Quick Answer:
Water at the expansion tank’s Schrader/air valve usually means the internal bladder has failed or the system pressure exceeded the tank’s air cushion. Press the Schrader briefly: if only air comes out, the bladder is intact; if water comes out, the bladder is breached. Replace the tank, set the tank’s precharge to the cold static system pressure while the system is depressurized, then run a full heat cycle and confirm peak pressure stays below the relief valve setting.
Why This Happens
- Most modern expansion tanks are bladder-style: an air cushion separated from the water by a rubber bladder. If the bladder tears or the air escapes, water can reach the Schrader valve.
- Excessive system pressure during heating (thermal expansion) can force water into the air side. Compare cold and hot pressures — see Pressure normal cold but high when hot for more on that pattern.
- Changes to the system — added radiators, closed-loop conversions, or a new boiler — increase expansion volume needs. That’s why you should also consider Thermal expansion after system retrofit.
Step-by-Step What to Do
1. Observe and confirm the leak
- Look at the Schrader valve on the tank. If water is actively appearing at the valve or the valve becomes wet after pressing, that indicates water is in the air chamber.
- Press the Schrader briefly with a small tool or cap:
- If only air or a quick hiss appears, the bladder is likely intact.
- If water or a water/air mix comes out, the bladder has failed or the tank is overfilled with water.
2. Shut down and depressurize the system before handling
- Turn off the heating appliance and any circulating pumps.
- Allow the system to cool if it was hot, then relieve pressure by opening a drain or a fixture until gauge pressure falls to the cold static reading.
3. Replace the tank if water is at the Schrader
- Remove the old tank after isolating and draining the connection. A wet air valve means the tank should be replaced; repairing the bladder is not practical for most homeowners.
- Install a new tank sized for the system. Use the same connection orientation and make sure fittings are tight and leak-free.
4. Set the precharge correctly
- With the system cold and depressurized, check the system’s static pressure with a gauge at a convenient location (typically near the fill valve). Set the tank precharge (using a tire gauge on the Schrader) to match that cold static pressure — common values are 12–15 psi for residential systems but match your measured static pressure.
- Do not change the precharge while the system is pressurized; that gives incorrect readings and risks under/overfilling the air side.
5. Repressurize and test with a heat cycle
- Bring the system back to normal operating pressure and run a full heat-up cycle to the usual operating temperature.
- Watch the system pressure gauge through the heat cycle. Confirm peak pressure stays below the relief valve setting (usually 30 psi on residential hydronic systems, but check your relief valve rating). If pressure still reaches the relief setting, the tank may be undersized or other expansion-control measures are needed.
What Not to Do
- Don’t ignore a leaking air valve—loss of air cushion means loss of expansion control. Leaving a failed tank can lead to repeated pressure relief valve discharges and stress on the system.
- Don’t add air to the valve while the system is pressurized. That gives a false precharge and can damage the replacement tank or cause overpressure.
- Don’t try to patch or permanently seal the Schrader valve as a fix; that hides the symptom and leaves the system vulnerable.
When to Call a Professional
- If you’re unsure how to depressurize the system safely, call a licensed plumber or heating technician.
- If pressure still spikes above the relief setting after replacing and setting the tank, the system likely needs a sizing review or repair by a pro.
- If the expansion tank is hard-piped in a way that requires special tools or brazing to remove, get professional help.
Safety Notes
- Always shut off power to the boiler or water heater before working on the hydronic system.
- Allow hot systems to cool before opening drains or connections to avoid scalding.
- Relieve system pressure before loosening any fittings. Working on a pressurized hot-water system is dangerous.
Common Homeowner Questions
- Q: What if I only see occasional dampness at the valve?
A: Any water at the air valve suggests the bladder is compromised. Monitor closely and plan to replace the tank soon. - Q: Can I buy a universal replacement?
A: You can buy compatible tanks, but match the type (bladder/diaphragm), connection size, and capacity to your system. - Q: How do I know the right precharge number?
A: Measure the system’s cold static pressure and set the tank precharge to that same value while the system is depressurized.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.