Central heating header tank is boiling

[t955i]

My central heating header tank is full of boiling water even though the the rads where cold, the thermostat was set low, this seams to have been doing this for some time as the attic is damp and humid any ideas

[Drusilla]

I believe the thermostat in your boiler has broken. I know this happened to my ex husband and I when we moved into our first home together. Sadly, I didn't pay as much attention to him as I now wished I had. I can hear you screaming, 'Typical!!'

[stanleyman]

Without more information about what boiler and how it has been running this could be the thermostat failed but usually there is a limit stat fitted to prevent this situation happening. It could also be a failed pump or valves, blocked pipe or poor design allowing water to pump up to the tank.

[stanleyman]

Typical!

[LATECOMER]

t955i . Not enough info to give clear answer. Could be your three way valve is jammed on the hot water programme, so when the pump turns on rads stay cool but hot water is pumping into header tank. Some jobs really DO need an expert.

[t955i]

Hi futher to the info about the header tank, the boiler is about 15 years old and is run by LPG, the rads and hot water function perfectly ok the room stat seems to work ok but i dont know about the one on the boiler, i instatlled this system myself about 15 years ago it has been regularly serviced by a corgi reg engineer, one thing i have found is the one rad is cold could it be the valve on this is sticking and causing the water from the rads to expand and empty into the tank, although the overflow does not drip any ideas thanks

[stanleyman]

If the system has been working ok for fifteen years and still appears to be apart from the hot tank, it could still be the boiler thermostat or even a slight alteration to the fill and vent pipework or pump speed can make the tank act as a radiator. One rad not working shouldn't make a difference. You should be able to turn the boiler off with the boiler thermostat throughout its range.