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

On 1 Mar, 22:29, wrote:
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

certainly sound like boiler stat failed
pete

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.

In an earlier contribution to this discussion,
wrote:

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

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.
--
Cheers,
Roger
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On Thu, 01 Mar 2007 14:29:59 -0800, kevinbecken wrote:

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

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.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

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

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.

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).