Heat doesn’t shut off

bakeer218

I have a Burham steam
boiler that doesn’t shut off one it reaches the set temperature on the wall thermostat . I have tried replacing wall thermostat but still having the same issues .  This does not happen all the time . The only way to shut furnance down is to crank temperature setting up then bringing it down to set temp on thermostat. Anyone have experience with this happening .

thank you 

EdTheHeaterMan

That sounds weird. Turn the thermostat up then back down to get the burner to stop.

When a thermostat calls for heat a switch inside closes contacts to complete an electric circuit. This makes the gas valve (after a normal ignition sequence) open to make a flame in the boiler. Once the thermostat is satisfied the switch contacts open to break the electricity to the gas valve. If the gas valve stays open with no electricity power, then that could be quite dangerous. In my 40+ years of HVAC work I have only found one "Run Away" gas valve. This is rare. the next time it happens, go the the boiler and tap on the valve to see if it is stuck open. If the tap shuts off the gas, then you need to replace the gas valve.

@Jamie Hall Said
"What boiler and burner and burner control do you have? There is a failure mode in some burner controls which is actually quite dangerous, and we need to figure out if this is happening..."

Agree with this!

Jamie Hall

Slightly odd. Let me make sure I have this right: thermostat calls for heat, and the boiler comes on. Temperature rises and the thermostat reaches the set point and stops the call for heat (you are quite sure that the thermostat is no longer calling for heat -- this is important). Boiler keeps running and the temperature keeps rising. You raise the thermostat setting well above the current temperature, boiler still running -- and now the thermostat is still calling for heat. You now turn the thermostat back down -- and the boiler stops.

And this same behaviour occurs with two different thermostats.

Have I got it?

Now the next question, which you may not have observed. Does the boiler shut off when it reaches the pressure set on the pressure control?

What boiler and burner and burner control do you have? There is a failure mode in some burner controls which is actually quite dangerous, and we need to figure out if this is happening...

Please get back as soon as you can.