Water heater leaks from the bottom only when heating
Usually not the tank
That timing rules the tank out. A corroded tank leaks constantly, because it is under pressure all the time. Water that appears only while the heater runs is coming from the T&P discharge pipe above it, or it is condensation.
Do not cap or plug a discharge pipe that is releasing water. It is the tank's pressure safety route.
How to tell it's this
- It appears within minutes of the burner or element starting, and dries up between cycles.
- The floor is bone dry overnight when nobody has used hot water.
- The discharge pipe running down the side of the tank is wet along its length.
- On a gas unit in cold weather, the tank's outer skin is beaded with moisture.
What to do
- Dry the floor and put paper towel under the end of the T&P discharge pipe specifically, not just around the base.
- Run enough hot water to make the heater fire, then watch that paper.
- Wet paper under the discharge pipe means the relief valve is opening. Work out which of the four causes it is — that is a real problem to fix, just not a tank problem.
- Dry paper under the pipe but a damp floor on a gas unit points to condensation, which needs nothing done.
What it costs
Depends what you find. Condensation costs nothing. A relief valve lifting costs $150–$350 for the valve, or $200–$450 for an expansion tank if the cause is thermal expansion.
Typical US ranges for 2026, not quotes. Labour is 30–50% of an installed price, so where you live moves these numbers more than anything else does. Get two or three local estimates.
If that's not it
The pool is there constantly, day and night
A tank that has rusted through →
You confirmed the relief valve is opening
Why the relief valve opens →
Not sure which of these it is?
Answer four questions — where it's leaking, tank or tankless, gas or electric, and how
old it is — for a specific cause, a repair-or-replace call, and a cost.