What Is the Purpose of a Water Heater Expansion Tank?

Because water will expand when heated, the excess pressure inside the water heater tank needs to be released. In the past, the expanding water in the tank would simply drain back out into the municipal water supply where it came from. Today, the water main is designed to prevent the backwards release of pressure, known as backflow, by employing a check valve. The check valve prevents waste water from inside the house returning to the water supply where it can contaminate the supply of fresh water. A water heater expansion tank is another small tank that is attached to the water supply pipe of the water heater. The expansion tank is designed to handle the thermal expansion of water as it heats up in the water heater, preventing excessive water pressure. If water pressure gets too high, it can damage valves in plumbing fixtures, joints in supply pipes, and the water heater itself. Expanding water from the water heater flows into the expansion tank, relieving water pressure in the system.  

What if my water heater doesn't have an expansion tank?

Most homes that have a check valve on the water main do not have an expansion tank, since it wasn't required until recently. This may or may not cause excessive pressure buildup, depending on the specific design of the plumbing in the house. If you are noticing that washers in plumbing fixtures are deteriorating rapidly, or water is dripping from the relief valve on the water heater, it may be wise to add an expansion tank. It can be low-cost insurance against more costly damage to your home's plumbing system.