Why would cold mains feeding the boiler need an expansion vessel?

Joef · Mar 9, 2020

British gas fixed a leak in the boiler and said the due to the recently installed water meter the fluxuation in pressure caused the leak. They advised to get a expansion vessel on the the cold mains entering the boiler.
I read on here someone say that Worcester wouldn't warranty boilers being fed by water meters if they didn't have expansion/accumalator valves on as it's a sealed system.
Why would the cold mains feeding the boiler need an expansion vessel?

Stigster · Mar 9, 2020

They prevent water hammer and shock in the pipework and boiler caused when water velocity is a little high and outlets are closed off after use. As the boiler manufacturer cannot control the water velocity in people's homes, I suspect they want a shock arrestor (a tiny little expansion vessel) fitted to mitigate this if it is occurring where one of their boilers is fitted. Hydraulic shocks can loosen fittings and cause leaks if severe enough. Even if you don't have this problem I can understand why they just ask for them for warranty purposes wherever their boilers go in.

It's not a big or particularly expensive job at all or at least it should not be. Easy for me to say when I'm not the one paying for it I know but that is the reasoning behind it I believe.

Joef · Mar 9, 2020

Stigster said:
They prevent water hammer and shock in the pipework and boiler caused when water velocity is a little high and outlets are closed off after use. As the boiler manufacturer cannot control the water velocity in people's homes, I suspect they want a shock arrestor (a tiny little expansion vessel) fitted to mitigate this if it is occurring where one of their boilers is fitted. Hydraulic shocks can loosen fittings and cause leaks if severe enough. Even if you don't have this problem I can understand why they just ask for them for warranty purposes wherever their boilers go in.

It's not a big or particularly expensive job at all or at least it should not be. Easy for me to say when I'm not the one paying for it I know but that is the reasoning behind it I believe.

For a regular sized terrace house what size tank would you suggest? If you could send a link that would be very much appreciated.

moonlight · Mar 9, 2020

It is a very small expansion vessel, and will go on the cold feed to the boiler, not necessary if its not a combi.

Mini Expansion Vessel Shock Arrestor - 1/2" BSP TM

not much bigger than your fist.