How to recharge expansion vessel on combi boiler

Combi boilers are the most preferred boiler in the UK because they come with high efficiency, energy, compact size and premium performance.

Combi boilers provide energy efficient hot water due to two main reasons. Firstly the boiler has very high operating efficiency and secondly they have the low internal power. In simple words, this means that in summers when you don’t need the space heating, you will only be paying for the heat required for hot water as per your domestic demands.

What is a combi boiler?

Mainly there are three types of boilers: regular boiler, system boiler and combination boiler. Combination boilers also known as combi boilers are very popular in homes because with saving space they provide the pleasure of saving money as well to their customers. A combi boiler is a compact, efficient boiler that provides heat and hot water to small homes mainly. They work with both hot water cylinders and central heating boilers and are so compact that it could be hung on the wall. Thus saves the space.

What is expansion vessel?

 Combi boilers need an expansion vessel with them. Also known as the expansion tank, this element helps for the proper pressure in the system. The expansion vessel handles the expansion and contraction of the water of the boiler as it heats and cools down. If this elements stop functioning properly than it can lead your system fluctuate a lot. Your system will not work properly after that. Resulting of no heating and hot water until the problem doesn’t get sorted.

So it is very important to maintain the expansion vessel and to recharge it timely.

Now the question arises how to recharge expansion vessel on combi boiler? But first we should know what pressure an expansion level should be set to? Let’s find it out

The compressed air in the expansion tank should be at a suitable pressure. Usually the pressure is one bar, but for more knowledge you can take the help of your boiler manual.

However you need to push down the Schrader Valve to check the pressure in the vessel. Remember you need to hear the air escaping. If you don’t then there is no pressure. Also draining and recharging the vessel is the important function of the boiler process.

How to recharge expansion vessel on combi boiler?

Recharge expansion level is also known as re-pressurizes the expansion tank. If you think you need to recharge your expansion level, you need to follow the following steps:

  1. Firstly switch off your boiler and remove the fuse.
  2. Drain off the extra pressure
  3. Check the pressure of the compressed air in the vessel. You can check it with any pressure gauge. If the pressure turns out to be low then recharge it with any air pump and attachment.
  4. Make sure that Schrader valve is not losing the air.
  5. You can check it with putting some washing liquid over it. If the bubbles get formed in the liquid, then you need a replacement valve.
  6. Also you need to check and re-pressurize the central heating system and may also have to bleed the radiators and recharge the pressure on the central heating side again.

How the expansion tank works?

The tank can contain the water and air together. As the water heats up it expands the tank. On the other hand the air compresses it. In this way it maintains the system stable by absorbing the extra pressure. Mostly the expansion tank is situated within the casing of the boilers making it invisible for the users.


If you find you have other problem with the expansion vessel, do not try to work and repair it by yourself. This can cause a severe damage to your boiler expansion vessel.

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