A puddle under the water heater sets off a fair amount of panic, some of it justified. But "leaking from the bottom" describes at least five different problems, and only one of them means buying a new heater. The four moves below come first, before any diagnosis: they stop the damage, make the area safe, and tell you which problem you actually have.
Find the cold water pipe entering the top of the tank — it's the inlet, usually on the right as you face the unit, and it should have a shutoff valve on it: a gate valve (round handle) or a ball valve (lever). Close it. The hot water in your house stops there; cold water to the rest of the house keeps working. If the valve is seized or keeps dripping through, close the house's main shutoff instead — our guide to shutting off the water to your house shows how to find it. This step comes first because every gallon the tank loses gets replaced by a fresh gallon from the street until you close that valve.
For an electric heater, switch its breaker off in your electrical panel. For a gas heater, turn the control knob on the gas valve at the front of the unit to "pilot" or "off." Two reasons. A tank that drains itself empty while the burners or elements keep firing will destroy those elements and can damage the tank lining. And electricity plus a growing puddle is a combination you don't want to be standing in.
This is the step people skip, and it's the one that decides whether you spend $20 or $1,500. Mop up the puddle, dry the base of the tank, then wait and watch where the water reappears. Water runs downhill and lies about where it started. Check these sources in order:
| Source | How to tell | Fixable? |
|---|---|---|
| Drain valve | Drips from the short spout near the bottom front of the tank | Yes — replace the valve, or thread a garden-hose cap on it as a stopgap |
| T&P discharge pipe | Water at the open end of the pipe that runs down the side of the tank and stops near the floor | The pipe is a warning, not the leak — see the safety note below |
| Top connections | Wet streaks running down the tank from the inlet/outlet fittings on top | Often — a fitting can be tightened or replaced by a plumber |
| Tank body | Water seeping from under the tank with no other source; rust or blistered paint at the base | No — the tank has corroded through and the heater gets replaced |
| Condensation | An even film of moisture on the lower tank on a humid day, no drip point, dries up on its own | Not a leak at all — a cold tank sweats in humid air |
The condensation row fools a lot of people in summer: a tank full of cold incoming water in a humid basement sweats like a cold drink. If the "leak" is worst right after heavy hot-water use on a muggy day and there's no single drip point, watch it for a day before you spend anything.
If the culprit is the drain valve, the fix is small: the valve can be replaced, and a $5 hose cap threaded onto it buys you time while you schedule that. If it's the tank body, the decision is made for you — steel tanks can't be patched from the inside, and a seeping tank only seeps more. While you line up a replacement, drain the heater so it stops feeding the puddle: connect a garden hose to the drain valve, run it to a floor drain or outside at a lower level, open the valve, and open one hot-water faucet in the house so air can get in. Treat the water as hot — it is. Then leave the supply off and the breaker or gas off until the new unit goes in. Our breakdown of water heater replacement cost shows what to expect, and the checklist of signs a water heater is failing will tell you whether yours was already on borrowed time.
Ignoring a slow drip. A quart a day still rots a subfloor, feeds mold, and rusts the tank's base ring. Small leaks at the tank body also tend to stay small until the day they don't. Assuming every bottom leak means a new heater. Drain valves and top fittings send plenty of perfectly good heaters to the curb. Ten minutes with a dry floor and a flashlight separates the $20 problem from the $1,500 one. Draining without opening a faucet. A sealed tank air-locks and barely drains — open the nearest hot tap first. Turning the power back on "to check." An empty electric tank burns out its upper element in minutes.
Water off, energy off, floor dry, source found — in that order. If the water is coming from the drain valve or a top fitting, you have a repair. If it's seeping from under the tank itself, you have a replacement, and the sooner it's scheduled the less water damage you'll be repairing alongside it. Either way, you'll know which one within about fifteen minutes.
It's urgent but usually manageable once the supply valve and the energy source are off — at that point the tank can only lose the water already in it. It becomes a true emergency if water is spreading where you can't contain it, the T&P valve is venting, or the tank is electric and standing in deep water. Those cases get a plumber now, not tomorrow.
Yes — closing the valve on the heater's cold inlet isolates just the heater. Cold water keeps flowing everywhere, and the toilets and cold taps work normally. You just won't have hot water until the repair or replacement is done.
Days, not weeks — and only if the leak is a slow seep you've contained and you're monitoring it. Tank leaks never heal, and a corroded tank can let go more suddenly than the drip suggests. The floor and framing under it are also soaking up damage the whole time you wait.
A plumber will happily repair a leaking drain valve, T&P valve, or inlet fitting. A tank that's leaking through its own steel shell is different — there's no repair for that, and any plumber who looks at one will quote a replacement. The tank is a sealed pressure vessel; patches don't hold.
Most tank leaks are the end of a long, invisible process: the glass lining cracks over years of heating cycles, water reaches the steel, and rust works through from the inside. Sediment buildup speeds it up by insulating the bottom and overheating the steel. It's also why age matters so much: most tank heaters live about 8–12 years, and a leak in year ten is rarely a surprise to anyone but the owner.