Expansion Vessels


Expansion vessels are commonly referred to as expansion or pressure tanks. They are used in domestic applications as part of pressurised heating systems. The expansion vessel contains water and air, which absorbs any excess water in central heating systems. It helps maintain optimum pressure levels in the system by allowing for a specific expansion of the water as it heats up. This acts as a protection mechanism from extreme pressure.

0239 226 8511

If you have any questions, feel free to call us or use our quote form

talk pumps, today.

To speak to one of our agents online, click here

How does an expansion vessel work?

You will have an expansion vessel if you have a closed-water heating system in your home, such as a combi boiler. By purchasing an expansion vessel, our customers can be assured that their heating system can withstand thermal expansion and water hammer effects when water heats up.

Expansion vessels accommodate changes in the water volume in the system as it heats up and cools down. An expansion vessel is comprised of two parts separated by a flexible diaphragm or bladder; one contains compressed air or typically nitrogen gas, which is charged to a certain pressure, for example, 5%, and the second contains water. When the water in the system starts heating and expanding, the air is compressed to absorb the extra volume of water, reducing the pressure in the system. When the water cools, the compressed air or nitrogen gas will push against the flexible diaphragm, returning the water into the system and maintaining the optimum pressure levels, which keeps your system running efficiently.

Without an expansion vessel, the pressure within the unvented water cylinder would rise, forcing the safety valves to open. This puts you at risk of burst pipes and damage to plumbed household appliances such as dishwashers and washing machines. It is essential to note that cold water systems also require expansion vessels. Without the vessel, the drop in pressure can leave a vacuum when the water cools down. Air valves are connected to the vessels, allowing the system to be depressurised and repressurised over time.

Tricel supplies a wide range of pumping solutions, including expansion vessels, from the top manufacturers in the industry, including Grundfos, Lowara and Flamco. We provide high-quality diaphragm expansion vessels from Flamco, including Flexcon, which is suitable for heating installations, or the Flamco Airfix P2, which is ideal for potable water systems. Lowara has fixed diaphragm expansion vessels, including the Lowara 8 LV Vertical Expansion Tank, an 8-litre vessel suitable for potable water.

What size expansion vessel do I need?

Sizing an expansion vessel can seem like a complicated task, but a few simple rules can help the process. A general rule of thumb is to use 10% of the system volume, rounding up to the nearest vessel size. This allows for 10 litres per standard radiator and an additional 10 litres for a boiler. For example, a system with ten radiators and a modern boiler would equate to an approximate system volume of 110 litres. 10% equate to 10 litres; therefore, a 12 litre vessel is suitable using this calculation.

Another rule of thumb customers can use to size their expansion vessels is using the boiler kW size. This method allows for a 1-litre capacity per kW. For example, 8kW output would require a 9-litre vessel.
Microfill has a helpful calculator tool to help you size the right vessel size you need. If you need to discuss further regarding the size, our expert sales team is on hand to help you size the suitable expansion vessel for your specific requirements.

expansion vessels


Designed to deliver sufficient volume and water pressure


Full range of Fire Protection Pumps from small to packaged systems


Pumping stations offer the ideal solution where gravity drainage is not possible


Recent Projects Completed 

Tricel is one of the largest UK pump distributors specialising in Commercial Building Services projects. We offer a wide range of products from circulator pumps to Wet Riser booster sets.

Our team includes experts which are happy to select and quote your equipment requirements, or attend site and size the equipment from scratch. We at Tricel pride ourselves on the highest possible levels of customer service, before, during, and after project completion.

We provide preventative maintenance contracts and also cater for specific repairs, including replacements seals, or complete overhauls.




Apartment Blocks

Commercial Buildings


Our skilled team of specialist engineers are ready to specify, supply, install, service and maintain pumps and their control systems


We provide preventative maintenance contracts and also cater for specific repairs, including replacements seals or complete overhauls


Our technical team will visit your location and complete an entire site survey to ensure your project is accurately specified, and the most suitable system installed and maintained to the highest quality


Through our 35 years of operation, we have accumulated a wealth of product knowledge and application experience that we use to support our customers