Last updated on September 23rd, 2023
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 quite a common job.
Here I’ll give you my best advice for recharging expansion vessels based on my everyday on-the-job experiences.
Faulty Expansion Vessel Symptoms
Faulty expansion vessel symptoms are easy to spot:
- Expansion vessel full of water
- Boiler pressure too high or 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 there is no air or water 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.
Expansion Vessel Pressure How to Check
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 on a Combi Boiler
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.
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:
- Open the pressure relief valve until the boiler pressure gauge is at zero
- Connect your pump to the expansion vessel
- Pump up the expansion vessel to the correct pressure (0.9 bar if you don’t know)
- The boiler pressure gauge will rise also, open the pressure relief valve again
- Pump up the pressure again with the pump
- Open the pressure relief valve again
- Keep pumping up the pressure and opening the pressure relief valve until the boiler pressure gauge doesn’t move
- Remove the pump from the expansion vessel and fit the cap on the Schrader valve
- Repressurise the combi boiler via the filling loop until the pressure gauge is around 1 bar
Recharging an expansion vessel with 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 leave feedback or ask me any questions in the comment section below and I’ll try my best to help.
Please share this post if you found it helpful.
Recharging Expansion Vessel FAQs
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.