Dry the floor. Towel-dry the tank. Wait 30 min. Come back with a flashlight. The first wet spot is where the leak originated. Don't guess from where water pooled — it could be running down the side from a top connection.
The 5 leak locations decoded
| Location | Probable cause | Severity | Fix |
|---|---|---|---|
| Top — cold or hot inlet/outlet | Loose union nut or failed gasket at the supply connections | Low — fixable | Tighten union nut 1/4 turn or replace gasket ($5 part, 20 min) |
| T&P relief valve discharge pipe | Pressure or temp exceeded valve threshold (150 psi / 210°F) — see below | Diagnostic — don't plug the valve | Find why T&P opened (thermostat setting, expansion tank, PRV) |
| Drain valve (bottom, brass) | Sediment crystallized inside, valve doesn't fully close | Low — fixable | Drain tank, replace valve ($15, 1 hr) |
| Bottom of tank (rust-colored water, steady drip) | Tank corrosion — glass lining failed, steel rusting | Critical — replace | Replace unit. No reliable patch. |
| Anode rod port (top center hex plug) | Anode rod gasket failed | Low — fixable | Replace anode rod + gasket ($25 part, 30 min) |
The T&P relief valve scenario (don't plug it)
The Temperature & Pressure Relief valve (T&P) is a safety device. It discharges water when tank pressure exceeds 150 psi or temperature exceeds 210°F. If it's dripping, something is wrong — but the valve is doing its job. Don't cap it. Capping a T&P discharge has resulted in water heaters exploding through roofs. Look it up — the videos exist.
Three causes of T&P discharge:
- Thermostat set too high. Lower from 140°F to 120°F. Check after 24 hrs.
- Expansion tank failed or missing. Modern closed-loop systems (with a check valve or PRV) require an expansion tank to absorb thermal expansion. If missing or failed, pressure spikes. Install or replace expansion tank ($60-$120 part, plumber call).
- PRV at main water entry stuck shut. Creates closed system. Test PRV per low water pressure diagnostic.
The anode rod check that extends life by 5+ years
The anode rod is a sacrificial magnesium or aluminum rod that corrodes instead of the steel tank. Replacing it at year 5-6 (when it's typically 75% consumed) extends tank life by 5-10 years. Most homeowners never check it. Cost: $25 part, 30 min DIY. Tools: 1-1/16" socket on a long breaker bar (the hex plug is torqued tight from the factory).
FAQ
Why is my water heater leaking from the bottom?
Tank corrosion. Glass lining failed, steel rusting. Replace the unit — no reliable patch.
Why is water dripping from the T&P valve?
Valve is doing its job — pressure or temp exceeded. Three causes: thermostat too high, expansion tank failed/missing, PRV stuck. Diagnose, don't plug.
How long does a water heater last?
Gas tank: 8-12 yrs. Electric tank: 10-15 yrs. Tankless: 20 yrs (with annual descaling).
Can I patch a leaking tank?
No. Internal corrosion. External patches don't address it.
Is a water heater leak an emergency?
Small drip — no, but plan replacement within weeks. Spraying or pooled water — shut off water + power/gas, plumber same-day.
Related guides
- Best tankless water heaters 2026 — for the replacement decision
- Best heat pump water heaters 2026 — beats gas tank on operating cost
- Tankless vs tank water heater — the comparison that decides the replacement
- Water heater banging popping — adjacent diagnostic
Editorial standards: Cited authorities include UPC/IPC plumbing codes on T&P valve and expansion tank requirements. Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer.