All you need to know about sealed system central heating expansion vessels

Installing an expansion vessel on your sealed central heating system will save you a whole load of soggy expensive hassle. They may not be pretty, but they are essential. Read on to find out why.

What is a central heating expansion vessel?

An expansion vessel is a pressurised and not very cuddly container with compressed air in one side, and heating system water in the other. The two sides are separated by a rubber diaphragm.

Why do I need an expansion vessel on a sealed heating system?

Trust me, you defo need one on a sealed system.

Vented (unsealed) systems have a vent pipe that takes all that expanded water up into your feeder tank. This tank is often called a feed and expansion (F&E) tank for a good reason.

However, in sealed (unvented) systems there is no F&E tank, but that expanded water still needs to go somewhere! That somewhere is an expansion vessel (better than bursting out of your pipes!), which needs to be plumbed into your heating system.

For the pros out there, watch my video on how to install a central heating expansion vessel

How does a central heating expansion vessel work?

Remember those two sides of the expansion vessel? Air can be compressed but water can’t: so when water heats up the pressure increases and it fills the diaphragm on one side of the vessel, which pushes it into the air side.

Watch me do a full-on dissection of a central heating expansion vessel in this video:

Central heating expansion vessel problems and how to fix them

First up, make sure you get your central heating expansion vessel sizing right. The bigger the heating system, the larger the vessel you’ll need.

If the pressure reading is way too high or the cut-off had popped, bob your finger on the vessel’s Shrader valve. If no air comes out, then repressurise (recharge).

If you leave your finger on the valve for a while the water comes out, that means the rubber diaphragm has failed and you’ll need a new expansion vessel.