Why is my water heater leaking from the bottom?

Updated August 2026

Water spreading across the floor under a tank water heater is, in the worst and most common case, a corroded tank that has rusted through from the inside and can no longer hold water. A steel tank cannot be patched: once it weeps from the shell, replacement is the only fix, and a new tank runs $1,200–2,500 installed. Before you assume the worst, though, rule out the cheaper sources, because water at the base can also come from a dripping drain valve, the temperature-and-pressure relief line, or a loose fitting above that ran down. Find the exact origin first, then act, because a real tank leak only grows.

Safety first: a leaking water heater combines water damage, scalding, and either gas or 240-volt electricity, so treat it seriously. If the tank itself is leaking, shut off the cold-water supply valve and turn off the power (breaker for electric) or set a gas unit to pilot, then drain it before it floods the floor or the room below. Never cap or plug the T&P relief valve to stop a drip; it is a safety device that prevents the tank from bursting, and blocking it is dangerous. The water at the outlet can be 120F or hotter, so avoid it, and if you smell gas near a gas heater, leave and call the utility. For the full downside, see the cost of a water heater failure.

Causes, most likely first

What makes a furnace do this, most to least common (Updated August 2026)
Cause	How likely	The fix	Typical cost
Corroded, rusted-through tank	Most common	The tank shell has failed and cannot be repaired. Shut off water and power, then replace the heater.	$1,200–2,500 installed
Dripping drain valve	Common	The bottom drain valve weeps. Tighten it, cap it, or replace the valve. An easy, cheap fix if the tank is sound.	$10–40
T&P relief valve discharge	Common	The safety valve is releasing water down its tube from high temperature or pressure. Test pressure and replace the valve.	$20–200
Loose inlet or outlet fittings	Occasional	A weeping connection above runs down and pools at the base. Tighten and reseal the fittings or nipples.	$0–150
Condensation on a new or cold tank	Occasional	A tank filling with cold water sweats and drips, mimicking a leak. Normal; monitor and let it warm.	Free

Check these first

Wipe every surface dry and lay paper towels under the tank, valves, and fittings to find exactly where water returns.
Check the drain valve at the bottom first, since a weeping valve is the cheapest and easiest fix.
Trace the T&P relief valve tube. Water there means the valve is releasing pressure or temperature, not a tank leak.
Look for rust streaks or a rusty puddle around the base, which is the signature of a tank that has corroded through.
If the source is clearly the tank shell, shut the cold supply and turn off the gas or breaker before doing anything else.

Prevent it next time

Tank life is largely a corrosion story, and the sacrificial anode rod is what corrodes so the tank does not. Replacing it on schedule is the single biggest thing that stretches an 8–12 year tank toward the top of that range; here is how often to change the anode rod. Draining sediment keeps the bottom from overheating and rusting, and testing the safety valve confirms it will relieve pressure rather than let the tank strain. Know how often to test the T&P valve and where to set the temperature, and when the tank is near end of life, plan the swap early: here is how often a water heater should be replaced.

When to call a pro

Call a plumber immediately when the leak is coming from the tank shell itself, when the T&P valve keeps discharging (a pressure or thermostat problem, not just a bad valve), or when a gas heater is involved and you are not comfortable shutting it down. A tank replacement runs $1,200–2,500 installed, and a T&P valve or drain valve swap is $150–350. A tank leak is one of the few plumbing symptoms where speed matters: the failure only worsens, and a full tank letting go can put 40 to 50 gallons across the floor at once.

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Frequently asked questions

Can a leaking water heater be repaired?

It depends entirely on where it leaks. If the source is the drain valve, the T&P relief valve, or a loose fitting, those are repairable for $10–200. If the tank shell itself has rusted through, no: the steel tank cannot be patched, and replacement is the only real option at $1,200–2,500 installed. That is why finding the exact origin matters before you spend anything. Rust streaks and a puddle from under the tank point to shell failure; a drip from a valve or fitting is the fixable kind.

Is a leaking water heater an emergency?

A tank leak is urgent, if not always a middle-of-the-night emergency. The leak only grows, and a tank that fully lets go can dump 40 to 50 gallons at once, flooding a basement or the floor below. Shut off the cold-water supply and the power or gas, then get a plumber out promptly. A slow drip from a valve or fitting is less pressing, but still worth fixing soon. When in doubt, treat water pooling under the tank as a shut-it-down-now situation.

How long do water heaters last?

A conventional tank water heater lasts about 8–12 years, while tankless units often run 15–20. Water chemistry drives the range: hard water and a neglected anode rod corrode a tank faster, so a unit in hard water without maintenance may fail before 8 years, while a flushed tank with a fresh anode can reach the top of the range. If your heater is leaking from the tank and is already past 8–10 years, replacement rather than repair is almost always the right call.

Why is there water only under my brand-new water heater?

On a new or recently refilled heater, that is usually condensation, not a leak. When the tank fills with cold water and the burner or elements start heating, the cold shell sweats and drips onto the floor, which looks alarming but clears once the water warms up. Wipe it dry and check again after a few hours of heating. If water keeps returning after the tank is fully hot, or you see it at the drain valve or a fitting, then look for a real leak.