Thermal expansion occurs when water is heated during non-use periods. The installation of a Pressure
Reducing Valve (PRV), Check Valve or Back Flow Preventer "closes" the water system, leaving water with no
room for expansion. National Standard Plumbing code: 10.5.7 requires backflow prevention, to prevent
backflow into the water main.
Thermal expansion in a closed plumbing system can be damaging, dangerous and costly. Its effects include damage to water heater connections, gas water heater flue tubes, pumps serving washers and dishwashers, leaking faucets, "weeping" of water through the water heater T&P Relief Valve, and noisy water hammer in the pipes.
A properly sized Expansion Tank eliminates these problems, by giving water a place to go when thermal expansion occurs. When a water heating cycle ends, or when any fixture is opened within the system, the impact of thermal expansion is reduced, and water drains out of the expansion tank back into the system.