How to Connect an Expansion Vessel to Your Combi Boiler

How to Connect an Expansion Vessel to Your Combi Boiler

The expansion vessel is an important part of the system that keeps the water pressure of a combi boiler at safe levels. It’s designed to absorb the extra volume of water when the system heats up, so that the pressure relief valve on the boiler doesn’t open to let excess pressure through.

The tank is made from a plastic or rubber membrane (known as a diaphragm), which is split into two compartments, one for water and the other for air or an inert gas, usually nitrogen. A Schrader valve similar to those found on motor car tyres is fitted to the air side of the tank, which allows the pressure to be re-charged whenever it gets low.

If the diaphragm fails, the vessel will not be able to re-pressurise and it will need replacing. It can also fail by rusting through and leaking, but this is less common and can be prevented using long-life corrosion inhibitor liquids.

Choosing the right size for your home

The most appropriate size of expansion vessel is based on the total water content of the heating system – including all pipework, radiators and the boiler itself. Ideally, the expansion tank should be a fraction of the volume of the hot water in your system so that the pressure will only increase to a small level.

Connecting the tank to the piping

To connect the expansion tank to your piping, you’ll need to install a tee where the flex line from the boiler was connected. Make sure to use plumber’s tape or pipe joint compound (liquid Teflon) on the nipple so that it will be firm and secure.

Mount the tank vertically with the piping connection at the top, instead of horizontally: This helps prevent air in the piping from getting trapped on the expansion tank’s water side when it’s first filled. It also reduces stress on the piping connection relative to a horizontally mounted tank.

Do not overheat the shell: The heat that is absorbed by the air pocket in the tank’s shell can cause it to expand more than it should. This will lead to an increased rise in system pressure between cold and hot, and a greater fall from hot to cold, causing the safety valve to open.

Check and re-charge the air/gas charge regularly: The air pressure in an expansion vessel should be checked and re-charged every 12 months. The pressure should be about 1 bar.

Recharging the tank is a simple task that can be done by yourself, or by a plumber. Most vessels have a bicycle tyre pump type adapter that makes it easy to do.

Test for leaks: The expansion vessel can be tested for a fault by unscrewing the plastic cap from the pressurisation valve (like a car tyre valve). Momentarily depress the pin to release a little pressure and see if any water comes out.

A faulty diaphragm may be the cause of an expansion vessel failure, so make sure you check it for any signs of damage or tear before disconnecting the tank. You can also tap the side of the diaphragm to see if it makes a ‘ping’ sound like you would with a perforated tyre, which can indicate that there is no air chamber in the tank.

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