Have you ever noticed that small, round metal tank installed above or near your water heater and wondered what it does?

It’s most likely a thermal expansion tank, and it serves an important purpose in protecting your home’s plumbing system.

What Does an Expansion Tank Do?

When water heats up, it expands. In a closed plumbing system, where a check valve, pressure-reducing valve, or backflow preventer prevents water from flowing backward into the main water supply, that expanding water needs somewhere to go.

A properly sized and functioning expansion tank absorbs the additional water volume, helping prevent excessive pressure that can damage plumbing components and contribute to leaks.

How Should Expansion Tanks Be Installed?

Expansion tanks are commonly installed vertically on the cold-water supply line near the water heater.

Depending on the manufacturer’s instructions, some models can also be installed horizontally. When installed horizontally, the tank must be properly supported to prevent excessive weight and stress on the piping and fittings.

An expansion tank should never be left hanging from piping that cannot safely support its weight when full of water.

Proper sizing, support, and air-pressure adjustment are essential to a safe and effective installation.

Are Expansion Tanks Required in North Carolina?

North Carolina’s Plumbing Code requires thermal expansion control when a storage-type water heater is supplied through a check valve, pressure-reducing valve, or backflow preventer.

These devices create a closed plumbing system, preventing expanding water from flowing backward into the public water supply.

When required, the thermal expansion control device must be properly installed on the cold-water supply side of the water heater, downstream of the devices that restrict reverse flow.

How Long Does an Expansion Tank Last?

Expansion tanks don’t last forever.

Like other plumbing components, they can deteriorate from age, corrosion, water quality, and repeated pressure changes. Their internal diaphragm can also fail, preventing the tank from doing its job.

We recommend having your expansion tank inspected annually, including checking for visible corrosion, leakage, proper support, and correct air pressure.

At Barry Williams Plumbing, we recommend evaluating expansion tanks for replacement around the five-year mark, particularly in installations exposed to challenging water conditions. Actual service life varies, and replacement should be based on the tank’s condition and the manufacturer’s recommendations.

Don’t Ignore Rust or Corrosion

One of the most important warning signs is visible rust, especially around the tank’s connection or seams.

I’ve personally seen the damage a failed expansion tank can cause. In one instance, an expansion tank failed while the homeowner was away, allowing water to spray into the garage and damage a substantial amount of drywall.

If you notice significant rust, corrosion, leaking, or other signs of deterioration, have the tank inspected by a licensed plumber promptly.

A relatively small plumbing component can cause expensive water damage when it fails.

Barry Williams Plumbing Co.

Protecting your home starts with understanding the components that keep your plumbing system operating safely.

How to Replace Your Water Heater Expansion Tank

• Turn off the power to the water heater.

• Turn off the cold-water supply valve to the water heater.

• Open a hot-water faucet in your home to relieve the pressure.

• Carefully unscrew and remove the existing expansion tank.

• Check and adjust the new tank’s air precharge to match the home’s water pressure, then install it using approved thread-sealing tape.

• Turn on the cold-water supply valve.

• Wait until a steady stream of water flows from the hot side of an open faucet.

• Check all connections for leaks.

• Turn the power back on to the water heater.