Why Does My Water Heater Leak From the Relief Valve?

Short answerUsually it's pressure, not a bad valve. Water expands as it heats, and in a closed plumbing system that raises pressure until the valve opens. A. O. Smith says to control water pressure and add a thermal expansion tank before replacing the valve. Never cap or plug the discharge line. A plumber can check this.

What this costs

Our national estimates, not quotes.
Option	Job	Low	High
Replace	Water Heater Replacement	$680	$1,900

Why the valve opens

The temperature and pressure relief valve is a safety device. A. O. Smith explains that water grows as it heats, and in a closed system, that increases pressure. The valve then releases water. Its manual says the valve isn't meant for constant relief of this expansion. That's what an expansion tank does.

What a closed system is

As we understand it, a closed system is one where water can't flow back toward the street, such as with a check valve or pressure-reducing valve on the supply line. A. O. Smith says a properly sized expansion tank should be installed on all closed systems, and that damage from thermal expansion isn't covered by its limited warranty.

What a plumber will check

Water pressure. A. O. Smith's target is 50 to 60 psi, with 80 psi as a maximum.
Whether you have an expansion tank, and whether it still works.
The valve itself. A. O. Smith says to replace it if it keeps dripping after pressure and expansion are fixed. Leaks at the threads may mean removing and resealing it.

Never plug it

A. O. Smith's manual says the discharge line must not be capped, blocked or plugged. It says not to plug or remove the valve, and warns that not following its instructions can result in death or explosion. If yours is dripping, we recommend putting a bucket under it and calling a licensed plumber. A. O. Smith says this water can be very hot.

Water Heater Leaking? What to Do Right Now

Common questions

Is a dripping relief valve dangerous?

We read the drip as a warning that pressure or temperature is too high, or that the valve is failing. We wouldn't ignore it, and never plug it. Call a licensed plumber.

How often should the valve be checked?

A. O. Smith says to check it yearly and have a professional inspect it about every 5 years. NC State Extension also suggests opening it to confirm it works. A. O. Smith warns the water can be very hot.