An expansion tank is required if a plumbing system is “closed”, which means that the back-flow that normally runs between the house and the main water supply, is blocked. However, when water is heated, it starts to expand, which causes a rise in water pressure.
The increased pressure must go somewhere. If there is nowhere for the pressure to go, it places extra stress and strain on your water heater and your home’s plumbing system. Increased pressure will shorten the life of your water heater. However, expansion tanks provide relief from this excess water pressure and protect your home.
A water pressure PSI test will determine if you need an expansion tank or not.