I purchased an older home about 3 years ago with radiant heat in the ceilings. Recently, one of the bedrooms had the temperature spike al the way up to over 90 degrees. I can't seem to get the temperature to come down. The heat is controlled by a Mears single pole thermostat, model M401. The thermostats are from the late 60's to early 70's, but I have never had any trouble with them before. They do no have an "Off" setting, just a temperature range area and a "Low" setting so I can't just shut that room off.

The only thing out of the ordinary that has happened recently is that I spackled and sanded in that room. Is it possible that the electric sander shook something loose in the wall or on the thermostat itself? I don't really know how this stuff works and am looking for any help I can get. Your help is much appreciated.

Sounds like the thermostat is bad. Just because you haven't had trouble with them doesn't mean that it hasn't failed.

All of your other suggestions must also be considered. This is a job for someone with a volt-ohm meter and some knowledge of electricity.

Thanks for the reply. I kind of had the feeling that the thermostat might just have gone bad. It is about 40 years old after all. I went out today and bought a new thermostat to replace it. I found the same kind of thermostat in an updated model that actually has and "off" setting as well. When is pulled the old one out I took the back off it and there was a small area that looked like an electrical burn on the "activator" switch. So it looks like it was just at the end of its life.

Thanks again.