Thermal Expansion

What is a closed system?
A closed system is created when an approved backflow prevention assembly or a single check valve (not approved for backflow prevention) is installed at a customer's service connection.  The backflow prevention assembly or single check valve does not allow water to flow backwards from the customer's private water system into the PWS's distribution system.
What is Thermal Expansion?
Thermal expansion is a result of heating water.  When water is heated, its density decreases and its volume increases.  Backflow prevention assemblies and other one-way valves installed at a customer's service connection eliminate a path for expanded water to flow back to the distribution system, resulting in increased system pressure.  This increase in pressure can result in: pressure surges, dripping faucets, chronic or continuous dripping of temperature and pressure-relief valves on hot-water heating tanks, and other mechanical problems with hot-water tanks, including distortion and rupture.
 
Thermal Expansion Diagram

Thermal Expansion Diagram