Water Heater Leaking: How to Find the Source and What to Do

Water under a water heater does not always mean the tank is done. A.O. Smith's leak checklist says many suspected "leakers" are not leaking tanks: the water is condensation, a relief valve doing its job, a drain valve left slightly open, a loose fitting, or another appliance. Those are fixable. A leak from the tank itself is not.

This page covers gas and electric tank heaters: shutting one down safely, finding the real source, the causes in the order to check them, and what belongs to a plumber. For no hot water, see no hot water; for popping or rumbling, see water heater popping noise.

First: Shut It Down and Stop the Water

A.O. Smith's order for a leak: heat off, then water off, then look for the source.

1. Turn off the heat. Gas: turn the gas control knob to OFF, then close the gas shutoff valve on the supply pipe; it is closed when the handle sits at a right angle to the pipe. If the heater has a power cord or switch, turn that off too. Electric: switch off the breaker. Some heaters have two, and both must be off.
2. Close the cold water supply valve on the cold pipe entering the top of the tank. A lever turns a quarter turn, crosswise to the pipe; a round handle turns clockwise until it stops. If it won't turn, or it's what's leaking, close the main shutoff; our shutoff guide shows where to look.
3. Contain the water with towels, a bucket, or a wet/dry vac. If it has soaked drywall, carpet, or the ceiling below, see our burst pipe guide.

Find Where the Water Is Coming From

Water runs along pipes and down the jacket before it drips, so the puddle is rarely under the leak. A.O. Smith says to check every possible source before replacing a heater.

• Dry everything: the tank, the fittings on top, both valves, and the floor. Look up, too, for a pipe or air conditioner coil dripping onto the heater.
• Use paper towels. Wrap one around each fitting and valve, lay a strip around the base, and check them over the next few hours. The first wet one is closest to the leak.
• Swab the openings. Where a fitting passes through the jacket into the tank, A.O. Smith's check is a cotton swab in the gap; wet cotton means the threads into the tank leak.
• Check the end of the relief valve pipe, which runs from the valve near the top down toward the floor. Water there came from the valve, not a tank leak.

Water that isn't from the heater

A.O. Smith's checklist also says to check other appliances, water lines, and ground seepage: a furnace or air conditioner condensate line or pump, a water softener's drain line or brine tank, a washer hose, a sweating cold pipe, or groundwater. A heat pump water heater also drains condensate through a hose or small pump, which can clog or come loose.

Most Likely Causes

1. Condensation, which is normal. Burning gas makes water vapor, which condenses on a tank full of cold water and drips onto the burner with a sizzle, sometimes enough to pass for a tank leak, A.O. Smith says. Electric tanks can sweat too after a fill or a heavy draw of very cold water, Rheem notes. How to tell: it follows a refill or heavy hot-water use, it's worst in winter, and it should stop once the water is hot, about 1 to 2 hours on A.O. Smith's gas models. What to do: if every fitting and valve is dry and the water began after a refill, restart the heater following the label and don't call it a leak until the tank has heated; if water keeps appearing, keep looking. Heavy condensation every day can mean an undersized heater.
2. The T&P relief valve is releasing water. It protects the tank from excess pressure and heat. The usual cause is thermal expansion: heated water expands, and if a check valve, backflow preventer, or pressure-reducing valve keeps it from flowing back to the main, pressure climbs with each heating cycle until the valve opens. Supply pressure over 80 psi, A.O. Smith's limit, does the same. What to do: never cap or plug the valve or its pipe; Rheem, Bradford White, and A.O. Smith all forbid it. Check your pressure with a hose-thread gauge on an outdoor faucet, and if the heater is set above 120°F, turn it down to 120°F (on an electric tank, only with the breaker off); A.O. Smith notes that can stop a pressure drip. When it's a plumber: any repeated drip. Rheem calls it an unacceptable condition that must be corrected and says a new valve alone won't fix thermal expansion; the usual fix is an expansion tank on the cold line, plus a pressure-reducing valve if pressure is high. If it releases very hot water or steam, the heater may be overheating: turn the gas control or breaker off and call a technician, as Bradford White instructs.
3. The drain valve is dripping. If the spout of the valve near the bottom drips, A.O. Smith says it may simply be slightly open, often after a flush. What to do: turn it clockwise until snug without forcing it; old plastic valves crack. A brass hose cap holds a slow drip, as our flush guide explains. When it's a plumber: a valve that keeps dripping, which usually means replacing it, or a wet swab where the valve enters the tank.
4. A loose connection or supply line on top. The cold inlet, hot outlet, their flexible connectors, and the anode rod fitting can all weep as thermal expansion and wear loosen them, A.O. Smith says. How to tell: a wet paper towel or corrosion at a fitting. What to do: tighten it gently with a wrench; A.O. Smith warns that overtightening can make the leak worse. When it's a plumber: a corroded or kinked connector, a leak that continues, soldered joints, and any leak where a fitting threads into the tank.
5. Water in the burner area of a gas heater. How to tell: sizzling, water under the burner access cover, or a pilot that goes out. Look through the flame window with a flashlight; don't open the burner compartment. After a refill it's usually condensation. If it keeps dripping once the tank is hot, the water is coming from above: A.O. Smith's checklist points to a loose connection or the relief valve first, then the tank. What to do: turn the gas off and call a technician. A.O. Smith says not to operate a flood-damaged heater, so a wet gas control or burner gets checked before it runs again.
6. The tank itself is leaking. Sediment, hard water, and a used-up anode rod let the steel tank corrode from the inside, and A.O. Smith says that corrosion cannot be repaired. How to tell: water seeping from the bottom of the jacket after everything above is ruled out, often with rust on the jacket or rusty hot water. Rheem says pooling around the tank body typically signals internal corrosion, and replacement becomes the only safe and cost-effective solution. What to do: leave the heat and cold supply off and call a plumber. If one can't come soon, draining the tank stops the leak. Leave the heat and cold supply off and let the tank cool for several hours first; the CPSC says 140°F water causes third-degree burns in six seconds. Then connect a garden hose to the drain valve, run it to a floor drain or outdoors, open a hot tap in the house to let air in, and open the drain valve. Don't reopen the cold supply to flush it; the flush guide's steps are for a tank that isn't leaking.

