It seems that the most common variant is a cadmium sulfide photoresistor. I found a spec sheet for one that's commonly used in oil furnaces, the Honeywell C554A. According to the spec sheet, it has a resistance of over 20,000 ohms in the dark, which drops to less than 1600 ohms in the presence of a flame. How you would use this device depends a lot on what sort of meter you intend to use. A problem with this particular device is it's only rated for 60 volts peak or about 40 Vrms (clearly intended only for 24 volt controls). But I suspect the ones used in torpedo heaters can handle 115 volts, if that's what you need to do.
You also need to find an hourmeter. Hobbs makes a 115V, 60 Hz hourmeter that draws 3 watts, which works out to be 26 milliamps. Put a CdS sensor similar to the C554A in series with a Hobbs meter, I think it will work. However, a solid-state meter with a higher input impedance would be a better choice, you could use a potentiometer to adjust the sensitivity of the sensor.
Obviously the exact components you use depend on what power you have available. Many systems like yours have 115 volt controls, while others use 24 volts. Of course you can always add a transformer to drop the voltage.
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