Quick answer
Water expands as it heats. In a closed system, where a check valve or pressure reducing valve stops water from flowing back to the city main, that extra volume has nowhere to go, so pressure spikes and the water heater relief valve drips. An expansion tank, a small tank on the cold line near the heater, absorbs it. Many local codes require one on a closed system. Tap the tank: a full, heavy, water-sounding tank, or water from the air valve, means it has failed. Call (888) 217-6781 to reach a plumbing company taking calls in your ZIP.
Follow the short menu, then your 5-digit ZIP. The line goes quiet for a few seconds while it connects; stay on. If it does not connect, call once more: companies taking calls change minute to minute.
Written & reviewed by Marcus ReedIICRC WRT
Reviewed November 4, 2026· Next review May 2027
Water expands when it is heated. In older plumbing, that extra volume simply pushed back into the city main. Many homes now have a check valve, a backflow preventer or a pressure reducing valve on the main line, which stop water flowing back. That makes the system closed, and every heating cycle pushes the pressure up until something gives, usually the water heater's relief valve.
An expansion tank is a small tank, usually on the cold line above the water heater, with an air cushion behind a rubber bladder. As the heated water expands, it pushes into the tank instead of straining the pipes.
Do You Need One?
| Your system | Expansion tank |
|---|---|
| Pressure reducing valve on the main | Usually needed; many codes require it |
| Check valve or backflow preventer at the meter | Usually needed |
| Open system, nothing stopping backflow | Often not required |
| Replacing a water heater in a closed system | Commonly required by the inspector |
The rule is set by your local code, so a plumber or the building department has the final word.
Signs Yours Has Failed
- 1Tap it. The top half should sound hollow. A solid sound all the way up means the bladder has failed and the tank is full of water.
- 2Feel it. A working tank's top half stays near room temperature.
- 3Press the air valve on the end briefly, like a tire valve. Air is right. Water means the bladder has burst.
- 4Watch the relief valve. A water heater relief valve that drips after heating cycles is a classic sign. See water heater leaking from top or bottom.
- 5Check pressure. A gauge on a spigot that climbs overnight points to expansion or the pressure valve; see signs of a bad pressure reducing valve.
Talk to a plumber about this
One call reaches a plumbing company taking calls in your ZIP.
Sizing and Replacement
A tank is sized to the water heater and house pressure, and its air charge is set to match the house pressure before it goes in. Replacement is a short plumbing job, often done at the same time as a water heater replacement. See water heater installation.
The Call
Call when the relief valve drips, the tank sounds full, or you are replacing a water heater on a system with a check valve or pressure reducing valve. (888) 217-6781 connects you to a plumbing company taking calls in your ZIP. Follow the short menu, enter your 5-digit ZIP and stay on while it connects; if it does not connect, call once more, because the companies taking calls change minute to minute.
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Frequently Asked Questions
Do I need an expansion tank on my water heater?
If your house has a check valve, backflow preventer or pressure reducing valve on the main line, the system is closed and many local codes require one. Ask a plumber or the local building department.
How do I know if my expansion tank is bad?
It sounds solid when tapped all the way up, feels heavy, or spits water from the air valve. A dripping water heater relief valve is another sign.
How long does an expansion tank last?
Often around 5 to 10 years, as the internal bladder wears out.
Sources
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Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.
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