when hot water or central heating are running bar pressure rises over 3 bar and overflow active .
when unit lazy bar pressure returns to normal pressure 1 bar.
have tried isolating cold water feed makes no diff

vw, October 2008

I need to top up or fill the expansion vessel on the 20/25 heat slave ,the expansion vessel is due to be replaced but at moment need to add more water as it is too low, I cannot turn or move the black tap on top of boiler as it seems to stiff or fixed I think it might break as is plastic

phyl H, September 2016

Yes. We've been very pleased with the boiler so far.
Now, when the boiler comes on not all the radiators heat up. We've bled them all but hardly air escaped. To heat up the downstairs we have to turn off all the upstairs rads.
Do you think this could be a worn out heat pump? You can hear it working but maybe its not working properly.

Jane, January 2015

I have no pressure on my heat slave and my heating isn't coming on. The power light is on but not a lot happening. Had to bleed all my rads as they were bubbling like made last night. I turned off most rads around the house because it was so noisy, I have re opened them all to full today and getting no noise or heat! Help please?

Ali, April 2013

expansion vessel flat, damaged or blockage to it.
drain and re-pressurise.

brett, January 2009

erm... sounds more like expansion vessel to me, (rectangular red thing on the front of the boiler) there is a Shrader valve on the top of it like a car tyre valve, press it, if water comes out, you need a new one. I do know, i have a garden full of these and i have 1 as my first job monday morning.

the diverter valve on these boilers are always open, even if it was blocked/broken, the pressure would never rise over 3 bar in a million years as the return pipework would take back the pressure.

You could swap it yourself easily, isolate mains power, allow boiler to cool, drain the pressure off, remove boiler front and top case, when pressure is zero, with the new expansion vessel nearby, undo the flexi pipe and point it into a bucket, grab the expansion vessel and lift it up and out the way, you may need to remove the old bracket if you have it on your boiler as it isn't required for the new expansion vessel. its just lifts out too. replace the expansion vessel with the new one, reverse removal, top up pressure, turn boiler on and your done. Takes 5 mins. Expansion vessel part number (Worcester Bosch part) is 87161425055. Parts centre should keep them there around £40. Hope this helps!

Ben the Oil Man, October 2008

Sorry no one could give advice in time.
Nevermind,found a decent plumber for once.
for those of you still scratching your heads, it was a faulty diverter valve...

pennyless, October 2008