How to repair the expansion vessel of a boiler


The expansion vessel is one of the main safety components of your boiler. It serves to absorb excess pressure, thanks to which avoids serious breakdowns and protects both the equipment and the human being. Be sure to read this article, because we will be seeing, step by step, how to repair the expansion vessel of a boiler, as well as what the expansion vessel is used for, how to know if it is broken and how to change it if necessary.

What is the expansion vessel in a diesel boiler and what is it used for?

Before going on to talk about the malfunctions and possible solutions, let’s analyze what the expansion vessel in a boiler is and what it is used for. This will help to better understand everything else we will be talking about.

An expansion vessel (something completely different to a hot water storage tank) is a container made of strong metal (usually steel plate is used), which is installed in a boiler. Its purpose is to absorb the excess pressure generated by the boiler while the water is being heated. Its interior is made up of two compartments divided by a membrane of elastic and impermeable material (usually elastomer or rubber). If its function is domestic hot water, it is usually painted white, while if it is used for heating, it is usually painted in a characteristic red color.

Since a boiler is a closed hydraulic system (heating pipes, boiler, radiators) containing a fluid that is heated (water), it needs a component such as the expansion vessel to avoid, among other things, explosions; although safety valves also exist for this purpose. As it’s a component that is kept closed, the expansion vessel prevents water losses by evaporation, as well as oxidation that would be produced by contact with oxygen from outside.


Where is the expansion vessel in a boiler?

The location of the expansion vessel will depend on the type of boiler and the type of installation. In cases of wall-hung boilers, the expansion vessel is usually housed inside the boiler, out of sight. But there are cases where an extra expansion vessel can be installed outside.

The installation of an external expansion vessel is especially necessary when new radiators are added to the original circuit, which increase the circulating water mass. As the higher pressure is generated by the increased water volume, the external expansion vessel becomes indispensable.


In cases of diesel boilers, the expansion vessel is usually located inside the boiler, in the lower area, near the safety valve. This may change with different brands, but it’s quite common.

How does the expansion vessel of a boiler work?

The raison d’être of a boiler’s expansion vessel is safety. By absorbing the excess pressure generated within the DHW (Domestic Hot Water) circuit while the water is being heated, it helps to prevent accidents or breakdowns resulting in damage to people or equipment. And how does the expansion vessel achieve this?

Normally boiler pressure surges (especially if they are abrupt) provide a strong impact against each component of the system through which the water flows. In fluid dynamics, these pressure surges can be caused by the so-called “water hammer”, one of the effects of abruptly shutting off the flow of water (ie. when turning off a tap, or when the water pump suddenly stops).

In these cases, water which is standing still is pressurized by the water coming from behind, and this causes a compression of the fluid and a consequent expansion of the components that circulate it (pipes, valves, radiators, etc.). The function of the expansion vessel is to take advantage of the absorption capacity of its structure, to absorb the excess pressure in the membrane, and thus protect the components against further damage.

How do you fill the expansion vessel of a boiler?

If you notice that air always comes out and there is a drop in the water level, it’s advisable to empty the boiler until there is no water left in the container, and then measure the air pressure with a manometer. If the pressure is below normal, it means that it may be necessary to inject air to achieve the pressure that corresponds to the geometric height of the installation.

There are several reasons why the expansion vessel may lose its normal pressure. You will notice it when you press the button to measure the pressure, and first the level is at 1 bar, but when the boiler starts to heat, the pressure level rises to 3 bar, the safety valve is activated and water starts to flow out. This may not be the case of a malfunction, but rather the need to fill the expansion vessel with air.

The procedure for filling the boiler expansion vessel is very simple. You don’t need any special tools. Only an air pump with a pressure gauge.


What we have to do is to inflate or fill with air (up to the recommended pressure) the area of the container designed for the gas. Of course, we will use air, because in most cases we will not have nitrogen. But air works almost as well as nitrogen. To refill, follow these steps:

  • Take out the front cover of the boiler, and look for the expansion vessel.
  • On top of the expansion vessel you will find a valve with a cap. Unscrew the cap and connect the nozzle of the air pump.
  • Before filling the vessel, you need to remove the water pressure from the boiler circuit and leave it at zero, leaving the drain open.
  • Start pumping air in with the air pump, until the pressure reaches 1.5 bars (this is the correct level for most diesel boilers with radiators).
  • Remove the pump, put the cap back on, lower the boiler cover, and close the drain.

What pressure should the expansion vessel of a boiler have?

The pressure levels will be indicated on the pressure gauge or display of the boiler. Normally, if the boiler pump is stopped and the water inside the heating circuit is at room temperature, the pressure may be between 0.8 bar and 1.2 bar pressure.


When the boiler starts to operate in heating mode, the radiators increase their temperature. At this moment you will notice that as the radiators heating up, the boiler pressure gauge (either pressure gauge or display) will start to increase the pressure level. This tendency for the pressure to increase with the boiler running is normal.

