Water expands when heated. This extra volume of water needs somewhere to go. When I started 30 years ago a expansion tank was mainly found only on a hot water boiler system. Today however it is mandatory that the inlet cold water supply line at your water meter assembly have a back flow or check valve device to eliminate any chance of contaminating the public water supply.
When your water heater heats water it creates thermal expansion. A expansion tank is installed on the cold water line by your hot water tank. Left untreated thermal expansion can be vary unsafe. The increased pressure in a closed system will make your safety relief valve leak and cause other flexible hoses or pipes to burst.
The thermal expansion can also cause the internal flue in your gas or propane water heater to collapse and cause carbon monoxide to enter your home.
Miller Plumbing Heating Cooling LLC Pittsburgh PA 15234 412-848-3253 www.callmiller.net