What is an expansion tank and why do I need one?

We had a home inspection and the home inspector said that the water heater needed an "Expansion Tank" installed on the cold water supply line.  What does this mean?

All tank type water heaters, regardless of heat source (gas, oil, electric, solar or indirect), can suffer the effects of thermal expansion. In every tank-type water heater, cold water is heated as it enters the water heater tank.. This increases the overall water volume and pressure inside the tank. For safety, the increase in volume and pressure must be relieved in some way. The most common is a dripping at the (temperature pressure relief)TPR valve. While this will reduce some of the pressure it can also damage the valve and prevent it from working properly.

We've never had to have one before, so why now?

Before major controls were placed upon city water supplies, it was possible for excess water pressure build-up in a water heater to flow back into the city water supply. This created a simple and efficient system for removing excess pressure in water heaters. Now most city water supplies are protected by backflow preventers at the meter or home. If a home has a pressure reducing valve (PRV) to reduce the city water pressure to a usable pressure inside the home this also acts as a backflow preventer.

The installation of a thermal expansion tank in the cold water line of the water heater can protect the system from the damaging effects of thermal expansion and increased pressure. Most plumbers will install one for around $200 and if it is installed with a new water heater the cost is usually around $50 to $75 extra.

The thermal expansion tank controls the increased pressure generated within the normal operating temperature range of the water heater. The small tank with a sealed, compressible air cushion provides a space to store and hold the additional expanded water volume.

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