Have fitted new central heating (cond/combi boiler) , worcester bosch recommended fitting an expansion vessel in the main water feed as my water meter will have a non return valve , which means any pressure build up cannot back up into the mains water pipe.

I have fitted a 5 litre watts vessel with 3 bar pressure relief valve unfortunately the pressure relief valve blows at 3 bar which is below my mains water pressure the result is water everywhere!!!

Do i need a relief valve or can i just fit the vessel direct to a tee from the main 15mm pipe?

How would this effect the preset pressure in the vessel?


The supplied filling loop and pressure gauge makes no sense in this arrangement , it hardly needs a filling loop when it is plumbed directly onto the main feed pipe.

What's the correct way to fit this thing!! in my application?

it sounds like you may have fitted an expansion vessel that is intended for use on the heating side of an installation (these are usually red in colour) there would be no need for a 3 bar safety valve in the posiion that you have described. What is sometimes used in the circumstances you are describing would be an expansion vessel suitable for connection to a potable water supply (drinking water/ mains supply), these are usually white in colour and are treated internally so as not to contaminate the water supply in some installations these are fitted inconjunction with a pressure reducing valve and a 6 bar pressure relief valve.

The accumulator and prv should be after the nrv and ball valve. If you try and charge your central heating to full mains pressure there is a good chance you may rupture the boiler.

Sorry didn't read properly, just ignore me!! That's the danger of forums, muppets half asleep talking rubbish

[Edited on 5/9/07 by daviep]

Potable water Vessels are also blue depending on the make

It is a white watts 5ltr potable expansion vessel as sold by Toolstation.

http://www.toolstation.com/images/library/stock/webbig/41665.jpg

http://www.toolstation.com/images/library/stock/webbig/49934.jpg

Hmmm, i'd be very careful. if you don't know what kind of expansion vessel to use I'm assuming you're not Corgi and therefore doing a DIY install. Whilst DIY on your own gas isn't illegal, you have to be what is woollily defined ni the law as "competent". Basically if anything blows up, then a court would decide that you weren't "competent" and you would be screwed.

Having said that if you are "competent" and don't blow anything up there's no issue

Worcester Bosch recommend a small 1 ltr vessel , I have a 5 ltr so should be more than adequate , I am surprised as lots of people have combi boilers and water meters yet there is not much info on how this install is best made , the CORGI guy who checked and commissioned my system was not at all interested in the extra recommended expansion vessel.

So I can only assume that not many of these are installed.

Whats it all about Alfie???

Dave the first thing to consider is the distance between the boiler and the stopcock/water meter if it is greater than say 8/10 meters away i wouldnt bother with the vessel as there should be enough pipe to take up the minute amount of hot water expansion you might experience.

The fittings pack you bought is for a pressurised central heating system which is designed to operate at only 1/1.5bar so thats why it keeps blowing, so bin them.

Even if you only had a very short run from the meter to your boiler you would only ever need just the expansion vessel on it's own anyway and then a max of 1 ltr, 5ltrs is defo overkill, not the extra bits which would only be required (set to a higher pressure) if you'd have installed an unvented hot water cylinder, which you haven,t.

Proffesionally i'd bin the vessel, you'd realy only need an expansion vessel on the cold water side of a combi if your experiencing water hammer. Otherwise they are totaly un-neccesary.

There isn't a lot of information about on fitting these generally as most situations dont require them. Manufacturers mention them to limit their liability when "exceptional" circumstances arise, so they cop out of the warranty.

[Edited on 6/9/07 by GasGasGas]

Am I missing something here?
The expansion vessel is for the heating side, designed to run at 1 - 1.5bar. The cold water inlet should run happily at mains pressure (up to 12bar in places). I appreciate you're going to get a little hot water in the cold feed, but a short run of plastic pipe would take up any expansion.
I'm no expert on boilers but I didn't think there was any form of pressure regulator in the inlet.
I can understand if you're getting water hammer though.
PS the expansion vessel needs to be inflated to about mains pressure to work effectively - it's only a rubber diaphragm with air one side and water the other.

Yes - gasgasgas , it is well over 10 metres to my water meter/non return valve.

But as you intimated I did not want to give any excuses to Worcester Bosch regarding warranty.

Having just spoken to WB again they have clarified the fitting for me and my suppositions where correct.

I do not need the fitting kit , they recommend just fitting it to the main via a tee , they DO recommend only a 1 litre vessel , but i couldn't find one ,so I got 5 litres .


The factory pressure is 2 bar so I hope this will be able to cope with my mains pressure (over 3bar).


Gis a job I've had hours of fun with this central heating.