Where is the water coming from?
Dry the floor and the outside of the heater with a towel, then watch for ten minutes with a flashlight. Water runs down, so the first wet spot you see is often below the real source. The table lists the usual places, in the order a plumber checks them.
| Where you see water | Most likely cause | Usual fix |
|---|---|---|
| End of the pipe running down the side | Temperature and pressure relief valve opening, or a worn valve seat | Find out why it opened; replace the valve if it keeps weeping |
| Drain valve near the bottom | Washer or plastic valve body leaking, often after a flush | Cap it with a hose cap, then replace the valve |
| Pipes and fittings on the upper side | Loose union, corroded dielectric nipple, old flexible connector | Tighten or replace the connection |
| Access panel on an electric model | Heating element gasket | Drain partly, replace the gasket or element |
| Under the jacket at the base, rust-colored water | Tank corroded through | Replacement; the tank can't be patched |
Sources: part names and requirements from IPC 2021 §504.4 and §504.6; diagnosis order is common service practice.
Water heater leaking from the bottom
Water at the base has two very different causes, and you can usually tell them apart in a minute. If the drain valve is dripping, the valve's spout will be wet and the drip will follow its outlet. If the valve is dry but water keeps appearing under the heater, the tank is leaking through its own wall.
Tanks fail from the inside. The steel shell is lined with glass, and a magnesium or aluminum anode rod corrodes in place of the steel. Once the anode is used up, rust starts at the welded seams, usually near the bottom, where sediment sits. By the time water reaches the floor, the damage inside the jacket is already done, which is why no plumber will quote a repair for a leaking shell.
Relief valve leaking: why it opens
The relief valve is a safety device, not a faucet. It opens when pressure or temperature in the tank goes past the rating stamped on its tag, and water coming out of the discharge pipe tells you one of three things happened: the water got too hot, the pressure got too high, or the valve itself is worn.
- Thermal expansion. Water grows as it heats. If your supply has a check valve, backflow preventer or pressure regulator, that extra volume has nowhere to go and pressure climbs with every heating cycle. The International Plumbing Code (IPC 2021 §607.3) requires a thermal expansion control device on the cold supply downstream of those valves; in practice that is a small expansion tank.
- High street pressure. Where static pressure in the building exceeds 80 psi, both the IPC (§604.8) and the Uniform Plumbing Code (§608.2) require a pressure-reducing valve. An inexpensive pressure gauge threaded onto a hose bib will tell you what you have.
- A tired valve. Mineral deposits keep some valves from reseating after they open. A plumber will replace the valve, but only after ruling out the first two causes, because a new valve on a system with runaway pressure will drip too.
Never plug the discharge pipe
Capping or plugging the relief valve line to stop a drip removes the only protection against a tank rupture. Fix the cause of the drip instead.
Water heater leaking from the top
Wet pipes above the tank are good news, because every part up there can be replaced without touching the tank. Look at the cold inlet and hot outlet connections, the steel nipples threaded into the tank, and the flexible connectors. White or green crust on a joint means it has been seeping for weeks. Corroded dielectric nipples are a routine find on older heaters, and a plumber will usually replace both at once.
What the plumber checks while there
A repair visit is also a quick code check. On the discharge pipe, IPC 2021 §504.6 requires that it not connect directly to the drainage system, that it run full size and downhill without traps or valves, that its end not be threaded, and that it end no more than 6 inches above the floor or a waste receptor. Where a leak would damage the building, §504.7 calls for a drain pan at least 1½ inches deep with a drain of at least ¾ inch. Where the UPC is in force, §507.2 requires the heater to be anchored or strapped against earthquake motion, within its upper and lower thirds.
If the tank itself is gone, the conversation turns to replacement, and the price depends on the fuel, the venting and those same code items. Our page on water heater replacement cost breaks the quote into its line items, and water heater repair covers the parts that are worth fixing.
Some plumbing companies answer 24/7, and some send a technician the same day; others take the call at night and schedule the visit for the morning. When you call, ask which one you have reached and what the after-hours trip charge is before anyone drives out.
What to do if you can't stop the leak
If the cold valve above the heater is seized, close the main shutoff for the house. If the tank is leaking and you want it empty before the plumber arrives, wait until the water has cooled; the U.S. Consumer Product Safety Commission notes that water at 140°F can cause a third-degree burn in about six seconds. Then run a garden hose from the drain valve to a floor drain or outdoors and open a hot faucet upstairs so air can enter the tank.
Short answers about heater leaks
Is a leaking water heater dangerous?
The water is the smaller risk. The real hazards are scalding water at the drain valve, electricity near a wet floor, and on gas models a burner that may be soaked. Turn off the power or set the gas control to off before touching anything, and keep children away from the discharge pipe.
Can I keep using the heater until a plumber comes?
If the drip comes from the relief valve pipe or the drain valve and the floor stays dry, many people leave the heater running for a day. If water is seeping from the tank body, close the cold inlet, shut off the fuel and leave it off: a rusted tank can open wider without warning.
Why is there water under a brand-new heater?
A gas heater that has just been filled with cold water can condense moisture from its own exhaust on the cold tank walls, and a few drops may fall onto the burner for the first hour or so. A steady puddle is different: that is a fitting, a valve or a connection, and the installer should come back.
Will homeowners insurance pay for a new heater?
Policies usually treat a heater that simply wore out as maintenance and do not replace the appliance itself. Wording differs from policy to policy, so call the insurer before the old tank is hauled away if you intend to file anything.
Sources
- International Plumbing Code 2021, Chapter 5: Water Heaters (§504.4, §504.6, §504.7), International Code Council.
- International Plumbing Code 2021, Chapter 6: Water Supply and Distribution (§604.8, §607.3, Table 604.3), International Code Council.
- Uniform Plumbing Code 2021 (§507.2 seismic strapping, §608.2 pressure regulators, §608.3 expansion tanks, §807.4 dishwasher air gaps), IAPMO.
- Avoiding Tap Water Scalds, Publication 5098, U.S. Consumer Product Safety Commission.
Reviewed September 27, 2026. Found an error? Write to admin@plumbingrepairquotes.com.