What is the purpose of an expansion tank on my water heater?

Expansion tank on top of water heater.

An expansion tank serves an important purpose in a water heater system by addressing the issue of thermal expansion. Thermal expansion occurs when water is heated, causing it to expand in volume. In a closed plumbing system, such as one with a water heater, this expansion can create pressure buildup within the system, which can lead to potential damage to pipes, fittings, and even the water heater itself.

The purpose of the expansion tank is to provide a space for the expanded water to go without causing excessive pressure in the system. Here's how it works:

1. Pressure Relief: When water inside the water heater tank heats up and expands, it increases the pressure inside the system. The expansion tank acts as a pressure relief valve, absorbing the excess pressure caused by the thermal expansion of water.

2. Protecting Plumbing System: Without an expansion tank, the increased pressure from thermal expansion can strain the plumbing system, leading to leaks, bursts, or damage to fixtures and appliances.

3. Prolonging Water Heater Life: By reducing stress on the water heater tank and associated plumbing components, the expansion tank helps prolong the life of the water heater and prevents premature failure.

Expansion tanks are typically installed on the cold water supply line to the water heater, either vertically or horizontally, depending on the space available and the configuration of the plumbing system. They contain a rubber bladder or diaphragm that separates air from water, allowing the tank to absorb the expanded water volume while maintaining a consistent pressure in the system.

In summary, the expansion tank acts as a safety measure to prevent excessive pressure buildup in a water heater system due to thermal expansion, thereby protecting the plumbing system and prolonging the life of the water heater.