How to Fit an external expansion vessel into a boiler
An expansion vessel is a small tank used in water heating systems that performs the important job of absorbing excessive water pressure. If the expansion vessel in your plumbing is not working, you could call up the plumber and get it fixed - for a very huge sum.
Or you can watch this video and do it yourself for way cheaper. Check out the tutorial to learn how to fit an external expansion vessel into a boiler.