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recharging expansion vessel

Recharging Expansion Vessel on a Combi Boiler Help Guide

Last updated on February 19th, 2024

Sick of your boiler losing pressure and having to keep repressurising it? well, recharging the expansion vessel might just be the job needed to get it properly fixed.

Recharging expansion vessels is hard to do when you do it for the first time.

I have been repairing and servicing boilers for over a decade and recharging the expansion vessel is a common job.

Faulty Expansion Vessel Symptoms

Faulty expansion vessel symptoms are easy to spot:

  • Expansion vessel full of water
  • Boiler pressure too high or too low

Expansion Vessel Full of Water

A surefire way to know if you have a faulty expansion vessel is to see if it’s full of water. You can check this easily by removing the cap on the Schrader valve (like on a car tyre) on the expansion vessel and pressing the valve in.

If water comes out of the valve then the diaphragm inside the expansion vessel is split and the vessel needs to be replaced.

There should be air coming out of the valve but if no air or water is coming out then you will need to recharge the expansion vessel.

Sometimes air will come out first then water so you need to make sure.

Boiler Pressure Too High or Low

The biggest faulty expansion vessel symptom is the boiler pressure being too high and then too low when the heating is on.

If your expansion vessel is faulty, then the heating system will have to take all the expansion of the heated water.

This will cause the boiler pressure to rise too high when the heating is turned on.

Having the boiler pressure low happens after being too high because the pressure relief valve lets the water (pressure) out to stop it from getting dangerously high.

This symptom tells you that the expansion vessel either has no air and needs recharging or the rubber diaphragm inside has split and the expansion vessel needs to be replaced.

You must check if the expansion vessel is full of water to find out.

Boiler expansion vessel pressure test
Checking for air in an expansion vessel

How to Check Expansion Vessel Pressure

To check the expansion vessel pressure you will need a pressure gauge. You must find the expansion vessel Schrader valve and connect the pressure gauge just like on a car tyre.

You might need to use an extension hose to reach the Schrader valve with the pressure gauge.

You will need to drop the pressure on the boiler to get the correct pressure of the expansion vessel, the boiler pressure gauge must be at zero to check the expansion vessel pressure.

Expansion Vessel Pressure Setting

To find the exact expansion vessel pressure setting you should check the sticker on the expansion vessel or in the boilers manual.

If I can’t find it, then I recharge the expansion vessel pressure on a combi boiler to 0.9 bar (13 psi).

Expansion Vessel Pump

To recharge an expansion vessel you will need an expansion vessel pump, a pressure gauge, and possibly a Schrader valve extension.


When I first started recharging expansion vessels I used to use a Regin hand pump specifically made for expansion vessels.

They do the job but with so much unnecessary work. Only electric pumps for me now.


The expansion vessel pump that I use the most is my 18v Makita battery-operated pump.

It blows car tyres and bike tyres up with ease but if you don’t already have Makita batteries and a charger then it can be expensive to buy everything.


Another electric pump I love and sometimes use on expansion vessels is a Cycplus pump.

It’s unbelievably small and light and can blow car and bike tyres up with ease also. I mainly use it for my mountain bike but it’s brilliant for expansion vessels.

Just set the pressure you want and press the button, simple. You charge it with a micro USB cable.

How to Recharge Expansion Vessel

To recharge an expansion vessel on a combi boiler you must keep pumping up and dropping the pressure on the boiler many times to get the water out of the expansion vessel and get some air in.

Recharging expansion vessel on combi boiler
Recharging expansion vessel on a combi boiler

You can do this by draining the water (pressure) from a hose and drain valve on the boiler or heating system, or you can use the pressure relief valve like I do which is much easier.

You might need to replace the pressure relief valve after it has blown the water out as sometimes they don’t close fully after being used.

How I recharge expansion vessels on combi boilers:

  1. Open the pressure relief valve until the boiler pressure gauge is at zero
  2. Connect your pump to the expansion vessel
  3. Pump up the expansion vessel to the correct pressure (0.9 bar if you don’t know)
  4. The boiler pressure gauge will rise also, open the pressure relief valve again
  5. Pump up the pressure again with the pump
  6. Open the pressure relief valve again
  7. Keep pumping up the pressure and opening the pressure relief valve until the boiler pressure gauge doesn’t move
  8. Remove the pump from the expansion vessel and fit the cap on the Schrader valve
  9. Repressurise the combi boiler via the filling loop until the pressure gauge is around 1 bar


Recharging an expansion vessel can be hard when doing it for the first time, but using a good electric pump is key.

