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What Is a Water Heater Expansion Tank? (And Why You Might Need One)

Tips & Definitions from George Brazil

A water heater expansion tank is a small tank that attaches to your water heater. It helps protect your water heater tank from leaking or bursting.

To show you how this works, we’ll look at the problem of thermal expansion and how an expansion tank helps.

The problem: Water expands when it’s heated

Water has a property known as thermal expansion which means water expands as it’s heated.

So as your water heater heats water, the water expands. And the extra volume needs a place to go. For most homes in Phoenix, the extra volume is pushed back into the cold water inlet and into the city’s water supply lines.

But there’s a problem. Many homes now have check valves or pressure-regulating valves (PRVs) on the water main. These are one-way valves that don’t allow water to flow back into the city’s main water supply.


These are good because they prevent contamination of the city’s water supply and/or keep your home’s water pressure at a more constant rate.

However, it also means that the excess water from your water heater needs somewhere else to go. And that usually means it drips out of your water heater’s T&P (temperature and pressure) valve. This valve is designed to release water from your tank when the pressure or temperature has gotten to unsafe levels that could lead to a leak or tank burst.

But that leaves you with a puddle on your floor constantly.

And in many homes the T&P valve sticks shut because of lack of maintenance. In that case, the pressure builds up in the tank and the tank eventually bursts or leaks to release the pressure.

The solution: Water heater expansion tank

An expansion tank is attached to the water line going into your water heater tank to absorb the excess water.

The expansion tank is able to absorb the excess water because it has a rubber-like diaphragm with an air cushion.

The excess water from the water heater flows into this expansion tank and the air in the tank is compressed to make up for the increased water volume.

Get a water heater expansion tank installed

If you have a tank water heater and a check valve or PRV on your water main, then you should consider a water heater expansion tank. In the Phoenix area, contact George Brazil Plumbing for more information.

Further reading:

George Brazil Plumbing and Electrical serves the entire Phoenix metro area, including Glendale, Ahwatukee, Queen Creek, Apache Junction, and everything in between.

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