(57) A gas boiler has a thermistor 12 located in a passage 51 opening into an inner enclosure
80 containing the burner 1, heat exchanger 4 and the inlet of an induced draught blower
5. Reduced pressure in the enclosure 80 generated by the blower 5 draws air along
the passage 51 over the thermistor 12. A comparator 17 receives the output of the
thermistor 12 and that of a second thermistor 13 protected from air flow. The output
of the comparator 17, together with outputs from other safety sensors 26, is connected
with a logic unit 30, which controls operation of the burner gas supply valve 3. If
there is a blockage, blower failure or the like, air flow over the thermistor 12 drops
causing a change in output of the comparator 17, which causes the logic unit 30 to
shut off the gas supply valve 3.
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