Boiler Safety Thermostat

pappyirl

I have an external boiler which is constantly giving problems . Rats have chewed wires, pump replaced three times, new thermostat, constant bleeding etc. This was last winter and eventually got sorted.

The latest one for the new cold season was a leaking overflow valve. This was replaced, only after alot of water was lost in and around the boiler.

The latest problem is the safety switch on the newish thermostat and is connected to the burner which keeps tripping. There are two cooper prongs from the thermostat which connect into the boiler and check the heat of the stack. Why does the safety switch keep tripping. If the stack is overheating, can it not turn off the burner without tripping this switch?

Any help would be appreciated.

JamesM

If it is not faulty, it is tripping because the temperature is going above 90 something degrees. Do you have the temp setting too high ? It should only be about 60, or 75 at the most.
If your pipes go straight from the boiler down into the ground, there will be no gravity flow away from the boiler and the temp in the boiler will shoot up if the boiler has been running when the timer is switched off. So if the temp is set for 90 degrees, it will go higher when the circulation stops and the safety stat will pop out. You could fit a pipe stat bypassing the timer, sending mains power to the pump, and sitting on the flow pipe out from the boiler. It could be set to keep the pump running until the water has cooled down to about 60degrees.
Maybe you just have a faulty limit stat
Jim.

pappyirl

James M,

Thanks for the reply, you seem to be keeping the DIY heating section going single handed.

The pipes do go into the ground. The stat was originally set at 75. I increased it yesterday to 90 to see what would happen. What seems to be happening is the hot water cylinder is heating up rapidly internally. The radiators do not heat at all. The stat trips and the overflow from the attic starts to flow.

It appears the boiler is heating only the hot water cylinder and not the rads. This would account for the stat tripping and the attic overflow, the hot water has nowhere to go.

Is it possible that when the valve was replaced either the radiator system did not re-pressurise or the whole thing is air locked. The leaking valve was more like a locked open valve. A huge amount of water was released from the system.

Thanks for the help.

JamesM

pappyirl;57456343]James M,
Thanks for the reply, you seem to be keeping the DIY heating section going single handed.

Burners are what I know, but I have some experience of the full system.
In your first post, you are talking about the safety valve. It can just leak, but it also blows when the temperature in the boiler goes too high. It has now been replaced.
When you turned the stat to 90, you boiled the water in the boiler, which tripped the stat and pitched up into the header tank, which then overflowed.

The pipes do go into the ground. The stat was originally set at 75. I increased it yesterday to 90 to see what would happen. What seems to be happening is the hot water cylinder is heating up rapidly internally. The radiators do not heat at all. The stat trips and the overflow from the attic starts to flow.
It appears the boiler is heating only the hot water cylinder and not the rads. This would account for the stat tripping and the attic overflow, the hot water has nowhere to go.

The stat tripping and the overflow is caused by you turning the stat too high and nothing else. Leave the boiler stat at 60 until you get everything working properly again.
If all the pipes go down into the ground and the hot water still heats in the cylinder, but not the rads, then there is probably a stat in the house which controls a motorised valve on the pipes to the rads. this valve may not be opening. Or a huge amount of air in the system.

Is it possible that when the valve was replaced either the radiator system did not re-pressurise or the whole thing is air locked. The leaking valve was more like a locked open valve. A huge amount of water was released from the system.

You do have to make sure that the system was fully refilled after the safety valve was replaced - bleed all the rads and see if there is air in them. If there is water in the header tank, there should be enough in the system - make sure that there is not a valve turned off on the fill pipe down from the header tank.
Jim.

JamesM

Another point : If the system was not filled fully after the safety valve was fitted there could be air at the top of the boiler. This would cause the limit stat to cut out, because there would be no water around the thermostat pocked. The water would boil and the stat click out. There are different types of safety valves. With the most common, you can twist the top of the valve and release the air until water comes out. If there is a lot of air here, this is most likely your problem.
Jim.