mobilehomerepair.com

so here's the issue, I have an older Intertherm Model MGH-065A gas heater (furnace?), regardless of whether the ambient temperature dictates the need (70+), and the thermostat turned down to 50, the heater comes on, the only way to resolve this is to turn off the heater with the switch inside the enclosure. I would really like to not have to turn off the heater outright since we still get random cold nights. the thermostat in use is a Honeywell CT30A1005 I believe.

any suggestions on what/how to resolve this??

btw the thermostat is an non-mercury type unit, though I did make sure that is was level, I have also thoroughly cleaned both thermostat (wasn't really dirty at all) and the heater control box (removed cover) as well as the heater cabinet.

When this happens is the burner actually firing?

yes, heater actually turns fully on

Some furnaces have a control board which could go bad and cause weird things to happen, like the blower kicking on & off

If you can get your hands another a used working thermostat or a new cheap one, I would suggest trying that first just to eliminate the Thermostat. Do you ever here the Thermostat click when the heat randomly turns on? That's where I would start I've come across a lot strange problems with thermostats being the culprit.

Pick up a cheap heat only tstat and wire that into just the furnace. Eliminate the original tstat and all it's wiring. See what happens. Sounds like either a varmint chewed through a wire and when they touch the circuit completes turning on the heat or a screw was run through the original wire and now years later the problem shows itself.