The short answer

A drip near an expansion tank is a location clue, not a diagnosis. Water can travel along a pipe and make the tank look guilty. Start by safely observing the first wet point, the rate of leakage, and whether the relief valve or discharge tube is involved. Do not cap, plug, or restrain a relief discharge.

Map the symptom

Observation What it may suggest Next safe step
Wet threaded connection Joint or fitting leak Photograph it without touching hot metal.
Water at the tank seam Tank shell failure Shut down according to the manufacturer and call a professional.
Relief discharge is wet Pressure, temperature, valve, or discharge issue Do not block it; obtain prompt service.
Water below the heater A higher leak, drain, or tank failure Check only from a safe distance and protect nearby materials.
Intermittent drip after heating Thermal expansion or a marginal valve Record timing; do not infer a pressure value.

Write down the heater fuel type, age, visible model information, and whether the drip changes after hot-water use. Keep children away and avoid opening panels or changing gas controls. If water approaches electrical equipment, use the appropriate emergency shutoff only if it can be reached safely and obtain help.

What a useful service visit should answer

Ask the technician to identify the first leak point, measure system pressure where appropriate, verify the expansion-tank condition, and explain whether the relief valve, piping, and discharge route meet the installation requirements. A replacement estimate should separate the tank, fittings, labor, disposal, and any code-related work.

Limits

This guide is not a pressure-setting procedure. Water heaters combine heat, stored pressure, electricity or gas, and scald risk; concealed or active faults belong with a qualified professional.