Heating 'boiling' & header tank overflows

[ChrisMedion]

Every few days - when the heating has been on for a few hours - all hell breaks loose. There are banging noises and water floods out of the header tank overflow. The header tank water is hot. The radiators and pipes also seem very very hot. 23 yr old Flamingo Boiler & no previous problems. Water feed not blocked (can drain radiators at high speed whilst header tank refills). System can go for days with no problem and boiler stat seems to work (it does switch boiler on and off although rads etc still seem very hot on low setting. Pump turns and seems OK. I wondered if thermostatic valves had all closed causing pump to force water down overflow but that's not so on latest episode (I have removed 3 of the valve tops to ensure rads are always open). Motorised valve seems to work OK - it cuts heating off when up to temp. Once problem happens it recurs until I get all air out of system (seems pump can run "dry" after an episode causing same problem again). Can take a while to get air out again. But what is causing it in the first place? Always happens only after system be running for a couple of hours. Any ideas gratefully received! Thanks

 

[brumylad]

sounds like boiler stat. (the one in the boiler not one on the (living room ?) wall )

Do you have a thermostat on the hot water cylinder.

Do you have a room thermostat.

 

[ChrisMedion]

Thanks for the reply Brumylad. I do have a h/w tank stat and a room stat (dining room). Room stat seems to correctly operate the single motorised valve. h/w stat seems OK (it clicks and if 'off' - and heating off - boiler does not operate).
1) I could try replacing the boiler stat - its just that it seems to go for days on end without a problem - and the boiler stat certainly turns the main burner on an off it you turn it up or down. Is it possible that the boiler stat can have this sort of intermitent problem?
2) Or is it perhaps that - normally - the tank and room stat effectively turn off the system before it can overheat & the biler stat is the culprit?
3) Its just odd that the main burner on the boiler does siwtch on and off as I would expect when the other stats are calling for heat - its not as if it stays on 100% of the time.
What do you think?
Thanks

 

[ChrisR]

Gravity HW or fully pumped? if FP pump sticking could make it boil if stat marginal.

Better with boiler stat turned down?

 

[felix]

It is possible for a boiler to overheat even though everything else is working. It's a simple case of the pump not carrying heat away as fast as the burning gas is putting it in. That's why the boiler has its own stat. The question is, does the thing always work when it's needed?

You can test this next time all hell breaks loose. Try turning down the boiler. If you get no click (or even if you do) and the gas stays on then either your boiler stat is a dud or the gas valve is jammed open. Test the gas valve theory by turning off the power instead. You should have a switch somewhere that isolates all the heating system electrics.

 

[ChrisMedion]

Chris R - Thanks. It is a fully pumped system. I turned boiler stat down and got rid of all air a week or so ago and though that fixed it - but I still got the problem tonight (boiler stat at '2' (of 5). Turned stat competely off for a couple of hours and pump has circulated the water no problem (and no great sound of air). Put heatng back on 1/2 hr ago and all quiet at the moment.

Felix - Thanks for your suggestions. The way I stop the whole thing when it goes pear shaped is to turn off the main heating electirc switch and that does the trick. From what you say I presume this means its not the gas valve stuck open.

Maybe I'll get a new boiler thermostat tommorrow and fit it and see if that helps - or is there anytrhing else worth tyring first? At least a stat will be a lot less than a gas valve!
Thanks to you both - Chris