What is an Expansion Vessel in a Boiler


The expansion tank is one of the most important parts of the boiler in terms of safety and equipment protection. But it is also one of the most sensitive components, and can often break down or stop working altogether. It is convenient that we know how to protect it, to extend its useful life. In this article, you will learn everything related to your boiler’s expansion tank. Here you also have (and at the end of the article) direct access to all the expansion tanks and their accessories for sale in the Suner store.

The first level of boiler protection

And why do we say that the expansion vessel is a very important part? There are many ways to lower the pressure of a boiler when it exceeds the normal range (which should be maintained between approximately 1 and 2 bar). But the first way in which the boiler itself maintains its stable pressure is through the expansion vessel.

As a boiler is a closed hydraulic circuit, it is convenient to have a component that deals with any excess pressure. As the water pressure rises, each element of the boiler that is in contact with the water receives impacts similar to “water hammer”. The expansion tank, thanks to its robust construction, receives these impulses and relieves the impact on more sensitive components. This also minimizes the risk of explosion and any further damage to the equipment.


To relieve large excesses of pressure there are safety valves. But these only work in extreme cases, during notable pressure excesses that the expansion tank has not been able to compensate for. And to lower the pressure manually throughout the system, periodic or special purges of the heating radiators are carried out. But while these resources do not come into operation or are not necessary to lower extreme pressure excesses, the pressure surge routine is compensated by the expansion tank, which keeps the system operating within its normal parameters.

That is why the material used to build the expansion tank is usually quite strong (it is made from steel plates most of the time). Its material and robustness make it capable of receiving and handling all those excess pressures. The logic of its design is also based on that purpose: the mechanism is purely internal, it is kept hermetically sealed, away from oxygen, so that the material does not rust, get damaged, or weaken because all that would go against its function of being an internal security measure that does not require continuous supervision, except during periodic maintenance of the boiler or in the event of breakdowns.

Expansion vessels usually have the most diverse colors and designs (cylindrical, tubular, flattened, rectangular, square, etc.), and come in a wide variety of capacities (18 liters, 35 liters, 250 liters, 600 liters, etc.), so that they can be adapted to the needs of any heating and domestic hot water system, whether small or large.


They can be installed inside or outside the boiler, depending on their factory characteristics or those of the boiler. If they go inside the boiler (this is almost always the case with wall-mounted boilers), the expansion vessel must correspond exactly to the space in which it should go in terms of size and installation. And in case they go outside, they can be fixed to the floor, to the wall or to a support. This all depends on the manufacturer’s design.

Depending on the design and dimensions of the heating system, depending on the characteristics of the boiler, and taking into account the real demands for hot water and heating in each place, even if the wall-mounted boiler has its own expansion vessel, it is sometimes necessary to install it a secondary external tank that absorbs greater pressure shocks.


What is the expansion tank-like and how exactly does it work?

The expansion tank is a hermetically sealed container made of steel (DIN 17100 steel welded in an inert atmosphere), which has two compartments inside separated by a membrane made of some flexible material, which tends little to harden with water (highly elastic neoprene is usual). One of the compartments contains air or nitrogen in a gaseous state, and the other compartment is in contact with the water in the installation.

Technically, the objective of the expansion tank is to maintain the pressure within a set range, by absorbing changes in the volume of the water contained within the closed heating circuit whenever the temperature of the liquid changes. It does this by absorbing the pressure from the compartment with water and sending this expansion onto the compartment with air or nitrogen.

By absorbing pressure shocks, avoids excesses and keeps the pressure of the heating and DHW system stable. As we said before, the expansion tank with its resistant steel body and its flexible internal membrane is the first level of safety with respect to increases in pressure. If the expansion tank works well, the safety valve will not have to react to excess pressure, unless it exceeds 3 bars.


Open (non-hermetic) expansion tanks were used in the past, but they have fallen into disuse due to the obvious advantages of the hermetically sealed expansion tanks: thanks to the fact that the circuit is 100% isolated from the outside, oxidation is avoided, there are no losses of water due to excessive volume or evaporation, the installation does not have to be refilled periodically, and the equipment can work with superheated water.

When the expansion tank does not work or breaks down

Being one of the most sensitive elements of the boiler, the expansion tank can break down in several ways and stop fulfilling its function. The two main ones are loss of gas and rupture of the inner membrane. Let’s look at these issues in a little detail.

Loss of gas and how to restore it

Sometimes the air or nitrogen compartment loses its contents, and because of this it is necessary to replace the gas. This can be done using a normal air pump, but care must be taken that the compartment is maintained at the exact levels of pressure. Check out this article for details on how to refill an expansion tank.

Hardened or torn inner membrane

But the element most prone to breakdown inside the expansion tank is the internal membrane that isolates its two chambers. If this membrane breaks, the individual compartments come together and the expansion tank can no longer fulfill its function of absorbing pressure shocks. Read this article of ours if you want to know how to check and how repair a boiler expansion tank.

Sometimes it can be fixed, but…

The repair of an expansion tank is not always possible. The repair that consists of filling the compartment with gas is a fairly efficient solution. However, the repair of a ruptured membrane is more of a palliative one. It can get you out of trouble if you can’t access a new expansion tank at the moment, but eventually, the problem will reappear.

That is why we say that an expansion tank can sometimes be repaired, but sometimes not; and in those cases, you have to think about replacing at least the membrane if it is possible (it is not always). The expansion tank is a component that is important enough to invest in, anticipating that otherwise, it can leave us without heating or hot water at any time.


Access the expansion tanks and accessories for sale at Suner

At Suner we have an extensive catalog of expansion tanks for all the main wall-mounted boiler brands. These are original parts made to the highest quality, manufactured with durable materials.

We also have accessories for expansion tanks (wall brackets, flanges, mounting kits, inflation hoses, load testers, membranes for multi-volume expansion tanks, caps, and floats).

Make Suner the boiler spare parts supplier for your business, and we’ll take care of offering you an up-to-date and always well-stocked catalog. Our store has a specialized finder that will make it easy for you to find all the parts you need. We offer you fast and safe shipments and an after-sales service that will always adapt to the needs of your business.


Subscribe to our Blog

You want to follow us?