On Fri, 02 Mar 2007 19:33:10 +0000, Andrew Gabriel wrote:

In article om,
writes:
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?

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.

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).