This article shows how to fit an external expansion vessel to a combination boiler. The combi already has an internal expansion vessel but the diaphragm inside of the internal expansion vessel has failed and so a new expansion vessel is needed. To replace the existing expansion vessel is often a job for an heating engineer and it should not be attempted by anyone who is not competent to work on Gas appliances. An external expansion vessel is a DIY job and can be fit by people with some plumbing experience. A replacement internal expansion vessel is also about three times more expensive than an externally fitted one.


switch off boiler

Switch off the boiler and remove the fuse as an extra precaution, to prevent it being switched back on by accident. The new expansion vessel needs to be located close to the boiler. In this case it is going in the corner of the kitchen, below the sink. The existing expansion vessel can be left in the boiler and there is no need to touch this.

central heating return pipe

Identify the return pipe to the boiler, this is the pipe that we need to Tee into.

expansion vessel tap connector

This is the new expansion vessel. You need to buy the correct size vessel for your particular boiler, this one is 8 litres. You also need a bracket to mount the vessel to and also a three quarter inch tap connector (that is the brass bit that screws onto the top)

drain down central heating

The central heating system will require draining down completely and so you will also need some inhibitor to put back in the system.

recharge expansion vessel

Before installing the vessel it is a good idea to pressurise it before it is fitted. Many vessels come pre-charged and so the pressure may need adjusting. To adjust the pressure either use a screw driver to press in the Schrader valve to release pressure or attach a suitable pump (such as a car tyre pump) and pressurise the vessel to the correct pressure as detailed in your boiler's user manual.

T valve

The new vessel pipe work needs teeing into the return pipe for the boiler. In this case we have an isolation Tee valve that we can connect to. It is unlikely that you will have such a valve on your installation so you would have to cut the pipe and insert a T piece.


Now the system is empty it is a good time to add the inhibitor, this can be poured in via the filling loop.

compression fittings

The new pipe work needs to connect the return pipe from the boiler to the Expansion vessel. Here we have used 15mm copper pipe (plastic pipe should never be used close to a boiler) and compression fittings. The connector that fits on top of the expansion vessel is a 3/4 inch tap connector. We used a 3/4 inch -15mm compression tap connector. You must ensure that the fibre washer is in the tap connector and that all connections are tight!

expansion vessel under sink

This is the new vessel which is in the corner of the kitchen underneath of the work top (sorry for the bad picture) Once the expansion vessel is fitted you can refill the central heating system and bleed the radiators-

How to fill a combi boiler

How to bleed radiators

You can then put the fuse back in and switch the boiler back on.