Expansion Vessel Maintenance Guide

Expansion vessels, sometimes called expansion tanks, are found in most domestic pressurised heating systems, such as combi boilers. They are crucial for helping systems work smoothly by adjusting to water pressure changes.

The most common type of expansion vessel in combi boilers is the bladder type system. This consists of a cylinder or disc which is split into two parts by a thin rubber sheet called a “diaphragm”. One side of the expansion vessel is filled with water from the central heating system, while the other side is filled with nitrogen. The vessel also includes an air valve that allows the system to be depressurised and repressurised.

Navien UK has discovered that around 1 in 5 of all warranty calls are related to problems that arise from overlooking the expansion vessel during routine servicing, despite its importance.

Problems can include the expansion vessel losing its air gap, which causes restriction by debris. As installers and gas engineers, if you don’t maintain the expansion vessel, it can have a negative impact on the heating system and can significantly reduce the system’s lifespan.

However, regular maintenance can help ensure optimal performance and longevity.

Best practice maintenance of the expansion vessel

As installers and gas engineers, you must identify these various issues to maintain expansion vessels. This can only be achieved by completing regular maintenance of the component to ensure it runs efficiently. This can be done by completing some of the following:

Clean the heating system during installation

When installing boilers, it’s best practice to perform an initial clean of the heating system. This prevents debris from building up, which can make the system less effective and cause parts to wear out. Without this initial cleaning, it can lead to costly repairs for the homeowner.

Giving the boiler system a thorough clean and maintaining high water quality will prevent rusting and allow heat to move around the home more efficiently.

Ensure the expansion vessel is the right size

It’s important to make sure that the expansion vessel is the right size for the boiler and system, as this affects system efficiency. Choosing the right size with the correct system volume calculations helps reduce wear on parts and pressure-related failures, allowing the system to run better for longer.

Any vessel too small for the system will struggle to handle large changes in water volume and pressure. The vessel fitted inside the boiler is ideal for a basic home pipework and protects the boiler. However, if the system size is larger, then a bigger expansion vessel is required to ensure it works effectively.

Check for leaks

Leaks from the air valve on the expansion vessel likely mean maintenance is required. Any leaks in the vessel will lead to intermittent heating system failures and damage to other components. Spotting and fixing leaks quickly is vital for keeping the system running smoothly.

Check the expansion vessel during annual service

Another common cause of boiler faults is not completing the correct annual service checks. This includes draining the system and ensuring the expansion vessel is working under the correct pressure.

Only by understanding these issues and conducting regular checks as part of the yearly maintenance can you prevent these problems from occurring.

How to do an expansion vessel test

As installers and heating engineers, you play a key role in taking preventive steps to keep your customers’ boiler systems in top condition. By cleaning well and following service guidelines, you help homeowners keep their systems running for a long time and avoid pricey repairs.

Expansion vessel testing should always be included in the yearly service. Follow these steps to maintain this component:

1.     Remove the water from the heating system

Draining the system allows for checking the component’s operational air pressure. Free movement of water ensures it is completely removed from the vessel when pressurising, providing an accurate air pressure reading. This avoids confusing your readings with water pressure readings.

2.     Attach the pressure gauge pump to the air test valve

This will allow you to check that the air pressure in the vessel is at 1.0 bar (100 kPa), adjusting where necessary using the attached pump. It’s important for the system that the pressure is no more or no less than this.

3.     Check for air leaks

Once complete, make sure that no air is leaking from the air test valve with the help of a leak detection fluid. If you experience constant water spraying out of the valve, it likely means that the vessel has failed. You can confirm this by pressuring with air and if the pressure doesn’t hold and instead continues to drop, this means component failure. This will require component replacement.

Failing to perform this check could lead to the expansion vessel operating incorrectly, which puts unnecessary pressure on the boiler and system. Taking these preventive steps during routine service helps heating systems last longer and operate more efficiently.

Most importantly for your customers, you provide peace of mind and could save them money in the long run.

Maintenance of expansion vessels is essential for boilers

After finishing maintenance tasks, refill the system and test it thoroughly to ensure it is all running perfectly. This allows you to check the system’s condition and reduces the chance of unexpected breakdowns.

In summary, preventive care and regular maintenance of the expansion vessel are essential for keeping boilers and heating systems in the best condition. From cleaning during installations to following service guidelines and choosing the right vessel size, all these preventive actions will help your customers avoid expensive repair bills.

For more support with installing or maintaining Navien boilers, check out manuals and brochures for all our products or discover our boiler training courses across the UK.