worcester 240 combi boiler pressure problems

hi there i have a worcester 240 combi boiler that is blowing its pressure release valve at just over 3 bar

hot water is all ok
the boiler pressure when cold is always and has been 1 bar

it does not loose any pressure, only when the boiler is in central heating mode does the pressure keep climbing until it blows the prv

have read the faq's/search pages so i know its something to do with the expansion vessell pressure(big red tank)

when i tested the inner value on the vessell there was No water coming out from it just a little bit of air----so i think i need to put some air in with a foot/bycicle pump.

but how much do i put in please if my needle is set to 1 bar on the gauge??

do i need to drain the boiler first of the water in it, so pressure is at zero then pump up the expansion vessell???

what is the correct and safest way

PS
i am a qualified electronics engineer and have fitted a diverted value and overheat thermostat and a fan unit in the last 9 years,so used to taking it apart,but want to make sure i do it correctly

step by step help would be very helpfull if someone could take me through it

many thanks in advanced

boiler must have no water pressure in to get correct reading whe pumping up ex vess to 0.75 bar

milkshaker said:

do i need to drain the boiler first of the water in it, so pressure is at zero then pump up the expansion vessel???

Click to expand...

Yes. The outlet to water of the expansion vessel should be at 0 bar. Them use a bicycle pump (schraeder valve) to pump up the air side, checking it with a pressure gauge (as used for tyres).

I think normal precharge pressure is 0.8 Bar, but it should be written on the vessel or in the boiler instructions. If the vessel loses it's precharge pressure soon you may need a new expansion vessel.

thanks for the very quick replies guys, just one point

i haven't got a car tyre pressure gauge, so do i just look at the pressure gauge on the boiler and pump to .75/.8 bar from the needle position on the gauge or do i need a proper tyre pressur gauge---need to be clear in my own mind exactly what to do

cheers

The pressure gauge on the boiler should not show any pressure. If it does rise when pumping, the EV diaphragm is broken and you will need a new EV. you must use a tyre type pressure gauge (as I said already). It's a pain to do, but that's why we earn such big bucks.

thanks for all your great advice, i did what you told me to do,drain boiler and pump back up to 0.8psi

put system back up to 1 bar as normal,ran central heating with all rads at max settings on the minium setting on the boiler and it went up to about 2-2.5 bar, so increased temperature setting on boiler to middle position(no 3)as i got to 3 and over i switched the boiler off as still rising

so with is still at a high pressure i turned off the mains and pressed in the schraeder valve slightly and instead of air coming out of it water was coming out instead.

So i'm 99% i need a new EV,if so where are the cheap places to buy them from on the net and when i get the new(if thats what i need) will i have to pump it up again or will it come ready pressurized??? so i can just pop it in

my boiler is a worcester 240 and looking in my manual the GC part number is 378 027 or the whs part number is zmmis082

if any of you have canon copiers/printers/scanners and you need advice on how to fix them just give me a shout as have 20 years experience, but sadly not in boiler repairs;each to there own profession

Easiest and cheapest thing is to fit a new generic expansion vessel to the pipework near the boiler. The 8 litre vessel will be about 400mm long by 300mm diameter, bright red, so should look good in kitchen. Try BES site for vessels.

sorry for delay in getting back to you all as have been on my hols
the boiler now works great with a new prv which was dripping and a new ev

many many thanks----what a great site this is----and fantastic people to help us out when we need it

very very great full