Did you know that a water heater could blast through a house like a powerful missile? Be sure your water heater’s T&P relief valve works!

Steam, if allowed to build in pressure, is an incredibly powerful force.

In the video below, Jamie and Adam of the popular television show, MythBusters, show just what can happen when the heat—and resulting pressure—inside a water heater surpasses the water heater’s rated limit.

When they crank-up the heat and pressure, the water heater blasts through the upper floor and roof of a demonstration house and soars hundreds of feet into the sky, leaving behind a demonstration house that’s been blown to smithereens.

A water heater launching through the roof of an outdoor shed, with debris flying everywhere.
Water heater blasts through the roof of demonstration house. See the video below. MythBusters


This is why storage water heater tanks have a simple little device called a temperature and pressure-relief valve (T&P relief valve) screwed into the top or side of the tank.

If a water heater overheats to a temperature above 210 degrees F, building pressure inside the tank in excess of 150 psi, this simple little valve automatically opens and releases the built-up pressure (and water), protecting against a disastrous explosion.

Obviously, it’s important to be sure this valve works properly.

water heater pressure relief valve
Water Heater T&P Valve Apollo

As T&P valves get old, they can begin to drip or leak and may need to be replaced. You can order a water heater T&P valve online or buy one at a hardware store or home improvement center.

How to Test a Water Heater T&P Relief Valve

Periodically test this device by lifting its lever to make sure it is working. Hot water should come out of the discharge pipe. When you release the lever, it should snap back down.

Unfortunately, doing this with an old valve can cause it to leak. If it does, you’ll need to replace it. So the solution can become the problem. That’s why we’re showing you how to replace it.

Because the pipes and the water will be very hot, wear sturdy gloves.

A defective TP relief valve, with rust around connections.
Replace a defective TP relief valve. ©HomeTips

Note: Be aware that a pressure relief valve is designed so that water will pour out of it if pressure builds up too high in the water heater. With this in mind, the discharge pipe from the valve should be emptied into a bucket or tray, or be piped outdoors or to a drain. It should never be capped!


How to Replace a Water Heater T&P Relief Valve

Here’s how to replace a water heater temperature-pressure relief valve:

1 Shut off the cold water to the water heater and gas or electricity that heats the water heater. Close the shutoff valve on the cold water inlet line that fills the water heater.

2 Drain the water from the tank until the water level is below the valve’s location. To do this, connect a garden hose to the water heater’s drain valve, open a hot water at any faucet, shower, or bathtub to relieve the water pressure in the system, and then open the drain valve. Remember that the water will be hot!

For a top-mounted valve, drain about 1 gallon. For a side-mounted valve, drain about 10 gallons. Then turn off the drain valve and the faucet.

Illustration showing a water heater drain valve emptying into a floor drain with a hose.
Attach a hose to the water heater’s drain valve and drain several gallons from the tank. © Don Vandervort, HomeTips


Caution: The water will probably be very hot unless you’ve allowed time for it to cool! 

3 Raise the lever on the valve to release any excess pressure. 

Gloved hand lifting the lever on a T&P valve on side of a water heater.
Lift the lever on the T&P valve to release in pressure in the water heater. © HomeTips

4 If the valve has a discharge pipe attached to it, unscrew the pipe, turning it counterclockwise (as viewed from the pipe).

Gloved hand turns a wrench on the discharge pipe, removing it from the T&P valve.
Remove discharge pipe from T&P valve by turning it counterclockwise. © HomeTips

5 Unscrew the old T&P valve from the water heater after you’ve unscrewed the discharge pipe from it. Turn it counterclockwise, using a wrench. Note: Slide a pipe over the wrench’s handle if you need more leverage to break it free. If hot water begins to pour out, screw the valve back in and drain more water from the tank.

Wrench unscrewing a T&P valve from the side of a water heater.
Unscrew the old T&P valve from the water heater, turning it counterclockwise with a large wrench. © HomeTips

6 Wind Teflon® pipe-thread tape on the threads of the new T&P valve, winding counterclockwise. Then screw the new valve into the water heater, turning it clockwise. Tighten it with a wrench, but don’t over-tighten.

Gloved hands winding pipe wrap tape onto the threads of a water heater T&P valve.
Wrap pipe-wrap tape onto the T&P valve’s threads, winding 5 or 6 times counterclockwise. © HomeTips

7 Wind pipe-thread tape onto discharge pipe’s threads and screw the discharge pipe into the new valve. Snugly tighten it with a wrench. Make sure the pressure-release lever on the valve is closed.

Gloved hand holding a wrench to screw the discharge pipe into a T&P valve, turning clockwise.
After winding the threads with pipe-wrap tape, screw the discharge pipe into the new T&P valve, turning clockwise (as seen from the pipe). © HomeTips

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8 Open one of your home’s hot water faucets. Then open the cold-water shutoff valve to the water heater. Put a bucket under the end of the T&P discharge pipe. When you hear the tank beginning to fill with water, lift the pressure-release lever on the T&P valve and watch water spill through the discharge pipe into the bucket.

When the water from the discharge pipe becomes a steady stream, close the T&P pressure-release lever. When hot water streams through the faucet that you opened in your home, turn off that faucet.

Turn the gas or electricity back on and, for a gas water heater, relight the pilot light or restart it with the electronic igniter if necessary. Give the water heater a few minutes to reheat, and then check the T&P valve again to make sure it is working and not leaking.

If everything is working properly, congratulations! You’ve handled this important improvement yourself!


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About Don Vandervort
Don Vandervort has developed his expertise for more than 30 years as a remodeler and builder, Building Editor for Sunset Books, Senior Editor at Home Magazine, author of more than 30 home improvement books, and writer of countless magazine articles. He appeared for 3 seasons on HGTV’s “The Fix,” and served as MSN’s home expert for several years. Don founded HomeTips in 1996. Read more about Don Vandervort