Correct expansion vessel selection – What installers need to know

Michael Pitt, UK Technical Advisor at Spirotech, answers key questions about the role of expansion vessels in maintaining heating system performance and longevity:

Maintaining the correct pressure ensures the heating system operates efficiently and avoids unnecessary wear and tear. Suboptimal pressures can lead to boilers cutting out, lifting of safety valves, water loss, and ultimately allowing gasses into the system. This reduces performance, efficiency, and can shorten the system’s lifespan. 

Isn’t the expansion vessel inside the boiler enough? 

For average domestic homes, the built-in expansion vessel is often sufficient. However, larger systems – especially those servicing buildings with multiple storeys – or commercial buildings will likely require additional expansion vessel volume. In these cases, a calculation on the required volume is made to select the suitable expansion vessel. 

What does an expansion vessel actually do? 

When liquid in sealed systems is heated, it will expand causing an increase in pressure. Conversely, a drop in temperature will cause the liquid to contract and therefore cause a decrease in pressure. The expansion vessel absorbs the increased volume of water as it heats and expands, and then pushes that liquid back into the system when it contracts. The result is the system pressure is maintained within a working pressure hysteresis. Without it, or with an expansion vessel that’s incorrectly sized, pressure in the system can spike or fall outside of the required operational range. This can lead to boiler shutdowns, safety valve activation, and constant topping up of the system, which in turn introduces air and causes long-term issues. 

How do installers determine the right size vessel? 

There are four main factors to consider when sizing an expansion vessel: 

• Total water volume in the system 

• Static height of the building or minimum pressure requirements 

• Boiler output in kW 

• Maximum system temperature 

The Spirotech EVN Vessel range offers many different sizes, giving installers flexibility to select the most appropriate size. 

Why is the gas charge so important, and how should it be set? 

The gas charge affects how well the vessel absorbs expanded liquid. If it’s too low, the bladder fills prematurely with cold water, reducing expansion volume left usable for thermal expansion. If it’s too high, system water can’t enter the vessel at all, leading to excessive pressure and system shutdowns. 

For the EVN range, Spirotech recommends setting the charge 0.2 bar below the cold fill pressure. This provides a small cushion, maintaining positive pressure even as the system cools down whilst maintaining optimal volumes for thermal expansion. 

How should engineers position the vessel in the system? 

Best practice is to install the expansion vessel on the return leg of the system, suction side of any circulating pumps. This location is known as the “pressure neutral point” and ensures the vessel reacts effectively to a change in pressure. Placing it here also exposes the vessel to cooler water, which is gentler on the membrane and prolongs its life. Additionally, the vessel should be installed in a vertical position with the port at the bottom. This orientation helps prevent debris and dirt from settling in the bladder, reducing the risk of internal corrosion and maintaining long-term reliability. 

What features should engineers look for in an expansion vessel? 

When selecting an expansion vessel, installers should consider both performance and practical features that support long-term system health and ease of servicing. The Spirotech EVN Pressure Expansion Vessel, for example, uses a high-quality bladder that helps reduce the risk of corrosion and helps retain pressure more effectively over time. It also includes an easy-to-access Schrader valve to simplify routine maintenance, and comes with a compact design and mounting bracket to make installation in tight spaces much easier.  

Any final tips for annual servicing? 

Yes, it is important to inspect the expansion vessel during annual boiler servicing. It is important to isolate and drain the vessel before checking the gas charge, as this can drift over time. Ensuring the correct pressure is maintained will help the system operate reliably, avoid unnecessary shutdowns, and prevent air from being drawn in, ultimately protecting both performance and efficiency in the long run. 

For more information about Spirotech’s range of water treatment solutions, including expansion vessels, visit www.spirotech.co.uk.