Quick Answer:
If the expansion tank feels heavy and solid when you tap it and doesn’t behave like an air cushion, it’s likely waterlogged. With the water shut off and a faucet open, the top of the tank should sound hollow when tapped. If it sounds solid and the Schrader valve shows no air pressure, the tank’s bladder is full of water and needs attention.
Why This Happens
- The internal bladder or diaphragm can fail from age, corrosion, or debris. When it fails the tank fills with water and stops providing an air cushion for thermal expansion.
- A failed expansion tank can let system pressure rise or cause the relief valve to cycle. If you’ve seen the relief valve venting, see the note about pressure relief opens after heating cycles for related issues.
- Regular pressure fluctuations, water hammer, or a bad pre-charge can accelerate failure.
Step-by-Step What to Do
1. Put the system into a safe state
- Turn off the water supply to the heater or boiler (isolate the unit). Open a nearby hot faucet to relieve system pressure and drain a little water from the system so pressure is low.
2. Tap the tank
- With the water off and the faucet still open, tap the top of the tank. The top should sound hollow or springy if air is present. If it sounds solid and heavy, that’s a clear sign of a waterlogged tank.
3. Check the Schrader valve air pressure
- Use a tire or pressure gauge on the Schrader valve (the small valve like on a bike tire). Compare it to the cold static system pressure. The tank’s air pre-charge should approximately match the cold static pressure.
- If the gauge reads 0 psi, or you see water coming from the Schrader, the tank is waterlogged.
4. Decide whether to recharge or replace
- If the bladder appears intact (no water at the Schrader and it holds pressure), you can recharge the tank’s air pre-charge to match the cold static pressure using an air compressor or pump.
- If the tank reads 0 psi, feels full of water, or will not hold a charge, it’s waterlogged — replace the tank. A tank with a compromised bladder cannot be reliably repaired.
What Not to Do
- Don’t keep ‘tapping’ the Schrader if water comes out—plan replacement; the bladder is compromised.
- Don’t remove or disconnect the tank while the system is pressurized. Always relieve pressure first.
- Don’t ignore repeated high-pressure or relief-valve activity; it may indicate a failed tank or other system issue.
When to Call a Professional
- If you find water at the Schrader valve, the tank won’t hold a charge, or you’re unsure whether the bladder is intact, arrange a replacement.
- If the pressure relief valve keeps opening after heating cycles or you notice frequent pressure spikes, call a plumber — these symptoms can mean a failing tank or another pressure-control problem. Also see expansion tank failed causing high pressure for more context.
- If you’re not comfortable working with the tank, or if the tank is integrated into a hydronic system that requires balancing, get a professional.
Safety Notes
- Always turn off the water supply and relieve system pressure before inspecting or removing the tank.
- If working on a water heater, shut off electrical power or gas before beginning any service.
- Wear eye protection and gloves. Beware of hot water in the system and avoid opening pressurized connections.
Common Homeowner Questions
- How will I know the tank is waterlogged? If it sounds solid when tapped and the Schrader reads 0 psi or produces water, it’s waterlogged.
- Can I recharge a waterlogged tank instead of replacing it? Only if the bladder is intact and it will hold pressure; if water comes from the Schrader or it won’t hold a charge, replace it.
- Is a waterlogged tank dangerous? It’s not immediately dangerous, but it can cause high system pressure and repeated relief-valve discharge, so fix or replace it promptly.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.