Thermal expansion is caused when water is heated. When a backflow assembly is installed, it creates a ‘closed system.’ Plumbing code requires a relief of the excess pressure caused by thermal expansion. The most common way to resolve this is a thermal expansion tank installed at the water heater. The expansion tank has an internal air bladder that compresses and contracts, thus relieving the expansion pressure when a plumbing fixture is turned on. Thermal expansion must be addressed due to the high volume of damage it may cause the plumbing system.