What is an Expansion Vessel?

The heating system of a UK home comprises various components, with the boiler as the central component. A boiler is designed to heat water, which is then circulated to radiators, taps, and showers. This type of heating system requires a preset boiler pressure. Boiler pressure plays a crucial role in circulating heated water to radiators and returning cold water for reheating.

The component responsible for regulating boiler pressure is called the "expansion vessel" or "expansion tank". An expansion vessel, or expansion tank, is the most critical component of the heating system. In most modern boiler systems, an expansion vessel is installed within the boiler casing; in some cases, especially in large heating systems, an external expansion tank is used. The external expansion tank is usually installed outside the boiler casing, near or alongside the water pipes.

In both cases, it maintains the boiler pressure. As the water is heated, it expands. Since the heating system is fully sealed, this water expansion increases the water pressure. The expansion vessel absorbs this pressure so that the heating system can continue to operate efficiently; if water starts leaking, the boiler will shut down due to an internal safety mechanism.

Keep reading this blog to learn what an expansion vessel is, how it works, and why it is critical to the safety of the heating system.

The Expansion Vessel – An Overview

The expansion vessel is a component that controls pressure in a sealed heating system. In that context, it is also categorized as a safety component. It is primarily found in combi boiler systems and rarely in conventional boilers.

By definition, an expansion vessel is a device that absorbs the volume increase of water as it heats and expands. As a result of thermal expansion, water expands when heated. It increases its volume or size. Since the heating system is sealed, the increased volume of water cannot escape, resulting in higher pressure. This increased pressure is accommodated by an "expansion vessel". In the absence of such an arrangement, pressure will continue to build up, which may be seriously dangerous for the boiler and the entire heating system.

In that context, an expansion vessel works as a "shock absorber". An expansion vessel comprises two sections or compartments, bifurcated by a diaphragm. A diaphragm is made of rubber and is flexible enough to accommodate the increased volume of water.

Understanding the Working of an Expansion Vessel

An expansion vessel consists of two compartments separated by a diaphragm. One compartment contains water, received from the boiler after heating. The other compartment contains air (mostly nitrogen), typically at a pressure of 0.75-1.0 bar. A nozzle, just like that of a car tyre, is provided for charging gas or air into it. It is known as the "Schrader Valve".

The diaphragm is the fundamental part of the expansion vessel. This is a flexible rubber part designed to absorb high pressure from heated water. When water expands, it exerts pressure on the diaphragm. As a result, the air or gas on the other side of the diaphragm is compressed, creating additional space for the expanded water. When water is cooled, it shrinks back to its original volume, causing the air to contract. This compression and contraction of air on the other side of the diaphragm controls the boiler pressure.

The entire process of working of an expansion vessel can be summarized as follows:

·         When the boiler is started, the water is cold. In that scenario, the water will remain at its original volume; hence, the diaphragm ideally rests at the water inlet.

·         When the boiler starts heating the water, its volume increases as it expands.

·         When water expands, it increases pressure because the heating system is sealed; the expanding water cannot escape.

·         The increased pressure, in the shape of expanded water, exerts pressure on the diaphragm. As a result, the diaphragm is pushed back.

·         As a result, the air or nitrogen gas on the other side of the diaphragm is compressed, creating space for the expanded water.

·         When water is cooled, its volume decreases, and the diaphragm returns to its original position, allowing the air or nitrogen to return to its actual volume (the process of contraction).

·         This process keeps on repeating, each time the water expands, hence the pressure remains within the required limits.

Importance of an Expansion Vessel in the Heating System

An expansion vessel is the most important and essential part of the heating system that comes with the modern combi boilers. It plays a crucial role in the central heating systems:

·         It controls the boiler pressure, preventing it from exceeding the safe limits. It helps maintain a stable boiler pressure.

·         If the pressure keeps mounting, it causes damage to pipework, valves, and radiators. An expansion vessel protects these components from damage caused by increased pressure. If the pressure is allowed to increase unchecked, the boiler shuts down due to its internal safety system.

·         An expansion vessel also plays a key role in protecting the heat exchanger of the boiler by reducing stress on it.

·         The boilers are also equipped with a pressure relief valve (PRV), which vents excess water to maintain pressure within limits. An expansion vessel also protects the PRV from frequent water discharge, as most of the pressure is absorbed by the vessel.

·         Generally speaking, an expansion vessel helps protect the boiler and the home from potential risks.

Types and Location of an Expansion Vessel

Most homes in the UK use a combi boiler, while some also use a system boiler. Depending on the boiler system, the location of the expansion vessel varies:

Internal Expansion Vessel (A Built-In Expansion Vessel)

An expansion vessel is typically fitted inside the boiler casing in most of the combi boilers. It is a sort of built-in expansion vessel, often fitted alongside the pipes, inside the boiler casing. Mostly, it is a round vessel, either red or blue. It can be easily identified by opening the casing. However, homeowners are not permitted to open the boiler casing themselves under gas safety regulations. Only a Gas Safe-registered engineer can do so.

External Expansion Vessel

An external expansion vessel is often found in large buildings and homes that have an older boiler. This type of expansion vessel is often a tank or cylinder, mounted away from the boiler, typically alongside the water pipes. It is mostly red, but it also comes in blue, making it easy to identify. It is usually much larger than the internal expansion vessel.

Mostly, this type of expansion vessel can be found:

·         Away but near the boiler.

·         Inside an airing cupboard.

·         Wall-mounted, adjacent to the main pipework of the heating system.

An external expansion vessel is often fitted in homes or properties with more radiators. But usually these are installed in commercial properties.

Symptoms and Consequences of Failure of An Expansion Vessel

A failure of an expansion vessel mainly results in various boiler pressure issues. Usual symptoms of a failed expansion vessel may include:

·         Abnormally high boiler pressure, typically above 3.0 bars.

·         Abnormal reduction in boiler pressure. Pressure drops to zero as the system cools.

·         The Pressure Relief Valve (PRV) may start dripping water.

·         Frequent drop in water pressure, requiring repressurizing repeatedly

·         Repeated lockout of the boiler due to pressure issues.

·         Unusual noises, such as gurgling sounds emerging from the boiler.

Common Reasons for the Failure of the Expansion Vessel

·         Loss of Air Charge – With consistent use, air may escape from the expansion vessel and require recharging.

·         Ruptured/Damaged Diaphragm – When the diaphragm is damaged or ruptured, water enters, replacing air from it. Resultantly, the expansion vessel fails to perform.

·         Incorrect Expansion Vessel Size – If the expansion vessel is too small, it may not perform as expected.

Important

·         Normally, an expansion vessel operates reliably for about 5 to 10 years, after which it must be replaced.

·         Only a Gas Safe-registered engineer is legally authorized to work on expansion vessels.

Conclusion

Boilers are designed to work at a specific pressure in a sealed central heating system. If pressure drops below or rises above that specific limit, the system locks out or shuts down due to an internal safety mechanism. Normally, a boiler operates at a pressure between 1.0 and 1.5 bar. However, if it drops below 0.5 or rises beyond 3.0 bars, it shuts down through activation of the safety system. To prevent this situation, boiler pressure must be regulated to remain within safe limits. The device that regulates boiler pressure is called an expansion vessel. That’s why it is often referred to as a safety device.

The main function of an expansion vessel is to control boiler pressure, which it does through a flexible rubber diaphragm. It accommodates the increased volume of water, as water expands on heating. Although the useful life of an expansion vessel is well beyond 5 years, it may need replacement if it is damaged or fails to perform as expected. This can only be done by a registered Gas Safe engineer as per the gas safety regulations.