You only need to ensure that the pressure does not exceed the upper set pressure levels of the boiler’s heating safety valve. Pressure excesses often indicate that the expansion vessel is damaged. The maximum operating pressure should always be slightly lower than the set pressure of the safety relief valve. The minimum and maximum pressures should be set taking into account the height at which the expansion vessel is placed.

A safety margin must be taken into account, which will be greater the higher the operating temperature (with a minimum of 0.2 bar for systems operating at temperatures below 90°C, and 0.5 bar for systems operating at higher temperatures).

How to tell if the boiler expansion vessel is damaged

It’s important that you learn how to detect the symptoms of a broken expansion vessel in your boiler. You can check it frequently, and in particular it should be checked during the yearly boiler safety check. A large part of the most common breakdowns in boiler heating systems happen due to malfunctions or failures in the expansion vessel. This is one of the most widespread problems.

A very frequent symptom giving misleading clues in heating installations occurs when the needle of the filling indicator goes down. In these cases people tend to continually refill the system with new water, because they think it is a leak. In reality this is precisely a symptom of a lack of pressure in the expansion vessel, due to lack of maintenance or failure.

The main symptom of expansion vessel failure is the continuous tripping of the safety valve. In wall-hung boiler installations, the safety valve is routed through a transparent tube located under the boiler. In diesel installations the safety valve must be in the vicinity of the expansion vessel.


Obviously one of the components of the internal vessel system which suffers the most fatigue or stress is the rubber or elastomer membrane. The expansion vessel membrane stretches and shrinks continuously, and this movement is prone to generate problems or even ruptures. Every time the heating system is turned on, the expansion has to be absorbed by the chamber, and that falls directly on the membrane like a battering ram.

Faced with symptoms like the ones we have just seen (and other similar ones), it’s advisable to check the structural and working condition of the expansion chamber. How can you do this? Press the valve core and check what comes out of it:

  • If only air comes out, this means that there is no rupture. The separating membrane of the container should still be intact and you only need to re-fill with gas or air in the dedicated compartment inside the expansion chamber.
  • If water comes out instead of air, this means that there is some sort of tear. The membrane of the expansion vessel is punctured and water from the boiler installation is flowing into the air chamber, which should normally be completely isolated from the water chamber.

The system components have to be arranged very firmly, because when the expansion vessel is too small in relation to the mass of water it must receive and expand, or when the elastomer wall loses its sealing condition and allows water from the heating circuit to seep in and flood the vessel where only the nitrogen should be, then it means that the vessel has lost its effectiveness.

If there is any kind of wear or damage, as the tank can’t receive the excess pressure when the circuit heats up, it can cause the heating safety valve to trip. Technically it’s assumed that if the diaphragm is not too weak to rupture under pressure, it will always receive some excess pressure and cause the safety valve to trip.

How to change the expansion vessel of your boiler

If the expansion vessel of your boiler is not working properly or is damaged, it may be time to replace it with a new one. Refilling the expansion vessel on a daily basis, or even systematically, is not recommended. These are not suitable solutions for malfunctioning, as they may cause deposits from new water, or may significantly increase the risk of corrosion due to the continuous contact between the inside of the expansion vessel and the oxygen in the environment, even though the material may be stainless.


The expansion vessel can be repaired up to a certain point, but from a certain point onwards it is better to replace it with a new one. In cases of partial failure, the elastomer membrane (or whatever material in the unit’s components) must be replaced, but only if it’s actually replaceable. If the membrane is not replaceable, the whole vessel must be replaced. In fact, this is the case in most cases. Beyond very punctual cases in which the problem is due to wrong pressures, when there are expansion vessel failures, the only real way to solve it is to replace the whole vessel.

To replace the expansion vessel of your boiler, you can follow the following steps:

  1. Find out the brand of your boiler and your expansion vessel, so you can purchase exactly the new vessel you need.
  2. Once you have the new vessel, turn off the whole system, cut the water supply to the boiler, or throughout your home. Wait for the boiler to cool down (about 1 hour).
  3. Open the drain valve and drain the boiler, in order to reduce the water pressure.
  4. Disconnect the old expansion vessel carefully, so that the connections and piping are not damaged, nor any other components.
  5. Normally new vessels are factory set to the proper pressure level. But it does not hurt to check this before installation as well.
  6. Tape the connectors with teflon tape (don’t wrap up to the end of the connection, to prevent the teflon from being inserted into the water system).
  7. Install the new expansion vessel exactly as the old one was fitted. Make sure that the connections are tight enough.
  8. Turn on the water supply, reconnect the power, turn on the boiler and wait for it to heat up.
  9. Finally, run a test: turn on a hot water faucet and check that the water is hot. If it is, the replacement works perfectly.

Available expansion vessels from Suner

Here we list the expansion vessels of the brands available from Suner. Just choose your brand of boiler from the list, click on the link, and you will be taken directly to the product you need to purchase:


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