Expansion vessels for Thermal Stores

Expansion vessels are needed when you have a sealed, or pressurised, heating system. Most new heating systems in the UK are sealed and indeed most boiler stoves and biomass boilers are installed on sealed systems on the continent. Over here there are now a growing number of boiler stoves suited to sealed systems and hence expansion vessels are needed. When a thermal store is involved the volume of water in the system becomes rather large and hence the expansion vessel volume also needs to accommodate.

An expansion vessel is essentially a metal tank, as you can see, with a balloon/membrane inside. As it warms up the water in the system expands and pushes into the balloon/membrane. When installing the system your heating engineer pumps air into the other side of the balloon/membrane to pre-charge this part of the tank. This pre-charge pressure is dependant on the height of your heating system. Essentially the higher your house is the higher the pressure will be at the base of your heating system because all that height of water is pressing down. By pre-charging the air on the other side of the membrane you ensure that there is somewhere for the water to expand into, otherwise the water would just fill full of water the moment it was connected.

The size of the expansion vessel is determined by the static pressure of the system (so the height of the whole heating system) and, most importantly, the maximum operating pressure. The maximum operating pressure of the system is decided by the element in the system with the lowest maximum operating pressure. As the water in the system heats up it expands and compresses the balloon in the expansion vessel. As the balloon compresses the air pressure in the balloon increases. We have to calculate the overall expansion vessel volume so that when the water has fully heated the pressure in the balloon is a little less than the maximum operating pressure of the system.

Our heating engineers can calculate the right size of expansion vessel for your system. You will need to tell them the total system volume, the height of your system from the lowest point to the highest point, the maximum operating pressure (so the element in the system with the lowest operating pressure), and whether the system is filled with anti-freeze. They can then do the rest.