Troubleshooting a Boiler That Won't Shut Off

A white boiler in a basement.

Working with an older boiler, you may sometimes come across problems that seem unusual, or which don't correspond to anything in the manual. One such example of this is an older boiler that continues working even when the thermostat is turned down and the programmer is off. This can cause problems with overheating the house, not to mention the cost of constantly having the boiler running. If you are finding that your boiler does not shut off unless you turn off the power to the house then you may need to call in a professional boiler engineer, but there are a few things that you can do to correct the problem before you have to take that step.

Check the Thermostat

Once it becomes clear that your boiler is heating the house when it isn't necessary, you should turn the thermostat down. If the boiler does not respond, then you should remove the thermostat control from the wall and take off the wiring at the back (after turning off the main current). Place the thermostat face down on a counter, and open the back. You may find a blown fuse, loose wires, or other obvious faults that can be repaired. Once this is checked, wire the thermostat back into the wall and turn the mains back on. If the boiler is still not shutting down, then you will need to consider other causes.

Examine the Programmer

The next area to look is at the programmer. This controls the operation of the boiler, even overriding the thermostat. Turn off the power again, and remove this from the wall. Check for loose wires, and replace the fuse. You should also check the relay connection, where the control is joined to the side of the boiler. This can come loose, or the relay itself can be damaged. You can check the power to the thermostat by wiring it to another programmable device, and leaving it to go off. In the meantime, check the rest of the boiler.

Look at the Valves

Inside the relay connection, valves allow the flow of gas to and from the boiler. If you are having trouble turning your boiler off, then the problem could be that these valves are stuck in the “on” position. Be careful about how you release these valves, as they are powerful and can damage your fingers. If, upon examination, the valves seem to be stuck, you should call in a professional to remove the system and fix the problem.

Check the Wiring

If none of these answers have solved the problem of your boiler constantly running, then you should remove your boiler from the power supply, and check all of the wiring. Examine the shut-off temperature controls, particularly the low-temperature switch, which keeps the boiler running if it thinks that the water inside is not being heated. You should also check the temperature of the water. If the two devices don't match, try removing the control switch, and then retesting your boiler. Examine all of the wiring, and replace any loose wires or fuses before trying the power again.