How often should you test the water heater pressure relief valve?

Short answer

Test the water heater pressure relief valve once a year. The valve sits on the top or the upper side of the tank with a small lever and a pipe running down from it. Its job is to open and dump water if the pressure or temperature inside the tank climbs too high. Lifting the lever to be sure it still opens takes 5 to 10 minutes and costs nothing, unless the test shows it's failed, in which case a new valve costs $0 to $30 to buy.

Why it matters

Water expands as it heats. In a closed tank with a failed thermostat, the pressure can rise far past what the tank was built for. The relief valve is the safety device that keeps that from ending badly: it opens at a set pressure or temperature and lets water out before the tank does. It's the same idea as the weight on a pressure cooker.

A valve that sits untouched for years can seize. Minerals build up on the seat and the spring, and the lever won't lift or the disc won't move. A seized valve can't relieve anything, and a tank with no relief and a stuck thermostat can rupture. That's rare, but it's violent when it happens. A yearly lift of the lever proves the valve still moves, and a valve that drips afterward is far cheaper to replace than to live without.

When to do it more often

Once a year is the schedule for the test itself. The things that bring it forward are signs the valve is already in trouble:

The valve starts dripping or the discharge pipe is wet when nobody has touched it. That can be a worn valve, or house water pressure that's too high. Test the valve, and if it keeps dripping, have the house pressure checked.
The heater has been replaced or repaired. Test the valve on a new or reinstalled heater once it's running.
The tank has made loud banging or rumbling noises. Those are often sediment, but a pressure problem shows the same way.

What you need

A bucket, placed under the end of the discharge pipe to catch what comes out

Steps at a glance

Find the valve on the tank and follow its discharge pipe to where it ends, usually a few inches above the floor or a drain.
Set the bucket under the end of the pipe.
Lift the valve's lever part way and hold it for a few seconds. Hot water should rush out of the pipe into the bucket.
Let the lever snap back. The flow should stop within a few seconds.
Watch the pipe for a minute. A drip that doesn't stop means the valve didn't reseat.
If the lever won't lift, or nothing comes out, or it keeps dripping, replace the valve.

You're done when water came out while the lever was up and stopped when you let go.

Signs it's overdue

The valve drips all the time, leaving a puddle or a mineral trail under the discharge pipe.
The lever won't lift, which means the valve has seized.
Nothing comes out when it's lifted, which means the valve is blocked.
A corroded discharge pipe, or one that's been capped, which is a sign nobody has looked at the valve in years.

DIY or pro

Testing is an easy job. The lever is there to be lifted, and the whole thing takes less time than reading this page. The only catch is that an old valve sometimes won't reseat after a test and starts to drip, which is the test doing its job: the valve was already worn, and now you know.

Replacing a valve is a moderate job, since the tank has to be partly drained and the old valve unscrewed. A plumber charges $100 to $200 to replace one. If the valve is seized, if the tank is old and the fitting looks corroded, or if the drip turns out to be high house pressure rather than the valve, let a plumber handle it.

Safety

Hot water under pressure: the water that comes out of the discharge pipe is as hot as the tank and under full pressure. Keep hands and feet away from the end of the pipe, use the bucket, and lift the lever from the side rather than standing over the outlet. Don't cap or plug the discharge pipe, and don't test a valve whose pipe ends somewhere that water can't safely go.

FAQ

What does the pressure relief valve on a water heater do?

It opens and releases water if the pressure or temperature in the tank gets too high, so the tank can't rupture. It's the heater's main safety device.

Is it safe to test the relief valve myself?

Yes, with a bucket under the discharge pipe and your hands clear of the outlet. The water is hot, so lift the lever from the side.

Why is my water heater relief valve dripping?

Either the valve is worn and won't seal, or the pressure in your plumbing is too high. Replace a worn valve, and if the new one drips too, have the house pressure checked.

How much does it cost to replace a relief valve?

The part is $0 to $30. A plumber charges $100 to $200 to swap it, including draining the tank enough to remove the old one.