A water heater expansion tank, also known as a thermal expansion tank, is a small, secondary tank that attaches to the water supply pipe leading to the water heater. To answer the question 'what does an expansion tank do,' its primary role is to absorb the extra volume created when water heats up and expands. The water heater expansion tank purpose is clear: by providing this additional space, it prevents dangerous pressure from building up in your water heater and plumbing system.

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The check valve ensures that the water inside your home stays separate from the external supply, but it also traps the expanding water within the system. Without a way to relieve the increased pressure, this can lead to damage in your pipes, water heater, and plumbing fixtures. This is where the water heater expansion tank comes into play.

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The expansion tank, also referred to as a potable water expansion tank, is divided into two chambers: one filled with air and the other meant to hold water. As the water in the heater expands, it flows into the tank, compressing the air in the other chamber. This compression relieves the pressure within the closed water system. By regulating pressure in this way, the tank protects vulnerable components like:

Without this pressure relief provided by an expansion tank for water heater systems, over time, these components can fail, resulting in leaks, broken pipes, or a damaged water heater.

If your home doesn’t have an expansion tank, you could still be at risk of pressure-related issues. Here's what you might notice:

As water heats up in a water heater, it naturally expands, a process known as thermal expansion. This expansion increases the volume of water, leading to excess pressure within the water heater's tank. In the past, this expanding water would simply flow back into the municipal water supply where it originated. However, modern plumbing systems are equipped with check valves that prevent water from flowing backward into the public supply, reducing the risk of contamination.

These symptoms may indicate that your system is struggling to manage the excess pressure. Installing an expansion tank provides a low-cost solution to mitigate these risks, protecting your plumbing system from costly repairs.

Many homes built before certain plumbing codes were enacted may not have an expansion tank, as they were not a standard requirement. In older systems, open water systems allowed the expanding water to flow back into the city’s water lines. However, in most modern homes with closed water systems (due to check valves or pressure-regulating valves), this is no longer possible.

Thermal expansion is a basic principle of physics. When water is heated, its molecules move faster and spread out, increasing the volume. In a confined space like a water heater, this expanded water has no room to go unless an expansion tank is present.