The expansion vessel has lost all of its air and is full of water, the rubber diaphragm inside is not doing anything.

When you recharge an expansion vessel you are forcing air into one side of it and forcing the water out of the other.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


How does an expansion vessel work?

An expansion vessel works by taking the expansion pressure of hot water when the water is heated. An expansion vessel has a rubber diaphragm inside which separates air and water.

When the water in a sealed system is heated, the air in the expansion vessel takes some of the expansion of the heated water so the pressure doesn’t rise too high.

What pressure should an expansion vessel be set to?

The pressure an expansion vessel should be set to is specific to each expansion vessel. You should find what pressure it should be set to on the expansion vessel or in the manual.

0.9 bar (13 psi) is what I usually set an expansion vessel pressure to on a combi boiler if I can’t find the exact pressure.

What happens if the expansion vessel pressure is too high?

If your expansion vessel pressure is too high then you could damage the rubber diaphragm inside which will lead to needing a new expansion vessel.

If you know your expansion vessel is too high you can lower it by pressing the Schrader valve (car tyre valve) in and letting some air out very easily.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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13 thoughts on “Recharging Expansion Vessel on a Combi Boiler Help Guide”

  1. Thanks for the guide, very easy to understand.
    Our system, as well as an internal expansion vessel, has an external retrofitted expansion vessel. The heating engineer that fitted it explained that the external vessel was necessary, as with it being a large CH system, the internal expansion vessel was always just about at capacity when the central heating was on.
    Do you have to charge both expansion vessels, and if so how does your procedure differ?
    Thanks – Paul

    1. Steven Reid

      Hi Paul, it’s the same procedure for recharging both expansion vessels. You only need to recharge them if they lose pressure which you can check with a hand held pressure gauge. If they are below what they are meant to be at, then you will need to pump them up to recharge. Each expansion vessels recommended pressure will be different so you will need to check the expansion vessel or the manual for each one.

  2. Excellent! Just what I needed to know. Thank you very much.

  3. I got a pro to do this and have noticed the pressure is slowly decreasing over weeks. Should I replace the expansion vessel or could it be something else?

    1. Steven Reid

      Sounds like it could be the pressure relief valve letting the water out slowly. This can happen after recharging the expansion vessel. You’ll probably need to replace the pressure relief valve. You should check the copper blow off pipe outside for any signs of water.

  4. Thanks so much for this guide. Dead easy to follow and sorted out the pressure issue on my baxi boiler.

  5. Hi,
    A very concise and clear description. I had tried this previously but only pumped it up once and clearly the expansion vessel was still mostly full of water.
    I redid this again today and probably recharged the tank 5 times (I’m not sure it was the full capacity of the tank, but as the only easy method was to reduce the pressure from a radiator I was worried about letting too much air into the system. The boiler was running at 3.5Bar (so was requiring frequent top ups as the PRV was venting). Now it its around 1.5Bar when at running temp so much better. I didn’t get to the point where the pressure in the loop was not increasing when pumping up the vessel (even on the last inflation the boiler pressure gauge was still rising to around 1Bar) – do you think I should continue to release pressure and re-inflate the vessel more or be happy with the current running pressure. The boiler is an Alpha CD 35C and doesn’t appear to have a manual release (the front of the boiler literally states releasing pressure from the radiators if needed!).

    1. Steven Reid

      Hi Chris,
      You need to keep recharging (pumping) the expansion vessel and releasing the pressure until the boiler pressure gauge stops moving completely. The expansion vessel pressure should be around 0.9 bar when the system pressure is zero. Then repressurise the system pressure to around 1.5 bar on the boiler pressure gauge.

      1. james k

        Hi. Same sort of situation. Got the rad valve open. Pumping to 0.9 bar. Pressure gage always goes up.
        Boiler was working fine since installed 2 years ago. greenstar cdi
        Any help would be really appreciated. I have pumped it over 20 times now still no change

  6. stephen freyne

    thanks for a c comprehensive guide to expansion vessel issues. Im sure ai have a problem with mine my boiler pressure goes up and down like a yo yo. A very small amount of water came out of the schraeder valve when pressed do you think this means it is faulty (membrane split)?

    1. Steven Reid

      Hi Stephen, if it was just a very small amount of water then the expansion vessel diaphragm might be good. They sometimes spit out little bits of water created via condensation that’s collected inside but should be good to recharge the vessel with a pump.

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