Central heating - water boiling

[kevinbecken]

I have a Myson boiler. The water boils, the overflow pipe make a noise like thunder as hot water overflows into the header tank in the loft with a tremendous gush. The overflow pipe is very hot to touch, and the water in the header tank is hot and steaming. Is this a boiler thermostat problem? The boiler does this after a couple of hours of being on for HW and CH and is up to temperature. When I turn the temperature control on the boiler front panel down to off, it stops. The pump is working. What's the likely cause, and is it a DIY job to fix it? Thanks Kevin

[gasman pete]

certainly sound like boiler stat failed pete

[Alex]

On the side of your hot water tank you should find a thermostat held onto the tank with a spring wire,this is likely to be your problem.

[Roger Mills]

What happens if you turn the temperature control down but not off? If the boiler keeps running, the thermostat is almost certainly duff.

The thermostat is actually a pressure switch, connected via a capillary tube to a bulb containing a liquid, which is plugged into the side of the heat exchanger. The switch responds to changes in vapour pressure from the liquid which depends on temperature. If the bulb or tube leak, they no longer produce any pressure - so the stat doesn't operate. Is there any sign of leakage? The bulb needs to make good thermal contact with the heat exchanger - usually using a conductive paste. If this has dried up, or if the bulb has come out of its socket, that would also stop the stat from working properly.

However, it's probably more likely that the electrical bit of the stat has failed.

[Ed Sirett]

Is this a boiler with a dial thermostat or simply a high/low switch? Either way the thermostat in the boiler has gone. This model clearly doesn't have any sort of over-heat stat.

[Andrew Gabriel]

As others have said, sounds like boiler stat has died.

If the loft tank is plastic and becomes very hot, beware it might soften and dump a load of boiling water through the ceiling. Someone was killed when this happened to their water storage tank.

[kevinbecken]

Thanks everyone. I'll check all the things you said, but the likely cause is unanimous. Can anyone recommend a good place to buy a replacement? Cheers Kevin

[kevinbecken]

The thermostat control is a dial. There's a little orange light next to it that is off when running normally. A couple of times the boiler has shut itself down and the light comes on. The two wires disappear into two tubes and held in place with k clips. Certainly seem very firm in there.

[John Stumbles]

Less seriously but more probably the plastic float on the cistern valve may pack up and lose buoyancy causing continuous overflow from the tank (and maybe through the ceiling if the overflow pipe system isn't well constructed).