Leaking water heater

First, stop the water

If water is actively spreading, deal with that before diagnosing anything. These steps work on almost every tank heater in the country.

1. Cut the heat. Turn a gas control knob to OFF. For an electric heater, switch off its breaker. Never run an electric tank that might drain, because a dry element burns out in seconds.
2. Close the cold supply. Find the valve on the pipe running into the top of the tank, usually marked cold or with a blue handle. Turn it clockwise until it stops.
3. Relieve pressure if it is the tank leaking. Open a hot faucet somewhere in the house. That lets the tank depressurize so the leak slows to a drip.
4. Protect the floor. Towels, a wet-vac, or a garden hose on the drain valve running to a floor drain or outside.

Our guide on what to do when your water heater leaks covers this in more detail, including what to photograph for an insurance claim.

Where is it leaking from?

Dry the area with a towel, then watch for a few minutes with a flashlight. Water runs down and pools under the tank no matter where it starts, so the puddle itself tells you little. Look for the highest wet spot.

A little water in the burner compartment right after a cold refill can be condensation, which burns off within an hour or two. If it is still dripping the next day, it is a real leak.

The relief valve: a leak that is trying to tell you something

The T&P valve on the side or top of the tank opens when the water gets too hot (about 210°F) or the pressure too high (about 150 psi). A valve that drips now and then often means one of two things: the valve itself is old and its seal is tired, or the system really is seeing pressure spikes.

Pressure spikes are common in homes with a check valve or pressure reducing valve at the meter. When the heater warms the water, the water expands and has nowhere to go. A small thermal expansion tank on the cold line, usually $200 to $500 installed, absorbs that expansion. Many local codes require one whenever a heater is replaced on a closed system.

Never cap or plug a dripping T&P valve or its discharge pipe. That valve is the only thing preventing a tank from rupturing if a thermostat sticks on. Replace the valve and find out why it opened.

When the tank itself is leaking

Steel water heater tanks are glass-lined and protected by an anode rod that corrodes in their place. Once the anode is used up, the water goes to work on the steel. The leak usually starts as a slow weep from the bottom and gets worse over weeks, though some tanks split and dump a lot of water at once.

Signs it is the tank: water coming from under the outer jacket rather than from a fitting, rust streaks down the side, a damp ring of insulation visible through the burner access, or rusty water from hot faucets only. At that point there is no repair. Plan a replacement soon and keep the cold supply closed until then.

Where the heater sits makes the timing matter. A tank in a garage with a floor drain can be lived with for a day or two. A tank in an attic, a closet over hardwood, or a finished basement should be dealt with the same day.

Leaks on tankless units

Tankless heaters leak less often, but when they do it is usually a failed fitting at the service valves, a cracked heat exchanger from freezing, or condensate trouble on high-efficiency models. Condensing units produce acidic condensate that must run to a drain through a neutralizer. A blocked or disconnected condensate line drips onto the floor and is often mistaken for a water leak.

A frozen heat exchanger is the expensive one. Units on exterior walls in cold climates can freeze during a power outage when their freeze protection loses power. That repair is often a heat exchanger replacement, which on some brands costs close to a new unit. See tankless repair for the details.

What waiting costs

A slow drip from a drain valve is a $150 to $300 repair if you deal with it this week. Left for months, the same drip soaks the subfloor or drywall around a closet heater, rusts the base of the tank, and attracts mold behind baseboards. Water damage restoration on a single room commonly costs more than a new water heater.

Tank leaks follow the same pattern, only faster. Small weeps from the bottom seam tend to grow once they start, and a 50-gallon tank can empty itself, then keep refilling from the cold line, if no one closes the supply. If you are leaving home for more than a day with a heater that has started to weep, shut off its cold supply and its fuel before you go.

Preventing the next leak

• Put a drain pan under any heater that sits over finished space, piped to a drain where possible.
• Add a leak sensor or an automatic shutoff valve. They cost little and they turn a flood into a puddle.
• Check the anode rod around year four or five and replace it when it is down to bare wire.
• Keep household water pressure between about 50 and 70 psi.
• Flush the tank once a year in hard-water areas so sediment does not overheat the bottom.

Ask the plumber about their own workmanship warranty on any valve or fitting repair. It is a fair question and the answer differs from one shop to the next.

Questions people ask