Short definition
A water heater leak is any visible water at or below the heater. The diagnosis is everything: a leak from the T&P discharge or drain valve is a $30 part. A leak from the tank itself — a puddle on the floor with no obvious source above — means the tank is rusted through and needs replacement. Identify the source before spending money.
What it is
“Water heater leak” is a homeowner search term, not a single failure. Water shows up under or around the heater for one of six reasons, and the fix depends entirely on which one:
- T&P discharge tube dripping. Almost always closed-system thermal expansion. Verify the expansion tank works; replace it if waterlogged. Don’t replace the T&P unless you’ve cleared the expansion tank.
- Drain valve dripping. The plastic seat at the bottom drain spigot has fouled or warped. Cap it temporarily; replace the valve.
- Tank itself leaking. Water on the floor with no obvious upstream source, often near the bottom of the tank. The steel shell has rusted through from inside. Replace the heater.
- Cold inlet or hot outlet drip. Dielectric union seal failed, or the flex connector gasket gave out. Replace the union or flex.
- Expansion tank leaking. Bladder ruptured. The tank weeps at its seam or Schrader valve. Replace the expansion tank.
- Pipe-thread leak at top connections. Insufficient pipe dope or Teflon. Rework the joint.
A condensation drip can fool you too. Cold supply lines sweat in WA’s humid summer crawlspaces; that’s not a leak. Touch the water — if it’s cold and the source is the cold inlet pipe, condensation is the most likely culprit.
Why it matters to a homeowner
The single most common bad outcome is replacing a $1,200 water heater for a $30 problem. T&P drips and drain-valve drips both look catastrophic — water on the floor near a heater feels like an emergency. Both are component-level repairs. The contractor who tells you “the tank is leaking, you need a new heater” should be able to point to the bottom seam of the tank itself, not the T&P discharge or the drain spigot.
The flip side is the slow seep at the bottom of the tank that gets ignored. Tank rust-through is end-of-life — once a pinhole opens at a tank seam, the rest follows fast. A small puddle today is a flooded utility room next week. Plan the replacement; don’t wait for the gusher.
For insurance and warranty purposes, photograph the leak source before any cleanup or repair, note the install date and permit number, and keep the manufacturer model and serial. WA water-damage claims hinge on documentation.
When you’ll encounter this term
- You spot a puddle on the floor under or beside the water heater.
- Drip stains in the drain pan that weren’t there last month.
- A wet floor only after running hot water for several minutes.
- A home inspector flags moisture or rust at the heater base in a sale report.
Common failure modes (where leaks come from)
- T&P discharge dripping. Closed-system thermal expansion (most common); fouled valve seat (less common).
- Drain valve dripping. Plastic seat gone. Ten-dollar fix.
- Tank seam at bottom rusted through. Anode-consumed end-of-life. Replace heater.
- Dielectric union or flex connector. Replace the failing fitting.
- Expansion tank waterlogged. Replace.
- Pipe-thread leak. Rework joint with proper sealant.
Washington note
WA water-damage insurance claims from water-heater leaks are common — the average residential claim runs into the thousands once flooring, drywall, and contents are factored in. Most WA homeowner policies cover sudden discharge but not gradual leak damage; if you’ve been ignoring a slow seep, the claim may be denied. Photograph and file early.
The other WA-specific item: many older homes in Seattle, Tacoma, and Spokane have water heaters in finished basements or under stairs without a drain pan or floor drain. WA-amended UPC requires a drain pan with a 3/4″ drain line to a code-approved location whenever a leak would damage finished space below. If your heater is on hardwood, carpet, or above a finished space, the pan and drain are not optional at replacement.
FAQ
My water heater is leaking from the top — what is it?
Top-of-tank leaks are almost never the tank itself. Likely culprits, in order: dielectric union seal at the cold inlet or hot outlet, flex connector gasket, T&P relief valve threads, or a pipe-thread joint above the tank. Trace the wet line up — the highest visible wet spot is your source. These are component repairs, not a heater replacement.
My water heater is leaking from the bottom — do I need a new one?
Maybe, but check the drain valve first. Half of “leaking from bottom” reports are drain-valve drips, not tank leaks. Run a dry paper towel around the drain spigot and around the base of the tank itself. Wet at the spigot only = $10 valve replacement. Wet at the tank seam with no upstream source = tank rusted through, time for a new heater.
How long should a water heater last in Washington?
Tank heaters typically last 8–12 years. Soft Cedar/Tolt water in Seattle and most of King County is gentle on tanks — original anode rods often go 8–10 years and tanks reach 12–15 with anode replacement. Eastside and Spokane on harder water see 6–10 year tank lives unless the anode is replaced at year 5. Tankless water heaters routinely run 15–20 years with annual descaling.