
In the simplest terms, a water heater expansion tank alleviates some of the intensive load that your water heater undergoes every day. Thermal expansion is the process of matter changing shape, area, or volume in response to temperature; for water, this thermal expansion process means that water expands when it is heated. So when a water heater heats up water within the sealed tank, the water volume inside the tank increases as the water expands, and also increases the pressure against the inside of the tank. Most homes have a closed water system, which means their supply of water only goes one way: from the municipal supply into the home. That means that the extra water and pressure don’t really have any outlet, and therefore, your water heater tank takes the brunt of that pressure increase.

While water tanks are specifically designed to withstand these temperature and pressure changes, in a long term fight of water and physics versus metal, metal will always take the bigger beating. Your metal water heater tank will eventually succumb to the intensive heat, water, and pressure it experiences everyday, but a water heater expansion tank can increase that tank longevity by a truly significant amount. A water heater expansion tank is an overflow receptacle that provides a way for your water heater tank to release the buildup of extra hot water and pressure in a safe and effective way, without losing any hot water or requiring an open water system. The excess hot water (and pressure) is stored in the expansion tank when the inner tank pressure is too high, and then fed back into the water heater tank when hot water is turned on.

A water heater expansion tank can do more than just increase the lifespan of your water heater tank. The extra pressure caused by thermal expansion can build up within your home’s water system, and can actually reach other water sources in your home. Most homes use a closed water system, which means that excess water and/or pressure cannot push back into the municipal water system; so when a water heater experiences the water expansion, the excess pressure is fed back into your home water system. The built-up pressure can cause turned-off faucets to drip, toilets to run, and put a lot of intensive stress and pressure on your home’s piping, potentially causing some premature damage to water fixtures all over your home.