External expansion vessel.

[Charles F]

Hi all,

Id appreciate a little advice on my next job....

No 2 son, the non-technical one of our two, has a Worcester Bosch combi boiler running a small simple system of five rads, all about ten years old. He has started to have to top up the pressure regularly, with the usual symptoms of rising pressure when warming up, and very little pressure next day, which sounds like a failing or failed internal pressure vessel to me.

He has a maintenance contract with British Gas, but they are saying that to change the pressure vessel the boiler has to come of the wall, and at that age other bits may fail while dismantling, and thus need a full replacement - and he doesn't want a new boiler just yet, esp as this one is otherwise working fine. BG also said that the boiler won't be harmed by regular top-ups - which may be true, but ignores the effect on the rads of regular additions of oxygenated limey water, which additionally will dilute any inhibitor that is there. So much for the experts.

So I proposes to add an external pressure vessel, and just ignore the failed one in the boiler. My query is where to add it. Ideally it should be near the boiler on the return pipe, but the kitchen is small, and the pipework difficult to access. His flat is a converted first floor in a semi, with an extra room in what had been the loft space, and that has a single easily accessible radiator. It seems to me that teeing in the PV to the return on that rad should be perfectly satisfactory - that way there is an open pipe all the way back to the boiler and the rest of the system. It also means that as it is the highest point in the system I've got less to drain down - useful as at present I don't know if the system has a drain c*ck.

So -

1) - is it OK to add a PV anywhere in the system to the return feed. 2) - I'm guessing a 12 litre PV will be sufficient for a small system like this.

Thanks for your thoughts, Charles F

[RJH]

I'd be looking for leaks outside the boiler - radiator valves and pipes.

Obviously diagnose properly first. I find it highly unlikely that the boiler needs to be removed - but I'm not familair with that particular boiler. On mine (an Ideal Logic) it's an easily replaceable part - maybe a 30 minute job IIRC.

Quite. Plus whatever damage the leaking water is causing. Living in a FF flat is of mild concern . . .

I'd be inlined to stick with the original arrangement for cosmetic reasons if no other - if indeed that's the cause.

[Jethro_uk]

On my Vaillant I am guessing they had some sort of rig in the factory that held the expansion vessel in place as they built the rest of the boiler around it.

Much like I imagine they built cars around the wiring looms in the old days. Or the heater matrix ...

However as the OP notes, you don't need to replace the failed one - just fit a new one alongside it. When I fitted mine, I did wonder vaguely why it wasn't supplied as a separate unit ?

[Charles F]

I need to do a proper "survey" before the job proper, and looking for leaks will be part of this. However, the system pipes appear to be mostly between the wooden first floor and the ground floor ceiling, and given the regular top ups I think there would be complaints from below if it was a leak.... There should be a copper pipe to the exterior for the pressure release, and if I can find that I'll plastic bag it to see if there are regular discharges.

We have the WB system boiler equivalent of this boiler - and yes, you do have to take it off the wall to get at the PV!

That would be nice - but may not be practical! Thanks for your thoughts though....

Cheers,

Charles

[John Rumm]

Yup location of the vessel in various boilers seems to vary from "right at the back and 'kin impossible to get at with the boiler in place", to the "it's right there in front of your face" kind of easy.

Most more recent Vaillants seem to have gone for the latter fortunately. On mine it's on the right hand side and comes out with removal of a couple of screws at the top and undoing the pipe coupling:

To keep installation simple and a "one box" solution probably...

[John Rumm]

Tying a plastic bag over the end of the PRV discharge pipe, is an easy way to see if the water is getting out from there.

[www.GymRats.uk]

I put a new one in my old Valliant a few years ago. Big square thing right at the back. :)

It was pretty straight forward to replace without removing the boiler. Had to remove the complete flue but the external bits were knackered anyway and the full flue kit was pretty cheap. Simple case of removing the the water pipe from the bottom of the pressure tank, remove flue, remove fan vacuum switch and the tank slid straight out of the top of the boiler. Looked like an impossible job but turned out to be quite simple really.

:)