Water Heater Expansion Tank Installation In A Boiler

water heater expansion tank

Its design to connect to the water heater expansion tank, the sanitary expansion vessel’s function, reduces the water pressure. Too much pressure risks damaging your piping, the safety unit, and your hot water calorifier.

For boilers, a heating expansion vessel absorbs the pressure of the hot water circulating in the radiators. You may have a bubbling expansion vessel or a bubbling expansion vessel. In other cases, it may be a leaking expansion vessel (i.e., an empty expansion vessel). Everything is solved, even an exploding expansion vessel! Hopefully, this never happens to you! So, lets see…

How to connect a vessel to water heater expansion tank?

Changing from a stable water temperature (between 10 and 15 ° C) in the water heater to a temperature for heating water (60 ° C to 80 ° C) causes a pressure increase of 3% if we are on the source. This quantity of water escapes from the balloon and sinks.

It’s like making pasta. If you heat the water, put the lid on. After a while, the water overflows. You cannot remove the “cover” of the water heater, and therefore the sanitary expansion vessel absorbs the overflowing water.

Every time you take a bath, you lose the equivalent of water to do the dishes. Not to mention the too high pressure which, as you know, damages the pipes and the safety group of your water heater.


The consists of a metal frame and a membrane inside are useful. It must be inflated with air or nitrogen to allow it to absorb hydraulic pressure. Typically, the interior membrane lasts for several years, provided the water heater and its components service. For this, it recommends descaling the water heater and change the anode every 10 to 15 years.

The Pneumatic vases from IMI differ in that they have no frame. Indeed, it is an ultra-resistant, waterproof membrane manufactured to last over time. These inflatable bladders take up much less space and are made of food-grade rubber (don’t try to eat it).



Blue, white, red, gray. Make no mistake! Apart from the often blue Pneumatex, sanitary vessels usually are white.

For heating, the vases are red and sometimes blue or gray.


The water heater provides you with the water you need to live but needs help to keep your environment safe.

The safety group makes it possible to protect the tank from overpressure, prevent hot water from returning to the cold water circuit, and drain the domestic hot water calorifier when necessary.

Without it, your water heater would overpressurize. Thanks to the safety group, a pressure of 7 bar is maintained. When this is no longer able to contain the tank’s pressure, the sanitary expansion vessel absorbs the excess water under pressure.


To choose the right domestic hot water expansion vessel (or even gas boiler expansion vessel), you must know the tank capacity and its setpoint temperature.

The average temperature of the water heater is equivalent to a certain coefficient of water expansion:

50 ° C: 0.01206

60 ° C: 0.01206

70 ° C: 0.023

80 ° C: 0.02899

This coefficient multiplier by the tank capacity to determine the boiler expansion vessel or sanitary expansion vessel. For example:

For a 200L tank operating at 70 ° C (therefore with a coefficient of 0.023):

0.023 x 200 = 4.6

You will then need a 5-liter expansion vessel. 

The expansion tank is one of the main safety components that your boiler has. It serves to absorb excess pressure, thanks to which it avoids serious breakdowns and protects both equipment and human beings. Do not stop reading this article, because we will see, step by step, how to repair the expansion vessel of a boiler, what this expansion vessel is for, how to know if it is broken and how to change it if necessary.

What is the water heater expansion tank for in a diesel boiler?

Before seeing the breakdowns and possible solutions, let’s analyze what the expansion vessel is and what it is for in a boiler. That will help you better understand everything else we will be talking about. 

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An expansion tank (which is different from a hot water storage tank) is a container made of strong metal (usually used sheet steel) installed in a boiler. Its objective is to absorb the excess pressure generated by the boiler while the water is heating. Its interior comprises two compartments divided by a membrane made of elastic and waterproof material (elastomer or rubber usually used). If its function is domestic hot water, it typically paints white, while if it uses for heating, it usually paints a characteristic red color.

02. Junkers boiler expansion vessel

As a boiler is a closed hydraulic system (heating pipes, boiler, radiators) that contains a fluid that heats (water), it needs a component such as the expansion vessel to avoid, among other things, explosions. However, for that, there are also safety valves. As it is a component that remains closed, the expansion vessel prevents water losses due to evaporation and the oxidation caused by contact with external oxygen.

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 the case of wall-mounted boilers, the expansion vessel usually housed inside the boiler, out of sight. But there are cases where You can install an extra expansion tank outside. 

The installation of an external expansion tank is essential when new radiators incorporate into the original circuit, which increases the mass of circulating water. As the greater volume of water generates more significant pressure, the external expansion vessel becomes essential.

