Not true. The expansion tank is not a water hammer arrester. The primary purpose is to maintain a constant and safe water pressure in your house and to protect the water heater from developing excessive water pressure. They also protect your plumbing fixtures from damage due to excessive water pressure. Expansion tanks are required by the plumbing code for all new construction.

Here is another potential issue if there is not an expansion tank on the water heater. Water heaters have a glass lining and without a properly functioning expansion tank the high pressure can damage the glass lining and reduce the life of the water heater.

The reason expansion tanks are now needed is because most municipalities now essentially have check valves which no longer allow water to back flow into the municipal water system when the water heater pressure is too high.