Where the leak is coming from tells you almost everything: water at a fitting or connection is often just a loose fitting that needs tightening, while water coming from the tank body itself, or from the air valve on top, means the internal rubber diaphragm has ruptured and the tank needs replacing. An expansion tank absorbs the extra pressure created as water expands during heating — once that diaphragm fails, the tank can no longer do its job.
Locate exactly where the leak is coming from
| Where the water is coming from | Likely cause |
|---|---|
| A pipe fitting or connection at the top of the tank | Loose fitting — usually a simple tighten |
| The small air valve on top of the tank (looks like a bicycle tire valve) | Ruptured internal diaphragm — water has entered the air chamber |
| The tank body itself (a seam or the shell) | Corrosion (external rust) or internal lining failure — tank needs replacing |
Leak at a fitting or connection
This is the easiest and most common fix on this list. Check that the fittings connecting the tank to the plumbing are fully tightened — thread sealant tape or compound applied during installation can settle over time. Tighten carefully rather than aggressively, since overtightening can crack the fitting or the tank connection point itself.
Leak from the tank body or air valve
Water at the air valve is a clear sign the internal diaphragm separating the pressurized air charge from the system water has ruptured. Once this happens, water floods the air side of the tank, and the tank can no longer compress or absorb pressure the way it is designed to — this failure mode is not something that can be repaired, and the tank needs replacing.
How a waterlogged tank happens
A tank that has taken on water through a failed diaphragm is often described as “waterlogged.” You can sometimes get an early hint before a visible leak appears: tap the tank at the top and bottom — a healthy tank sounds hollow near the top (air side) and solid near the bottom (water side). A tank that sounds solid all the way up has likely already taken on water internally, even before it visibly leaks.
Checking the air pre-charge
- Isolate and fully drain the tank
Follow the manufacturer’s procedure to shut off water to the tank and relieve pressure before checking it.
- Check air pressure at the valve with a tire gauge
It should roughly match your home’s static water pressure — check the tank’s label for the exact factory spec.
- Add air with a hand pump if low
A manual bicycle-style pump is preferable to a compressor here, since it is less likely to over-pressurize and rupture an already-weakened diaphragm.
Running the tank with incorrect air pre-charge pressure puts extra mechanical stress on the diaphragm every time the system cycles, which shortens the tank’s working life even before a leak becomes visible — checking pre-charge periodically is worth doing as routine maintenance, not just after a problem appears.
When to call a plumber
Any leak that is not resolved by tightening a fitting should be treated as a failed tank rather than something to keep monitoring — a professional can confirm whether replacement is needed and correctly isolate and drain the system during the swap. See our water heater repair cost guide for how tank-related failures are generally priced.
