I posted a reply about a problem with my recently installed loading unit in a previous "dump zone design "thread. I thought the thermostatic valve stuck, causing no circulation thru return protection line or supply and return and resulting in an overheat condition. AS USUAL, I should have looked before I typed. Dissassembled the valve last night. No way a stuck valve would have caused lack of circulation. The unit is very well constructed (no sheet metal). Threre is brass ball on a temp sensitive spring. It allows flow either one way or another or both, so no way it could prevent any circulation. The only thing I think it could have been is an air lock at pump or some small piece of stuff in pump rotor. If there was a stuck themostat on this unit, it would allow only return protection circulation, or supply and return circulation. After looking at it, I'm more comfortable with the safety of this thing. This is a very nice piece of equipment, easy to service and well constructed. It's expensive, but is all brass, includes 3 unions, 3 shutoff valves, pump and thermostative valve.