Where does an expansion tank connect?

An expansion tank connects to the cold-water line near the water heater. The connection point should allow the tank to stay supported and accessible for inspection.

Does an expansion tank need maintenance?

An expansion tank needs inspection for leaks, corrosion, loose connections, and loss of air charge. A plumber can check these conditions during water heater service.

What happens when an expansion tank fails?

A failed expansion tank may stop absorbing expanding water. Pressure changes can then place more stress on valves, fittings, and the water heater.

Glossary

What Is an Expansion Tank?

An expansion tank is a small pressure vessel connected to a water heater system. The tank contains air and water separated by a flexible diaphragm. As heated water expands, the tank accepts the extra volume and helps limit pressure changes in a closed plumbing system.

Quick Facts About Expansion Tank

Key Takeaways About Expansion Tank

A diaphragm separates air from the water supply.
The tank connects near the water heater.
An expansion tank isn't a storage tank for extra household water.

How Expansion Tank Works

A hand holds a gray metal tank beside copper pipes in a utility room

An expansion tank has an air chamber and a water chamber. A flexible diaphragm keeps the chambers apart. When the water heater warms the supply, the water expands and pushes against the diaphragm.

Related glossary terms: Thermal Expansion, Water Pressure, Pressure Reducing Valve.

The tank takes in that added volume, which helps reduce pressure swings in the piping. A plumber connects the tank to the cold-water line near the heater. The tank's air pressure must match the system's needs, and the connection must remain secure.

Thermal expansion happens in any water heater as water heats. A closed system gives expanded water less room to move, so an expansion tank provides a controlled space for that change.

A plumber points at a water heater while talking to a woman inside a utility room

Expert Note

When we install a water heater, we check whether the plumbing system can absorb heated water as it expands. We inspect the tank connection, support, and air charge. We also look for pressure symptoms that may point to a sizing or installation issue.

Expansion Tank in Practice: A Real-World Example

A new water heater may heat water in a closed plumbing system, which raises pressure as the water expands. An expansion tank connected to the cold-water line gives that extra volume a place to go.