Expansion Tank Pinhole Leak: Replace It
The Short Answer: A pinhole in the steel shell of a potable expansion tank is a dead tank. The bladder may also be gone. Do not epoxy, Flex Seal, or clamp it. Isolate the tank, keep the water-heater T&P open to atmosphere, and replace the expansion tank. Then set precharge to static house psi — typically 50–80. A waterlogged bladder with no hole in the shell is a different job: full of water.
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Search Intent & The Real Problem
You found a rust pit weeping on the little tank teed into the cold line — not on the 40- or 50-gallon heater. Water is on the pan or the ceiling below a joist-hung Amtrol / Watts / State tank. That vessel sees city pressure plus every heat cycle. A hole in the steel is not a “slow leak you can watch.” It is a pressure vessel with a failed wall.
Do not confuse it with T&P discharge on the heater — overflow discharging and Rinnai T&P. A wet heater jacket is bottom of the tank. This page is a hole in the expansion tank shell.
What other guides get wrong
They recommend JB Weld on a 150 psi-class tank. They also mix a pinhole (steel) with a dead bladder (Schrader spits water, shell dry). Pumping air into a tank that is already leaking water from the jacket just atomizes the leak. And they skip why the shell rusted: precharge left at the factory 20 psi against 70 psi street, tank hanging in a wet pan, or a unit that sat waterlogged for years.
Step-by-Step Troubleshooting
Step 1: Confirm it is the expansion tank — not the heater
| Wet location | What it is | Next |
|---|---|---|
| Rust pit / pin stream on the small tank’s steel | Shell pinhole | Replace the expansion tank |
| Schrader (air valve) spits water, shell dry | Dead bladder | Full of water |
| Tee, dielectric, or flex at the tank nipple | Fitting | Tighten or remake — not the shell |
| Brass T&P on the water heater, pipe to the pan | Heater relief | Pressure / expansion / overheat |
| Heater jacket / foam | Heater tank | Replacement of the heater |
- What to look for: Wipe, wait, wipe. A pinhole returns as a bead on bare steel, often at a seam or a rust blister on the bottom head.
- Required tools: Paper towel, flashlight.
Step 2: Gauge the house vs the air charge
- What to look for: Hose-bib static. IPC/UPC want a PRV when street pressure can exceed 80 psi. Precharge on a healthy expansion tank, measured with the tank isolated and drained, must match that static number — typically 50–80 psi, often 50–60 in houses with a working PRV. Factory air is commonly ~20 psi. Leave 20 against 70 and the bladder is crushed on day one; the shell then lives wet and rusts.
- Required tools: Hose-bib gauge. Tire gauge on the Schrader only if the tank is isolated and empty of water.
If the Schrader already spits water, the air chamber is flooded — do not add air. That is a bladder failure plus, here, a hole in the wall.
Step 3: Isolate. Do not patch.
- What to look for: Ball valve on the tee. Close it. The water heater can run short-term without the expansion tank, but the T&P may start weeping on a closed system — that valve is doing the expansion tank’s job. Do not cap it. Do not leave a streaming T&P overnight.
- Required tools: The isolation valve. Bucket. If there is no isolation valve, shut the house cold and hire the replacement — do not spin a live 3/4” tee over a live tank.
Epoxy, clamps, and “tank repair kits” are not listed repairs on a potable expansion tank.
Step 4: Replace with the right size and a real precharge
- What to look for: Match or upsize the volume (a 2.0–2.1 gal tank is common under a 40–50 gal heater at moderate psi; higher static and larger heaters need more acceptance). Potable-rated (not a hydronic Extrol on domestic). New tee, new dielectric if the old one rusted with it. Precharge before you open the water side, to static psi.
- Required tools: Two wrenches, potable dope or listed tape, bicycle pump or compressor with a regulator, tire gauge.
Hang it where you can see the Schrader. A tank buried against the heater jacket is how the next pinhole rots unseen.
Step 5: Fix why it died, or the new one pinholes too
- What to look for: Missing or failed house PRV (static still over 80 psi). Check valve or meter check that made a closed system with no working expansion tank for years. Wet pan with the tank sitting in water. Water-heater T&P that has been weeping — the expansion tank was already the victim, not the cause. Typical heater T&P is 150 psi / 210°F; it should not be the daily drain.
- Required tools: Gauge after the new tank is in. If static is 90 psi, the PRV is the repair.
When DIY Ends: Calling a Licensed Pro
Stop if the closet is flooding, you cannot isolate the tee, the water-heater T&P is streaming, or the heater jacket is also wet. You may close an isolation valve and photograph the pinhole. You may not epoxy the shell or cap the T&P. A licensed plumber replaces the expansion tank, sets precharge to measured static, and installs or rebuilds the PRV so the next tank is not a 20 psi balloon on a 80 psi street.
Frequently Asked Questions
Q: Can I repair a pinhole leak on an expansion tank? A: No. A hole in the steel shell is replacement. Sealant on a pressure vessel is a delay until the ceiling fails.
Q: Why did my expansion tank get a pinhole? A: Corrosion after the tank sat waterlogged, factory 20 psi precharge against much higher city pressure, or the tank hanging in a wet pan. Gauge static and replace the tank — do not patch it.
Q: Is a pinhole the same as a waterlogged expansion tank? A: No. Water from the Schrader with a dry shell is a dead bladder. Water from a rust pit in the steel is a dead shell. Both are replacement; they are not the same failure.