A.O. Smith treats a leak as urgent. Until the new heater is in, an alarm at the base, on the side the drain valve and relief pipe face, tells you if a slow weep gets worse.

What You Can Fix Yourself

• Shut off the heat and the cold supply, and contain the water.
• Trace the water with paper towels, and rule out other appliances.
• Give condensation an hour or two with a hot tank.
• Close a drain valve that was left slightly open, and cap a dripping spout.
• Gently snug a weeping fitting.
• Check your water pressure, and set the heater to 120°F.

When to Call a Pro

Call a plumber for a relief valve that drips more than once, pressure over 80 psi, a valve or fitting that still leaks after a gentle tightening, a leak where a fitting enters the tank, or a leaking tank. Call a gas technician for water in the burner compartment or on the gas control. Have the model and serial number ready.

Prevent It

Keep the heater at 120°F, flush it yearly, and test the T&P valve yearly as Rheem and Bradford White direct, with the precautions in our flush guide. Have a plumber check the anode rod; Rheem says yearly, and Bradford White says a water softener uses it up faster. On a closed system, have an expansion tank, its air charge matched to the house water pressure, A.O. Smith says. And plan the replacement before the leak: Rheem puts a conventional tank's life at around 10 years (other sources say 10 to 15), and the manufacture date is on the rating plate, often coded into the serial number.

Where a leak would damage floors or rooms below, Bradford White's manual requires a drain pan at least 4 inches wider than the heater, piped to a drain with at least 3/4-inch pipe; Rheem recommends one too. Pan rules vary by model: one A.O. Smith gas manual caps pan depth at 1.75 inches, and Bradford White says a pan must not restrict combustion air, so a deeper pan like the one below may not be allowed under a gas heater. Check yours first. A pan means lifting the tank, so it's a replacement-day job. A.O. Smith adds that a leak detector with the pan cuts damage further; our leak alarm guide compares the options.

Frequently Asked Questions

Can I still use a water heater that's leaking?

Not if the tank, a fitting, or the burner area is leaking; A.O. Smith says to shut it off and have it checked first. Condensation that clears once the tank is hot is normal, and a dripping relief valve is doing its job, though the cause needs fixing.

Why is it leaking from the bottom right after I flushed it?

Usually the drain valve isn't fully closed or won't reseal, or, on a gas heater, the cold refill is making condensation for an hour or two. Water from the bottom of the jacket once the tank is hot is more serious: on an old tank, a flush can loosen sediment that was plugging a weak spot.

Should I repair it or replace it?

Valves, fittings, connectors, and an expansion tank are repairs; a leaking tank is a replacement. Past about 10 years, even a repairable leak is a reason to price a new heater, since A.O. Smith says replacing an old heater is usually more cost-effective than repairing it.

The Bottom Line

Gas or power off, cold supply closed, then dry everything and find the source. Condensation fades once the tank is hot. A dripping relief valve is a pressure problem for a plumber, never a plug. Drain valves and fittings are small fixes; a leaking tank is a replacement. Keep it at 120°F, flush it yearly, and put an alarm at its base.

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