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Water heater relief valve leaking or dripping: causes and fixes

The temperature and pressure relief valve (T&P valve) is the safety valve on the top or upper side of your tank, with a pipe running down toward the floor. Its job is to open and let water out if the tank gets too hot or the pressure gets too high. When it drips or spits water, it's either doing that job because something is wrong, or the valve itself is worn out. Either way, it's a signal to investigate, not something to plug.

Safety first. Never cap, plug, or remove the relief valve or its discharge pipe. It's what keeps an overheated or over-pressurized tank from failing violently. Water coming out of it can be scalding hot, so keep hands and faces away from the pipe outlet. If the valve is blowing steam or very hot water continuously, turn off the heater (breaker off, or gas control to OFF) and call a plumber.

Step 1: Figure out how it's leaking

A steady drip all the time usually points to a worn or dirty valve seat that no longer closes fully.
Water shows up after hot water use, or overnight, then stops, usually points to thermal expansion: pressure climbs while the heater is recovering and nothing is drawing water.
A gush or surge of hot water, especially if the water at your faucets is extremely hot, points to an overheating problem with the thermostat or gas control. Treat this as urgent.
Water around the valve threads, not out of the pipe means the leak is at the fitting, not the valve opening. That's a different repair (and could be a corroded tank fitting). See water heater leaking.

Common causes and fixes

1. Water pressure in the house is too high

Most residential T&P valves are rated to open at 150 psi or 210°F; check the tag on your valve for its exact rating. If street pressure coming into the house is high, it doesn't take much extra to push the valve open. You can check with an inexpensive pressure gauge that screws onto an outdoor hose spigot or the heater's drain valve. Check it a few times, including at night when demand on the water system is low.

Fix: if pressure is consistently high, a pressure-reducing valve (PRV) on the main line is the usual answer. If you already have one, it may have failed or need adjustment. Your local water utility or a plumber can tell you what's normal and allowed in your area.

2. Thermal expansion in a closed system

Water expands as it heats. In older homes that extra volume could push back into the city main. Many homes now have a check valve, backflow preventer, or PRV at the meter that stops water from flowing backward, which makes the plumbing a "closed system." With nowhere to go, the expanding water raises pressure every time the heater runs, and the relief valve is often the first thing to give. A gauge that shows pressure climbing well above its normal reading while the heater recovers is a strong sign of this.

Fix: install a thermal expansion tank on the cold supply line near the heater, sized for your tank and water pressure. If you already have one, it may be waterlogged. Tap it: a working tank sounds hollow on top and solid on the bottom. Water coming out of the air valve stem, or a tank that sounds full all over, means the bladder has failed or the air charge is gone. Many local codes require an expansion tank on closed systems, so check with your local plumbing authority.

3. The water is getting too hot

If the thermostat or gas control valve fails to shut off, the water can overheat and the valve opens on temperature rather than pressure. Signs include water at the taps that's far hotter than usual, steam from the discharge pipe, or the valve opening even though house pressure looks normal.

Fix: turn the heater off and have the controls checked. On an electric heater, a stuck thermostat or a high-limit switch that keeps tripping needs attention. On a gas heater, a gas control valve that won't regulate temperature should be replaced by a qualified technician.

4. The valve itself is worn or dirty

Mineral deposits or debris can keep the valve from seating, and valves wear out over time. If pressure and temperature check out fine and the valve still drips, replacing it is a common and inexpensive repair.

Fix: shut off power or gas and the cold supply, let the water cool, relieve pressure by opening a hot faucet, drain a few gallons so the water level is below the valve, then unscrew the old valve and install a new one with the same pressure and temperature rating (and one suited to your heater's BTU or wattage input, listed on the tag). Use thread sealant suited for potable water, reconnect the discharge pipe, refill the tank with a hot faucet open to purge air, then restore power or gas.

Some manufacturers recommend lifting the test lever periodically to make sure the valve isn't stuck. Be aware that on an older valve, testing it can dislodge debris and leave it dripping afterward, so have a replacement on hand.

Check the discharge pipe while you're there

The pipe from the relief valve should be the same size as the valve outlet, run downhill with no valves or caps on it, and end in a safe spot near the floor or a drain as your local code requires, with the end left open so you can see when it discharges. A pipe that's undersized, capped, or routed uphill is a safety problem on its own, even if it isn't leaking now.

When to call a plumber

The valve discharges very hot water or steam, or your tap water is scalding hot.
Pressure is high and you need a PRV or expansion tank installed.
You're not comfortable draining the tank or working with gas controls.
The leak is at the threads where the valve meets the tank, which can mean corrosion in the tank fitting.
The heater is older and showing other problems. See how long water heaters last before paying for repairs.

Tip: Before you call, note your house water pressure from a gauge, whether the leak is constant or only after hot water use, whether there's an expansion tank, and the heater's age and fuel type. That helps a plumber get to the cause quickly.