Water Heater Leaking
Where it leaks from tells you if this is a $20 part or a new tank.
Compiled by DIY Repair Guide · reviewed 2026-09-05 ·how we research these
A puddle under the water heater is not one problem. It is five different problems that happen to end up on the same patch of floor, and they range from a twenty-dollar part you can change with a wrench to a tank that has rusted through and is going to flood the room.
The good news is that telling them apart is mostly a matter of looking in the right place. Water runs downhill, so the puddle is almost never under the actual leak, you have to trace it back up. The five leak points below run from cheapest to most expensive, with what each one means and the price of putting it right.
Before You Diagnose Anything, Make It Safe
A water heater holds forty to eighty gallons at around 120°F under household pressure. Two things need to happen before you put your hands near it.
Kill the energy source. On a gas heater, turn the gas control knob on the front of the unit to Off. On an electric heater, switch off the dedicated double-pole breaker in your panel. Never drain a tank with the power still on. An electric element that fires with no water around it burns out in seconds.
Shut off the cold water. There is a valve on the cold inlet pipe at the top of the heater. Turn it clockwise until it stops. If that valve is seized, and on a fifteen-year-old install it often is, shut off the main instead.
Never cap, plug or thread a valve onto a T&P discharge pipe. That pipe is the only thing standing between a runaway thermostat and a tank that becomes a projectile. If it is discharging, the answer is to find out why, never to stop it.
The Five Places a Water Heater Leaks
Dry the whole unit and the floor around it, then put down a few sheets of paper towel and come back in an hour. Paper shows you exactly where the first drop lands, which is the single fastest diagnostic there is.
1. Inlet and Outlet Fittings at the Top
The two pipes entering the top of the tank are joined with threaded nipples, and on most tanks those nipples are where dissimilar metals meet. Copper against steel in the presence of water is a small battery, and over a decade or so it eats the joint. You will see green-white crust around the fitting and a slow weep rather than a stream.
This is a repairable leak. A plumber will cut the connection out and reinstall it with dielectric unions or a brass nipple to break the galvanic path. Expect one to two hours of labor plus a few dollars in fittings. It is worth doing on a tank under about eight years old, and rarely worth doing on one over twelve.
2. the T&P Relief Valve. Usually Not the Valve
The temperature and pressure relief valve sits near the top of the tank with a pipe running down toward the floor. It is a safety device: it opens if the water exceeds roughly 210°F or 150 psi.
When people find water at the bottom of that discharge pipe, the instinct is to replace the valve, but that is usually the wrong fix, because a discharging T&P valve is generally doing its job. Something is over-pressurising the tank, and the most common cause is thermal expansion in a closed plumbing system. One with a check valve, a pressure-reducing valve or a backflow preventer on the incoming main. Water expands as it heats, and with nowhere to go the pressure climbs until the T&P lifts.
The real repair in that case is a thermal expansion tank on the cold supply rather than a new relief valve. If your static pressure is also high, a pressure-reducing valve may be needed as well.
| What you see | Most likely cause | The fix |
|---|---|---|
| Occasional drips after a hot shower | Thermal expansion, closed system | Add an expansion tank |
| Steady dribble, tank not especially hot | Failed valve seat, often from mineral scale | Replace the T&P valve |
| Full discharge, water near boiling | Thermostat or gas control failure | Shut it down now, call a plumber |
| Water at the pipe but valve is dry | Condensation running down the pipe | Nothing. See below |
3. the Anode Rod Port
The anode rod is a sacrificial magnesium or aluminum rod hanging inside the tank that corrodes so the steel does not. It threads into a port on top, sometimes hidden under a plastic cap or the insulating jacket.
Leaks here are almost always a tired thread seal rather than anything serious, and they are worth chasing down for a second reason: if you have the port open, check the rod. A rod worn down to the steel core means the tank has been unprotected for a while and is on borrowed time. Replacing a rod on a tank that still has good steel is one of the few genuinely life-extending things you can do to a water heater.
4. the Drain Valve. the Cheap One
At the bottom of the tank is a spigot for draining it. Many are plastic, and plastic in permanent contact with hot water eventually weeps at the stem or fails to seat after someone has flushed the tank.
First try snugging the valve closed. If it still drips, the part costs under twenty dollars and a brass replacement is a genuine upgrade over the plastic original. Changing it means draining the tank, so budget an hour or two, and be aware that a valve that has never been touched on an old tank can break off in your hand, which converts a small job into an urgent one. If that possibility makes you nervous, this is a reasonable one to hand to a plumber.
