Hydroflame BRC 10A - Heat won't shut off

[Fiberglass Gregg]

This is a pretty basic gravity furnace in a 77' Boler. I've been using this heater for years with no trouble. Until this past week. The furnace lights with no trouble, but now the heat will not shut off. The little white temperature control knob on the left does not appear to be regulating the temperature. I can turn it to 'low' but the heat comes on full and never stops. There used to be a little bit of resistance in the knob; so when it got too warm, and I'd turn down the heat and just before it shut off, there was a little 'twang' inside -that is now gone. I've pulled the knob assembly apart, and all the rubber bladders appear to be in excellent condition, and all the machined parts are in good order. I wonder if perhaps the copper piece that comes from the external temperature sensor is worn out ? At any rate, apparently, I am the first person to have this happen, because I cannot find any information about it on Dr. Google. Anyone have this happen before? I am not really sure what part has failed, so I wouldn't know what to order even if I could find a supplier. I should mention that I do a lot of back country boon-docking with it, and it can take some rough thumps on those back roads--maybe something else broke inside. Thanks for reading.

[Dudley]

Check that copper bulb and tubing running to it for wear points. It is filled with a gas and if it leaks, its ruined. Sounds like its no longer sensing the temp change or the gas valve is not working. They were used in tent trailers and truck campers from that time frame and should be showing up on Kijiji or craigslist for cheap. Parts from a Coleman gravity furnace MAY also work but not sure.

[Fiberglass Gregg]

I wasn't aware of it containing gas. I was trying to understand how the warmth of air could expand the brass inside the control set enough to switch off the flow, but yes, now I see that the gas inside would do that. I'm pretty sure you are right, and you have have confirmed my suspicions that is likely the part in question. Thanks very much...now I just need to find that part.

[Dudley]

Hopefully it available separate from the valve assembly.