Why Is My Water Heater Leaking?

A puddle under a water heater has only a handful of possible sources, and the location of the water sorts them almost completely. Water coming from the top of the heater is usually a connection or a valve, and those are repairable. Water running out of the relief valve's discharge pipe is a pressure or temperature problem that needs diagnosing, not plugging. Water seeping from the bottom is either the drain valve, harmless condensation, or the tank itself failing, and that last one ends in water heater replacement. Before you mop up, take a dry paper towel and trace the wet path upward until you find where it begins.

What should you do right now about a leaking water heater?

Stop the water and the heat first, then investigate. A leaking water heater keeps refilling from the cold supply, so the leak will not run dry on its own, and a heater that keeps firing while it loses water is working against you.

1. Close the cold water supply valve on the pipe feeding the top of the water heater.
2. On a gas heater, turn the gas control knob to off or pilot. On an electric heater, switch off its breaker at the panel.
3. Put a pan or towels under the leak and move anything stored nearby up off the floor.
4. Open a hot water faucet elsewhere in the house to relieve pressure in the tank.
5. Note where the water was coming from, and when you first saw it, before you call.

Why is water leaking from the top of the water heater?

Water on top of a water heater almost always comes from a connection, not the tank. The cold inlet and hot outlet each screw into the tank through short threaded nipples, and those nipples corrode, especially where copper meets steel without a proper dielectric fitting. A slow weep shows up as a crust of white or green mineral, or rust streaks running down the jacket. Compression nuts and flexible connectors on the supply lines can also loosen or crack.

The anode rod port is another source. The anode rod screws into the top of the tank, and its threaded fitting can seep, particularly after someone has had the rod out. A wet top can also be the relief valve's threads rather than the valve discharging. All of these are repairs on a tank that is otherwise sound, though heavy corrosion at the nipples is often a sign the tank's internal protection is spent and the tank itself is not far behind.

Why is water coming out of the relief valve pipe?

Water from the temperature and pressure relief valve's discharge pipe means the valve has sensed too much temperature or too much pressure in the tank and opened to release it. That is the valve doing its job. The real question is what pushed the tank over the limit.

The most common cause is thermal expansion. Water expands as it heats, and in a house with a check valve, backflow preventer or pressure-reducing valve on the incoming service, that expanded water has nowhere to go, so pressure climbs every time the burner or elements run. An expansion tank on the cold supply gives it room. A heater that drips from the relief pipe after each heating cycle, then stops, fits that pattern. Other causes include a thermostat or gas control that is not shutting off, high incoming water pressure, or a worn relief valve weeping at normal pressure. Boilers carry the same kind of safety relief, and a leaking boiler often turns out to be the same story.

Why is water leaking from the bottom of the water heater?

Water at the bottom of a water heater comes from one of three places: the drain valve, condensation, or the tank. Telling them apart matters, because the first two are minor and the third is the end of the heater.

The drain valve

The drain valve near the base is often plastic and can drip from its outlet or around its stem, especially after it has been opened to flush the tank and sediment has lodged in the seat. A drip from the end of the valve can sometimes be stopped with a hose cap, and a worn valve can be replaced, so this is a fixable problem on a sound tank.

Condensation on a gas heater

A gas water heater full of very cold water can sweat. When the burner fires, the combustion gases carry water vapor that condenses on the cold tank walls and drips onto the burner, sometimes with a sizzle, and can leave a small puddle. It is most common after a big hot water draw, when the tank refills with icy winter water from the main, and it dries up once the tank is back up to temperature. If the water appears only after heavy use and is gone an hour later, condensation is the likely answer.

The tank itself

A steady leak from under the jacket or around the burner area, with rusty water and no obvious fitting above it, usually means the steel tank has rusted through. Tanks are protected by a glass lining and a sacrificial anode rod, and once the anode is consumed the steel corrodes from the inside out. A leaking tank cannot be repaired. There is no patch, sealant or weld that makes a pressurized, rusted-through tank sound again, and the leak only gets bigger.

Leak location, likely cause, fix or replace

Where the water is on a leaking water heater, and what it usually means

Where the water is	Likely cause	Fixable or replace
Top, at the pipe connections	Corroded nipples or loose supply fittings	Usually fixable
Top, at the anode port	Seeping anode rod threads	Usually fixable
Relief valve discharge pipe	Thermal expansion, excess temperature or pressure, or a worn valve	Fixable once the cause is found
Bottom, at the drain valve	Worn or sediment-fouled drain valve	Usually fixable
Bottom, only after heavy use on a gas heater	Condensation on a cold tank	Normal, dries up
Bottom, rusty and steady	Tank rusted through	Replace the water heater

What happens when a leaking water heater has to be replaced?

Replacing a leaking water heater is a chance to fix whatever shortened the old one's life. If the relief valve had been discharging, the new installation should include an expansion tank where the plumbing is closed. If the old gas heater shared a chimney with a furnace that has since been replaced, the venting deserves a look, because an orphaned water heater on an oversized chimney is a common problem in older houses. Some households use the moment to compare tank and tankless options, and our guide to tankless water heaters in a cold climate covers how very cold incoming water affects that choice.

In the rural houses around Esko, Gnesen and Midway, a water heater often sits in a finished basement or beside a well pressure tank, so a slow leak can do real damage before anyone notices. A drain pan piped to a floor drain, and a glance at the base of the heater now and then, are cheap insurance.

When to call about a leaking water heater

Shutting off the water, the gas or the breaker, and putting down a pan, is where the homeowner's part stops. Replacing nipples and valves on a pressurized tank, working out why a relief valve discharges, and anything involving gas connections or venting belong to a technician. A tank leak is less an emergency repair than an urgent replacement, and knowing where the water starts makes the call quicker. Our diagnostic is $89 and is credited toward the work when you approve it, and water heater repair and replacement explains what each path involves, with typical installed ranges of $1,600–$6,500 depending on the heater.