Why is my water heater relief valve leaking?

By Dana Whitlock. Updated October 2026. Repair or replace?

The temperature and pressure relief valve is the safety device that keeps a water heater from building up dangerous pressure. When it drips or discharges, it is telling you something. Usually the house water pressure is too high or there is no room for expanding hot water. Occasionally the tank is overheating, which is serious. In every case, never cap or plug the valve or its discharge pipe.

Likely causes

High household water pressure Pro job

A. O. Smith's manual says a small amount of water dripping from the T&P valve usually means water pressure in the home is too high. It recommends a pressure reducing valve set to 50 to 60 psi. Installing one is plumbing work.

Thermal expansion with no expansion tank Safe to check

Water expands as it heats. In a closed system with a check valve or pressure reducer, it has nowhere to go but out the relief valve. The manual points to adding or checking a thermal expansion tank. You can look for one (a small tank near the heater) but sizing and installing it is for a plumber.

The tank is overheating Pro job

A control that fails to shut off the burner or element can push water temperature too high. Large amounts of very hot water from the discharge pipe are the sign. This is a stop-and-call situation.

A worn or debris-fouled valve Pro job

After years in service, or after being opened for testing, a valve can fail to reseat. Replacing it is routine for a licensed plumber.

Safe things to check first

• Look at the end of the discharge pipe. Note whether it is a slow drip or a steady flow, and whether the water is hot.
• Check whether a small expansion tank is plumbed near the heater.
• Confirm the discharge pipe is open at the end. It must never be capped, plugged or reduced in size.
• Check the temperature dial. If it has been turned to a very hot setting, return it to the normal mark.
• If a hose-thread pressure gauge is already on hand, a plumber can show you where to read it. Otherwise leave pressure testing to them.

When to stop and call

If the valve is releasing large amounts of very hot water, the manual says to turn off the supply and call a qualified person. For an electric unit, switch off its breaker. On a gas unit, leave the gas valves and control alone and let the plumber handle them. A slow drip still needs a licensed plumber soon, because the cause (pressure or expansion) stresses the tank and every fixture in the house. If you smell gas, leave the house before calling the utility.

Repair or replace?

Start with its age. Type the serial from the label:

A new relief valve, expansion tank or pressure reducer is a modest repair and is worth doing on a heater that has life left. On an old tank the repair still protects the next heater, so the money isn't wasted. Date the heater with the age decoder above. DOE puts the average gas tank at about 14.5 years, and a 2007 builders' survey at about 10. If yours is near the end, ask the plumber to price the valve work as part of a replacement using the range above.