03. Boiler installation with expansion tank and accumulator

In diesel boilers, the expansion vessel is usually inside the boiler, in the lower zone, near the safety valve. It can change with different brands, but it is quite right.

How does the water heater expansion tank of a boiler work?

The reason for being of the expansion vessel of a boiler is safety. By absorbing the excess pressure generated within the DHW (Sanitary Hot Water) circuit. While the water is heating, it helps avoid accidents. Or breakdowns that cause damage to people or equipment. And how does the expansion vessel achieve this?

Usually, the boiler pressure rises work as a strong impact against each component of the system through which the water circulates. In fluid dynamics,

In these cases, the water that stopped receives the water pressure that comes from behind. Which causes a compression of the fluid and a consequent expansion of the components. That make it circulate (pipes, valves, radiators, etc.). The expansion vessel’s function is to take advantage of the absorption capacity of its structure. It absorb excess pressure in the membrane, and thus protect the components against further damage.

What you have to do is inflate or fill with air the glass area intended for 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 fill in, follow these steps:

Altogether remove the cover from the front of the boiler, and look for the expansion tank.

In the expansion tank, you will find a valve with a lid at the top. Unscrew the cap and connect the mouth of the inflator.

Before inflating the glass, remove the water pressure from the boiler circuit and leave it at zero, with the drain open.

Begin to insert the air with the inflator until the pressure reaches 1.5 bars (it is the correct level in most diesel boilers with radiators).

Remove the inflator, put its lid on, lower the boiler’s top, and close the drain thread.

What pressure should the expansion vessel of a boiler have?

It will indicate the pressure levels on the boiler pressure gauge or display. Usually, if the boiler pump stopped and the water inside the heating circuit is at room temperature, the pressure can be between 0.8 bar and 1.2 bar pressure.

When the boiler started in heating mode, the radiators increase their temperature. At this time, we will notice that as the radiators heat up, the boiler pressure indicator (either a pressure gauge or a display) will begin to increase the pressure magnitude. This tendency to increase in pressure with the boiler running is normal. 

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You have to ensure that the pressure does not exceed the boiler heating safety valve’s upper set pressure levels. Excess pressure often indicates that the expansion vessel is defective. The maximum operating pressure should always be slightly less than the set pressure of the safety valve. It must establish the minimum and maximum pressures considering the height at which the expansion vessel locates.

Take a safety margin 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 using at higher temperatures).

How to know if the expansion vessel of the boiler broken?

It is essential to learn to detect the symptoms that indicate if the boiler’s expansion vessel is broken. You can check it frequently, and You should mostly check it during the boiler’s annual maintenance. It is known that a large part of the most common breakdowns in boiler heating systems. That is due to malfunctions or studies in the expansion vessel. 

A persistent symptom that gives false clues in heating installations is when the fill indicator’s needle drops. In these cases, people tend to continually refill the building with new water because they believe it is a leak. In reality, this is precisely a symptom of a lack of pressure in the expansion vessel. It is due to lack of maintenance or due to failure.

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

FAQ of water heater expansion tank

1. How to know if the expansion tank is defective in the car?

To check its condition, do not hesitate to take a regular look at your expansion tank. If your engine is running, the coolant is boiling, which indicates abnormal pressure due to clogging or leaking coolant.

2. Why is my boiler losing water?

If a boiler is leaking, it may be because the pipes’ pressure is too low or too high. A loss of stress often linked to a leak. If the pressure gauge indicates too much pressure, the safety unit opens to allow the overflow of water. It is to pass, hence the leak.3. What is the price of an expansion tank?

An expansion vessel costs between $24 and $120, including tax for volumes up to 100L. It will sometimes be necessary for larger volumes for collective boiler rooms to count up to $250. The price depends on the work, the type of enclosure, the brand, or the expansion vessel’s robustness.

4. Can my boiler explode?

In short, a gas boiler can explode. However, this risk is low, easy to control, as long as you follow the safety rules and adopt acceptable practices.

Conclusion of water heater expansion tank

If we notice that air always comes out and there is a drop in the water level. It is advisable to empty the boiler until there is no water left in the glass. And then with a pressure gauge. You must measure the air pressure. If the pressure is below average. It may be necessary to inject air until the corresponding strain reaches the installation’s geometric height.

For various reasons, the expansion vessel may lose its normal pressure. You 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 bars. The safety valve is activated, and water begins to come out. In these cases, it may not be a fault but rather a need to fill the expansion vessel with air.


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