Search guides

Fix a thermal expansion tank that fails

We'll confirm the tank has failed, 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)
Step 1 of 6

Quick triage — pick your path

Get started

Choose the option that matches what you see. You can jump straight to that section.

Show full guide

Steps

Goal: Confirm the expansion tank has failed, check the air pressure, then replace it or call a plumber.

  • Check if the pressure relief valve on the water heater is dripping. Check the pressure gauge if you have one.
  • If pressure is high or the relief valve drips, the expansion tank may have failed.
  • Good: Valve dripping or pressure high—proceed to Tap the tank.
  • Bad: No drip and pressure normal—different issue.

Tap the tank

Goal: Quickly check if the tank sounds waterlogged.

  • Tap the expansion tank with your knuckle. A properly charged tank sounds hollow when you tap the top half.
  • A waterlogged tank sounds full, dull, or solid—no hollow sound.
  • Good: Tank sounds full—failed. Proceed to Check air pressure.
  • Bad: Tank sounds hollow—check air pressure to confirm.

Check air pressure

Goal: Verify the tank pressure with a gauge. Zero or very low means the bladder has failed.

  • Press a tire gauge onto the Schrader valve on the tank. The air pressure should match incoming water pressure (typically 40–60 psi).
  • If the gauge reads zero or very low, the bladder has failed. The tank must be replaced.
  • Good: Pressure matches water pressure—tank may be OK. Check for leaks.
  • Bad: Zero or very low—replace the tank. Call a plumber.

When to get help

Call a plumber if:

  • The tank sounds waterlogged when you tap it.
  • The air pressure is zero or very low.
  • The tank is leaking.
  • The pressure relief valve drips.
  • You need to replace the tank.

If you smell gas near the water heater, evacuate and call 911 or your gas utility from outside.

Verification

  • The tank sounds hollow when you tap the top half.
  • Air pressure at the Schrader valve matches incoming water pressure.
  • The pressure relief valve does not drip.
  • No leaks at the tank fitting.

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 failed.
  3. Check air pressure Use a tire gauge at the Schrader valve—zero or very low means failed.
  4. Call a plumber A failed 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.

  • Tank sound when tapped (hollow or full)
  • Air pressure reading at Schrader valve
  • Whether the pressure relief valve drips
  • Incoming water pressure (if known)

Is the pressure relief valve dripping or is pressure high?

The pressure relief valve on the water heater drips when pressure is too high. A failed expansion tank causes high pressure.

Check the pressure relief valve on the water heater. Check the pressure gauge if you have one. Good: valve dripping or pressure high—expansion tank may have failed. Bad: no drip and pressure normal—different issue.

Yes No

You can change your answer later.

Does the tank sound hollow or full when tapped?

Tap the expansion tank with your knuckle. A good tank sounds hollow; a waterlogged tank sounds full.

Tap the expansion tank with your knuckle. Hollow: tank may be OK—check air pressure. Full or dull: tank waterlogged—replace it.

Hollow Full or dull

You can change your answer later.

Is the air pressure at the Schrader valve zero or very low?

Check air pressure with a tire gauge. It should match incoming water pressure (40–60 psi).

Press a tire gauge onto the Schrader valve. Zero or very low: bladder failed—replace tank. Matches water pressure: tank may be OK—check for leaks or other causes.

Yes No

You can change your answer later.

Replace the expansion tank

The tank has failed. Call a plumber to replace it. The new tank must be pre-charged to match incoming water pressure before connecting.

Done

Tank may be OK

The tank sounds hollow and holds pressure. If the relief valve still drips, check incoming water pressure or call a plumber.

Done

Call a plumber

Call a plumber if the tank has failed, is leaking, or you need to replace it. If you smell gas, evacuate and call 911 or your gas utility from outside.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a thermal expansion tank fail?
The bladder or diaphragm inside the tank can fail, allowing water to fill the air side (waterlogged). The tank can also leak at the fitting or develop a hole. Once failed, the tank cannot absorb pressure when water heats and expands.
Can I fix a failed thermal expansion tank myself?
You can check the tank by tapping it and testing air pressure. Replacing the tank requires shutting off the water heater, relieving pressure, and plumbing—call a plumber if you are not comfortable.
When should I call a plumber for an expansion tank?
Call a plumber if the tank sounds waterlogged, the air pressure is zero or very low, the tank is leaking, the pressure relief valve drips, or you need to replace the tank. Replacement requires plumbing work.

Rate this guide

Was this helpful?

Continue to