Expansion Tank — tank that absorbs heated water pressure
An expansion-tank is a pressure-control tank that absorbs thermal expansion in a closed plumbing or hydronic heating system.
What It Is
When water heats up in a closed system, it expands and raises pressure. The expansion tank provides an air-cushioned space that absorbs that extra volume so the system pressure stays in a safe range.
Residential homes commonly use expansion tanks on water heaters and hydronic boilers. Without one, pressure relief valves may drip and components can wear out faster.
Where It Is Used
Expansion tanks are used on domestic water heater systems with backflow prevention and on hot-water heating boilers. They are usually mounted near the water heater or boiler piping.
How to Identify One
Look for a small metal tank, often cylindrical or oval, connected to the piping near a water heater or boiler. Many residential units are painted white, gray, or blue.
Replacement
Replacement is needed when the internal bladder fails, the tank becomes waterlogged, or corrosion appears at the connection. Symptoms can include pressure swings, relief valve discharge, or a heavy tank that sounds full.
Frequently Asked Questions
Expansion Tank — FAQ
- What does an expansion tank do on a water heater?
- It absorbs the extra pressure created when heated water expands in a closed plumbing system. That helps protect the water heater, valves, and piping.
- How do I know if my expansion tank is bad?
- A failed tank may feel unusually heavy, sound full of water, or coincide with pressure relief valve dripping. Pressure fluctuations in the system are another common clue.
- Is an expansion tank required on every water heater?
- Not always, but many homes need one when a pressure-reducing valve or backflow device creates a closed system. Local plumbing code and the piping setup determine the requirement.
- Can a waterlogged expansion tank cause leaks?
- Yes. When the tank can no longer absorb expansion, pressure can rise and force water out of the relief valve or stress fittings elsewhere in the system.
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