Water Heater Pressure Relief Valve Leaking: Causes & Fixes

The pressure relief valve — technically the temperature and pressure (T&P) relief valve — is a small brass or plastic fitting near the top of your water heater, with a pipe running from it down toward the floor. When it drips or sprays water, it's easy to assume the valve itself is broken. Sometimes it is. Often, it's doing exactly what it was designed to do, and the real problem is somewhere else.

What the T&P Valve Actually Does

As water heats, it expands. In a sealed system, that expansion raises pressure inside the tank. The T&P valve is designed to open if pressure or temperature exceed the safe limits set by the manufacturer, releasing water through its discharge pipe before the tank itself is damaged. That discharge pipe should always terminate a few inches above the floor — never capped, threaded shut, or reduced in diameter.

Why It's Leaking: Possible Causes

1. Excess Thermal Expansion

If your home has a check valve or pressure-reducing valve on the main water line, your plumbing system is "closed" — expanding water has nowhere to go except back out through the T&P valve. This is often the most common reason a T&P valve drips on an otherwise healthy heater.

What helps: A licensed plumber can install a thermal expansion tank on the cold-water supply line. This small tank absorbs the extra volume and is generally associated with resolving dripping caused by a closed system.

2. Water Temperature Set High

If the thermostat is set well above your unit's manufacturer-recommended level, the tank runs closer to the valve's trigger point more often, especially during high-demand periods.

What helps: Check your unit's manufacturer-recommended temperature setting and adjust if it's set unusually high, following the manufacturer's instructions. This can also reduce scalding risk and energy use. If dripping continues afterward, the cause is likely elsewhere on this list.

3. High Municipal Water Pressure

If incoming water pressure from the street runs consistently high, it can push the tank closer to the valve's threshold even without much thermal expansion.

What helps: A plumber can test your static water pressure and install a pressure-reducing valve on the main line if it's found to be too high.

4. A Genuinely Failing Valve

T&P valves have a seat that can wear out over time, or accumulate mineral scale that prevents a full seal. If the valve drips steadily regardless of temperature settings or expansion tank presence, the valve itself may need replacement.

What helps: Replacement is generally a minor repair, though cost varies by location and unit type. It's not something to defer, since a failing valve may not open properly in a true overpressure event. Get written quotes before authorizing any work.

How a Plumber Diagnoses the Cause

Rather than guessing, a plumber will typically check several things in sequence: whether your plumbing system has a check valve or pressure-reducing valve that closes the system, your static water pressure, your thermostat setting, and the physical condition of the valve seat itself. Narrowing down the actual cause matters, because addressing a symptom without the underlying cause — for example, replacing the valve without addressing high water pressure — often just leads to the same drip returning within months.

When To Call a Plumber Right Away

• The valve is discharging continuously, not just an occasional drip.
• You hear hissing or see steam near the valve.
• The discharge pipe is hot to the touch or making noise.
• You're unsure whether the cause is thermal expansion, pressure, or valve failure.
• You're seeing water from more than just this valve — our guide to diagnosing a water heater leak can help you trace the fuller picture, and if the tank itself turns out to be the source, our guide to help you decide whether to repair or replace covers that decision.

What Not To Do

Don't tighten, cap, plug, or install a shutoff valve anywhere on the T&P discharge line. Don't ignore a steadily dripping valve hoping it stops on its own — an underlying pressure or temperature issue doesn't typically resolve itself, and a valve that's already marginal can fail to open when it's actually needed.

Bottom Line

An occasional drip after a hot shower, in a home with a closed plumbing system, is common and is often associated with needing a thermal expansion tank — generally a moderate-cost fix. Continuous discharge, hissing, or a valve that won't reseat is a same-day call to a licensed plumber, and worth getting a written quote for before any work begins — this is one component of your water heater you don't want to gamble on.