Water heater leaking: first steps, repair cost, and when to replace

If there's standing water around your water heater right now, skip the diagnosis for a minute. Shut off the cold water supply valve at the top of the tank — turn the handle clockwise until it stops. For a gas unit, turn the gas control dial to "off" or "pilot." For an electric unit, flip the breaker labeled for the water heater at your main panel.

If the floor is actively wet and the unit is electric, don't wade in. A wet electric water heater is a shock hazard; keep your distance and call a licensed plumber or electrician.

Once the water and power are off, lay down towels or a shallow bin to contain the pool. Move rugs, cardboard, and stored items away from the base. Water heaters usually sit in basements, garages, or closets, and the drywall, baseboards, and subfloor nearby will soak up water fast. A properly sized water heater drain pan↗ plumbed to a drain is the cheapest insurance against a slow leak turning into a floor-replacement job.

Find where the leak actually starts

People say "the water heater is leaking" when the real source is a fitting above it. Dry everything off and watch for a minute or two. There are four places worth checking.

• The cold and hot water connections at the top. A drip here usually points to a loose fitting, a failed flex connector, or corrosion at a dielectric union. Fixes tend to land in the low hundreds of dollars.
• The temperature and pressure relief (T&P) valve on the side or top, with a discharge tube running down. If it's leaking, the valve is either releasing excess pressure (doing its job) or the valve itself has failed. Either way, don't ignore it — pressure problems in a water heater can be dangerous.
• The drain valve near the bottom. Plastic or brass drain valves can weep, especially after a recent flush. A hose cap tightened onto the outlet will sometimes slow the drip until a plumber can swap the valve.
• The tank itself. If water is seeping from under the unit with no fitting above it, and a drip pan (if you have one) is filling, the inner tank is corroded through. At that point, the heater is finished. Tanks can't be patched in any lasting way.

What usually causes these leaks

Sediment buildup is the quiet killer of tank water heaters. Minerals settle at the bottom over the years, the burner or heating element works harder, and the steel shell corrodes from inside. A sacrificial anode rod is supposed to rust in the tank's place, but once it's gone, the tank itself starts to go. That's why most standard tank water heaters run roughly 8 to 12 years, give or take, depending on water quality and whether anyone has flushed the tank or checked the anode.

Beyond that, the usual suspects are a failed T&P valve, a cracked nipple fitting at the top, loose flex lines after nearby plumbing work, condensation on a cold tank that looks like a leak but isn't, and a tired drain valve. Tankless units leak much less often; when they do, it's usually at the inlet/outlet fittings or the relief valve, and the diagnosis path looks different.

Gas water heaters: the CO angle

If your unit burns gas, the same conditions that cause leaks — corrosion, sediment, or a failed vent — can also interfere with how the flue draws combustion gases out of the house. A leaning, wet, or corroded gas heater deserves a look at the venting, not just the water side. At minimum, confirm you have a working combination carbon-monoxide and smoke alarm↗ on the same level as the water heater and on every sleeping floor. CO from a backdrafting flue is odorless and easy to miss.

If you smell gas at any point, leave the house and call your gas utility from outside.

What's reasonable to DIY — and what isn't

Tightening a loose water connection with a wrench, replacing a drain valve cap, or swapping a flexible supply line are within reach for a confident DIYer who is willing to shut off water and gas/power first. Replacing a T&P valve is technically doable, but you're working with a pressurized hot water system, and a mistake can scald or cause a tank rupture. Fair to hand off.

Not reasonable DIY: replacing a tank water heater. It involves gas lines or 240V wiring, venting, code-required expansion tanks and drip pans in many areas, and hauling a 150-pound corroded tank out of a basement. A botched install can cause carbon-monoxide issues, hidden water damage, and problems at the next home inspection.

Rough cost ranges

These vary by region, labor rates, and the details of your setup. Treat them as ballpark, not quotes.

• Minor fitting or flex-line repair: about $150 to $350 with a plumber.
• T&P valve replacement: about $200 to $400.
• Drain valve replacement: about $150 to $300.
• Full replacement of a standard 40–50 gallon tank water heater: roughly $1,500 to $2,800 installed, more in high-cost metros or where code upgrades (expansion tank, pan, drain, venting) are needed.
• Tankless replacement or new install: often $3,500 to $6,000 or more installed, because of venting and gas-line sizing.

When you're comparing quotes, ask what's actually included: permit, haul-away, expansion tank, new shutoff, sediment trap for gas, and whether the price assumes the existing location.

When replacement beats repair

Replacement usually wins when the tank itself is leaking, when the unit is past 10 years and needs a significant repair, when the anode rod has never been looked at and the water runs visibly rusty, or when you've already been unhappy with recovery time or capacity. Spending $600 on a 12-year-old tank that will fail within a year or two is money better put toward the replacement.

Call for same-day help if…

• The tank is actively discharging water onto the floor.
• The T&P valve is releasing on its own.
• You smell gas.
• The area is wet and electrically live.
• Water is reaching finished floors, subfloor, or a ceiling below. If it already has, our guide on what a ceiling water stain actually means walks through the next steps.

None of those wait until tomorrow.

Document for insurance before you clean up

If water has reached flooring or framing, take photos of the unit, the affected area, the drip pan (or the absence of one), and anything stored nearby — before you mop up. Note the date and time. Homeowners insurance will often cover sudden water damage from a failed appliance, but not the appliance itself, and not long-term seepage. Clean records help if it becomes a claim.

A failed water heater is rarely a same-day disaster if you shut things down quickly and think clearly. Once the immediate water is under control, the real decision is usually repair versus replace — and on most older tanks, replacement is the quieter, cheaper long answer.