Fitting an expansion vessel?

9 Dec 2015
Reaction score
United Kingdom
Hi, I hope someone can help.

I'm going to be fitting an external expansion vessel as the one inside the combi boiler is shot.

I've been doing a fair amount of googling and there is various advice given. A few things I'd like to work your brains on.

- I gather it is recommended to install the vessel close to the boiler on the return pipe. Though others state it's not essential. Can anyone explain why closeness to the boiler is beneficial?

- Is there an issue with installing the vessel on it's side?

- How is air prevented from being trapped on the water side of the diaphragm? Especially since there is no flow through, let alone an air vent.

- I've seen vessel kits which also have a filling loop with them. Are these to allow easy connection to the filling loop section of the return pipe?
Sponsored Links
  • Depends on the boiler - but close as possible fro neutral point.
  • No
  • It isn't. Doesn't matter anyway - air will eventually be absorbed by the water anyway.
  • No.
The EXV is filled with air first either at manufacture or at installation.

The pressure of that air forces the diaphragm against the opposite side of the EXV thus leaving no space for any air there.

When filled with water the diaphragm moves across as the air is compressed until the volume of air is reduced to about 2/3.

Then as the system water expands the increase in pressure compresses the air further and the diaphragm is pushed further over.


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links