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  • Writer's pictureG&C Plumbing and Heating

What Is the Purpose of An Expansion Tank on A Water Heater?

Updated: Jun 5, 2023

Wondering what a water heater expansion tank is, and why it’s an important element of any tank-based water heater? In this guide from G&C Plumbing, we’ll discuss the basics of this water heater component, and help you understand how it works – and why it’s important. Let’s get started.

Water Expands When It’s Heated in Your Water Heater

Like the vast majority of other substances, water expands when it’s heated.

This is because when heat is applied to water, this causes the molecules to become less dense, which means that the volume of the water itself becomes higher.

If you’ve ever heard of a water heater exploding, this is why – uncontrolled increases in pressure have caused the tank to rupture.

As water heats up, it can expand dramatically, and if it’s hot enough to turn into steam, this can add even more pressure to the water heater.

If the relief valve is malfunctioning, this can lead to an explosion.

In the past, expanding water in the hot water heaters would simply be released back into the main water supply where it came from – relieving excess pressure.

Today, though, this is prevented by a “check valve.”

This valve prevents water backflow, which is necessary because wastewater sent back to the water supply by a home could contaminate the fresh water supply.

An Expansion Tank Gives Excess Pressure A Place to Go – Keeping Your Water Heater Safe

So, if your plumbing system’s check valve prevents water from flowing back into the water supply, how does your water heater relieve excess pressure?

If you have a newer water heater, the answer is an expansion tank.

This is a secondary tank that attaches directly to your hot water heater.

Water heats up, and if the pressure ever exceeds a certain set threshold, some of the expanding water can be drained directly into the expansion tank. There is a mechanism inside the expansion tank, along with pressurized air that need to be calibrated to work harmoniously with the same incoming pressure of your direct water main line.

What If I Don’t Have A Water Heater Expansion Tank?

If you don’t have an expansion tank, your hot water heater may be under excessively high pressure.

If you notice that plumbing components like washers wear out quickly, or if you notice a lot of water dripping from the relief valve on the water heater, you may want to add an expansion tank. Doing so is a quick and easy job, and helps prolong the lifespan of your water heater – while also keeping your family safe.

Need A Water Heater Expansion Tank or A New Water Heater? Contact Us Now!

At G&C Plumbing, we’re here to help you with plumbing projects of all types and sizes. If you’d like to learn more about expansion tanks or you’re in the market for a new water heater, just contact us online or give us a call at 508-541-8783.

What Our team of expert plumbing contractors is always here for you.

1 Comment

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Anne Stewart
Anne Stewart
Aug 24, 2022

I first read about water heater expansion tanks from a post by American Home Water & Air, you can click here to view the post. It got me interested to have one installed at home but I'm doing a bit more research first to know how it works and see if it really is a must for me. Based on the information that you shared here, it seems like it really is beneficial to have a water heater expansion tank at home.

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