Yeah I can explain, I have a small bronze circ going 24/7 (Taco 003 I think) to take hot water off the top and return it to the bottom to even out the temps in the boiler. I have found it stratifies somewhat and the sensor is on the top naturally. It would not turn on until every bit of hot water was gone, so I made sure it is full of hot water. At any rate, this 3/4 copper return line was about 1 inch from the cold water supply to the domestic coil. It melted from both sides probably as the coil was full of exceedingly hot water also. It was so hot I had to tighten up the metal crimp rings on the pex tubing on both sides of the coil.
It would appear now that the malfunction was not the valve but the cube relay that controlled its operation. I can hear LSFarm now! Damn Rube Goldberg device using switches and relays that go bad!
I made a bunch of changes while it was drained down and I am going to add a backup plan. Still working on that. Probably include a Honeywell aquastat as I have not been happy with the Johnson controls.
Thanks for the suggestions, I'll surf fleabay and pic up something to play with.