My buddy and I were brainstorming about how to sense an outfire when the 520 stoker boiler is used to circulate to another boiler. In this particular case, the gas boiler would automatically come on and take over when the stoker had an outfire. The problem was that if the gas boiler was making hot water and circulating through the coal boiler during an outfire, an L6006 wouldn't sense the outfire because the water in the pipes would remain hot at the stoker.
So, we're going to experiment with an L6006C, strapped onto the burner pipe, either inside the base or just outside the base, wherever it could sense a drop in temperature, and not be damaged by excessive heat. In my own case, with a Modine heater, I used an L6006A, with the bulb, and taped the bulb directly to the pipe and insulated around it. I'll report back once we try it.
I wonder if this same set-up wouldn't also work with the AF-150 stoker furnace.