Scale in coils or thermostat?

Hey John,
We have a hydrotek unit that has a diesiel fired burner. Burner is working but heating the water to a very high temp and then shutting off. Burner kicks back on after water gets cold. I am thinking thermostat or coil scale. Any other ideas or which is the most likely cause?

I ain't John, but, hey...

If you take the outlet fitting off the coil, do you see a scale build up? You say "very high temp"...is it steam? Or just very hot? Does your machine have steam capability? Anything else changed lately? New nozzle (possibly wrong size), pressure dropped off a little? Does the burner smoke black at all? Could be the thermostat seeing as how it cools completely before refiring. I'll help all I can.

Thanks for the reply. it is actually a backup unit and has been doing it for a while but we would use it for cold water applications or something where I didnt need heat all the time. However, I would now like to get it fixed while things are a little slow. The burner fires and runs nice and clean. Scale build up is not visible. The machine is supposed to steam if you use the steam nozzle (temps to 250). Not sure how hot it is getting but near steam. I dont really run it enough to tell if the flow has decreased much. Seems to me that it is putting out very close to the 5gpm that it is supposed to. Pressure is good. I think I am going to put in a new thermostat and see what happens. If it doesnt work after that than I dont know.