the system, a resistance adjustment signal having a magnitude related in a predetermined manner to the timer output signal is created and added to the sensor resistance output signal. In another embodiment, the timer output signal is used to appropriately adjust the minimum-maximum received sensor signal magnitude range, based on the installed age of the sensor, which will preclude combustion in the appliance. Additionally, in each embodiment thereof the system is provided with an improved initial minimum-maximum sensor signal magnitude range. Other types of shut-off gas sensors may be alternatively utilized.