5. the Tank Itself. the Expensive One
If water is coming from the body of the tank, or seeping out from under the jacket with no fitting above it, the steel has corroded through from the inside. There is no repair. Welding is not an option, sealants do not hold against the pressure, and the hole only gets bigger.
A tank that has started weeping can go from a drip to a full failure with no warning, which is why this is the one leak you should not sleep on. If the heater sits above finished space or anywhere without a floor drain, treat it as urgent.
Check the manufacture date before you spend anything. It is on the data plate, encoded in the serial number, and every manufacturer does it differently. A. O. Smith and State put a two-digit year then the week at the start. Rheem and Ruud put the month then the year, so a Rheem serial opening 0618 is June 2018 rather than week 6. Bradford White uses a two-letter code with no digits at all. Look up your own brand decoder before you judge the age, because a gas tank past roughly twelve years is at the end of its expected life and money spent on fittings is money you will spend again.
The Thing That Is Not a Leak
Condensation fools a lot of people, and it is worth ruling out before you call anyone. A gas heater that has just been refilled with cold water, after heavy use, or right after installation, will sweat heavily on the outside of the tank and drip onto the floor. It looks exactly like a slow leak.
The tell is timing. Condensation appears during and shortly after a big hot-water draw and dries up once the tank has recovered. A real leak does not care what you are doing and does not dry up. If your puddle disappears on its own by the next morning and only shows up on laundry day, you are watching physics rather than failure.
What Each Repair Costs
Prices below are typical US ranges for a standard 40–50 gallon storage heater. Labour rates vary widely by region and go up sharply for emergency and after-hours calls.
| Repair | Parts | Typical installed cost | Worth it on an old tank? |
|---|---|---|---|
| Drain valve replacement | $15–20 | $120–250 | Yes. Cheap either way |
| T&P valve replacement | $15–25 | $150–300 | Yes |
| Thermal expansion tank | $40–90 | $250–500 | Yes. Protects the next tank too |
| Anode rod replacement | $25–60 | $150–350 | Only if the tank is under ~8 years |
| Re-plumb inlet/outlet fittings | $20–50 | $200–450 | Not past ~12 years |
| Full tank replacement (gas, 40–50 gal) | — | $1,200–2,500 | This is the answer for a leaking tank |
Do It Yourself, or Call Someone
The line runs roughly where gas, electricity and structural water risk begin.
- Reasonable to do yourself: tightening a drain valve, swapping a drain valve on a tank you are willing to drain, checking the anode rod. Replacing a T&P valve is also within reach, but only after you have shut off the gas or the breaker and drained the tank below the valve port. Pulling that valve on a full hot tank puts scalding water across your hands.
- Call a plumber: anything involving the gas line, re-plumbing the inlet or outlet, diagnosing high system pressure, and every tank replacement, which in most jurisdictions needs a permit and has to meet current code for venting, seismic strapping and expansion control.
- Call today, not tomorrow: water coming from the tank body, a T&P valve discharging near-boiling water, or any leak above finished ceilings.
Common questions
Can I still use a water heater that is leaking?
It depends entirely on which leak you have. A weeping drain valve or a fitting that drips a few times a day is not an emergency, and you can keep using the heater while you arrange a repair. A leak from the tank body is different. It will get worse, sometimes very suddenly, and it should be treated as urgent.
How long can a leaking water heater last before it fails completely?
There is no reliable answer, which is the problem. Some pinhole tank leaks weep for months. Others open up within days. Because the failure mode is uncontrolled water rather than a gradual decline, the sensible planning assumption is that a leaking tank could dump its full contents at any point.
Why is my water heater leaking only when the hot water runs?
That pattern usually points at pressure or expansion rather than a hole. Drawing hot water brings cold water in, the burner fires, the contents expand, and pressure rises. If the leak tracks that cycle, look at the T&P valve and whether the system is closed and needs an expansion tank.
Is it worth repairing a 15-year-old water heater?
Generally no, beyond the cheap items. At fifteen years a storage tank is past typical service life, the anode is likely long gone and the steel has been corroding unprotected. Spending several hundred dollars on fittings buys you an unknown amount of time on a component that is already overdue.
Where is the serial number that tells me the age?
On the data plate stuck to the side of the tank, encoded in the serial number. There is no universal scheme. A. O. Smith and State use a two-digit year followed by the week. Rheem and Ruud use the month followed by the year. Bradford White uses two letters and no digits. Find your brand on the manufacturer support site and use its own decoder, because reading a Rheem plate as though it were an A. O. Smith one can put you out by most of a year.