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Expansion Vessels

Expansion Vessels

Expansion Vessels

Sealed central heating systems are the most commonly used form of heating system installed throughout most of Europe and the rest of the developed world. To accommodate for increases in water pressure sealed central heating systems require an expansion vessel to be installed.

An expansion tank or vessel is a small tank which is used to protect cold water heating systems from excess levels of pressure when the heating system is at its maximum heating capacity. When water is heated in a closed heating system it expands creating an increase in pressure. To protect the heating system from becoming damaged an expansion tank is an essential part of the installation process.

Expansion vessels can come in a variety of shapes and sizes however the majority of expansion vessels are designed with the same principles in mind. The expansion vessel itself is separated in to 2 sections by a flexible diaphragm in the middle.

Half of the expansion vessel is empty and the other is pre pressurised with air. When an expansion vessel is installed as part of the heating system the water enters the empty half of the tank, allowing extra room within the system. As the temperature of the water increases so does the water pressure. Once the water reaches its maximum temperature and pressure the diaphragm flexes against the pre pressurised air cushion providing more room within the expansion vessel.

At Direct Heating Supplies we stock a great selection of boiler expansion vessels in sizes ranging from 8 to 80 litres, so you are bound to find one perfect for your personal heating needs. Contact our sales team on 0800 181 4488 for friendly advice and information on our expansion vessels or explore our range today.

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Expansion Vessels FAQ’s


Boiler expansion vessels can sometimes seem complicated to fully understand, however they are an exceptionally important part of a sealed central heating system to ensure boiler performance and efficiency. Take a look below at some of the most popular questions surrounding boiler expansion vessels.


Is it essential to have an expansion vessel?

Depending on the type of heating system you have installed an expansion vessel may be an essential component of your heating system. Closed heating systems require an expansion vessel to be installed to help accommodate increased water pressure, failure to do this can result in water heater tanks or pipes becoming damaged or splitting. Other signs that you may require an expansion vessel to be installed include:


  • Having a closed heating system
  • Noticing fluctuating high and low water pressure
  • Having a pressure regulating valve
  • To help extended the lifespan of your boiler

  • Still not sure? We would advise contacting a gas safe registered engineer who will be able to advise on the best course of action for your heating system.


    Who can install an expansion vessel?

    To ensure your safety at Direct Heating Supplies we would recommend using a gas safe registered heating engineer to install your expansion vessel.


    How does an expansion vessel work?

    An expansion vessel is separated in to 2 sections by a flexible diaphragm in the middle. One half of the vessel is empty ready to be filled with water and the other is pre-pressurised with air.

    When an expansion vessel is installed the water enters the empty half of the tank, making more room within the heating system. When the boiler is working at its full capacity and the temperature of the water increases so does the water pressure, this is when extra space is required. Once the water reaches its maximum temperature and pressure the diaphragm flexes against the pre pressurised air cushion making more room within the expansion vessel.


    Does my expansion vessel need maintaining?

    Like the rest of your gas heating appliances and equipment, it is recommended that expansion vessels have an annual safety check by a gas safe registered engineer.