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.