Boiler pressure

26 Jan 2010
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United Kingdom
A couple of weeks ago my boiler cut out completely (Worcester junior 28i combi installed around 4-5 years ago). The fuse had blown on the circuit board, after getting a corgi guy in he found the fan was to blame and duly replaced the fan which seems to work fine and the fuse is OK.
Problem is, since then when I turn the boiler on for heating, it will ignite, heat the rads for about 20 mins, during which time the pressure increases to the point where it vents outside and then the boiler cuts out. As a result I lose some water from the system. I then have to top it up, reset the boiler and this cycle starts again if I want heating. No good at all I know. Before the repair the system was absolutely fine regarding pressure. I can't even see how replacing a fan can cause this, is it co-incidence?

Cheers all for any info and help.
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coincidence, your expansion vessel probably needs recharging, see faqs
First things to check is:
Is the filling loop left on (see boiler manual or silver hose or info on that) not alot of chance in that.

IF the pressure rises alot and dumps outside this can be 2 things, 1 he removed a stat on the pipework which is causing the boiler to over heat as the boiler gets hot water expands and the pressure relif valve dumps water outside.

2. The pressure vessel has gone and will need a replacement as a gauge rise as the boiler gets hot is normaly a sign of this.

As you say the boiler runs for 20mins and then cuts out and dumps water i would advise you to switch the boiler off or only use in an emergency and get a installer out to check as its over heating or if he changed the fan did he correctly commisson the appliance gas/air mix as required.

I would strongly advise getting someone out asap and advise them of the problem the work done on the appliance and the fault.
For info on the air gas mix check the manual i think on the junior books its page 46.

twgas could be right but i would personaly advise you get it seen to asap by a engineer to ensure its working correctly if number 1 and 2 are not related to your fault.
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There was no checking of mixture after the fan was changed, i'll try the method on the faqs page for pressurizing the PV.

Thanks for the info all.
Thanks twgas and all, the pressure vessel was re-pressurised to 11 psi, after topping back up and bleeding rads, pressure now reads 1 bar when cold and between 1 and a 1 and a 1/2 when hot, there is now another problem though. If the heating is put on, it runs at normal pressures but cuts out after about 3/4 to an hour. I then have to switch it off at the fuse spare, switch it back on and then it re-ignites and all is well for approx the same period of time and then the cycle repeats of me switching it back on again.

My plumber suggested a new pcb, but I feel its an expensive item for a guess even if it is an educated one.

Any Ideas? :(

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