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Fix an expansion tank that is waterlogged

We'll confirm the tank is waterlogged, check the air pressure, and tell you when to call a plumber to replace it.

Troubleshooting · Home maintenance
10–20 min (diagnosis only; replacement requires a plumber)
What you'll need
  • Tire pressure gauge (for checking air pressure at the Schrader valve)
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Steps

Goal: Confirm the expansion tank is waterlogged and know when to call a plumber.

  • Check if the pressure relief valve is dripping or system pressure is high.
  • Good: Pressure is normal and the relief valve does not drip—the tank may still be good. Bad: Relief valve drips or pressure is high—the expansion tank may have failed. Proceed to Tap the tank.

Tap the tank

Goal: Quickly check if the tank sounds waterlogged.

  • Locate the expansion tank on the pipe near the boiler or water heater.
  • Tap the tank with your knuckle. A good tank sounds hollow; a waterlogged tank sounds full, dull, or solid.
  • Good: The tank sounds hollow. Proceed to Check air pressure.
  • Bad: The tank sounds full or dull—waterlogged. Call a plumber to replace it.

Check air pressure

Goal: Confirm the bladder has failed by checking air pressure.

  • Press a tire gauge onto the Schrader valve on the tank. The air pressure should match system pressure (12–15 psi for boilers, 40–60 psi for water heaters).
  • Good: Pressure matches system pressure—the tank may be OK. Check other causes for the relief valve drip.
  • Bad: Pressure is zero or very low—the bladder has failed. Call a plumber to replace the tank.

When to get help

Call a plumber if:

  • The tank sounds waterlogged when you tap it.
  • The air pressure at the Schrader valve is zero or very low.
  • The pressure relief valve drips and the tank has failed.
  • System pressure is high.

Do not attempt to replace the expansion tank yourself unless you are experienced with plumbing. If you smell gas near the boiler, evacuate and call 911 or your gas utility from outside.

Verification

  • The tank sounds hollow when tapped (good) or full/dull (waterlogged).
  • The air pressure at the Schrader valve matches system pressure when the tank is good.
  • A waterlogged tank is identified and a plumber is called for replacement.
  • After replacement, the pressure relief valve stops dripping and system pressure stays normal.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Check if the pressure relief valve drips or system pressure is high.
  2. Tap the tank Tap the tank—hollow means good; dull or full means waterlogged.
  3. Check air pressure Use a tire gauge at the Schrader valve—zero or very low means failed.
  4. Call a plumber A waterlogged tank must be replaced—call a plumber.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • System type (boiler or water heater)
  • Tank sound when tapped (hollow or full)
  • Air pressure reading at Schrader valve
  • Whether the pressure relief valve drips

Is the pressure relief valve dripping or is system pressure high?

A dripping relief valve or high pressure often indicates a failed expansion tank.

Check the pressure relief valve on the boiler or water heater. Check the pressure gauge. Dripping or high pressure: expansion tank may have failed. Normal: tank may still be good—monitor.

Yes No

You can change your answer later.

Monitor the system

If pressure is normal and the relief valve does not drip, the expansion tank may still be good. Tap the tank periodically to check—if it starts to sound full, it is failing.

Done

Does the tank sound hollow or full when you tap it?

A good tank sounds hollow; a waterlogged tank sounds full or dull.

Tap the expansion tank with your knuckle. Hollow: tank may still be good—check air pressure. Full or dull: tank is waterlogged—call a plumber to replace.

Hollow Full or dull

You can change your answer later.

Does the air pressure match system pressure?

Use a tire gauge at the Schrader valve. Boilers: 12–15 psi. Water heaters: 40–60 psi.

Press a tire gauge onto the Schrader valve. Matches system pressure: tank may be OK—check other causes. Zero or very low: bladder failed, tank waterlogged—call a plumber.

Yes No

You can change your answer later.

Check other causes

If the tank sounds hollow and pressure is correct, the relief valve drip may have another cause—stuck fill valve, blocked discharge pipe, or worn valve. See Fix a pressure relief valve that leaks or How to fix a boiler pressure leak.

Done

Call a plumber

A waterlogged expansion tank must be replaced. Call a plumber to shut off the system, drain, remove the old tank, and install a new one. Do not attempt replacement yourself unless you are experienced.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would an expansion tank become waterlogged?
The bladder or diaphragm inside the tank can fail, allowing water to fill the air side. Once waterlogged, the tank can no longer absorb pressure when the system heats—pressure rises and the relief valve may drip.
Can I fix a waterlogged expansion tank myself?
No. A waterlogged tank must be replaced. You can check the tank by tapping it and testing the air pressure. Replacing the tank requires shutting off the system, draining, and plumbing—call a plumber.
When should I call a plumber for an expansion tank?
Call a plumber if the tank sounds full when you tap it, the air pressure is zero or very low, the pressure relief valve drips, or system pressure is high. Expansion tank replacement requires professional work.

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