Relief Valve

Potable water boilers, water heaters and storage tanks are limited to a maximum temperature of 210°F. Most of them supply water in the range of 120 to 140°F. Therefore they can make use of pressure-temperature type relief valves for overpressure protection.

The pressure-temperature relief valve provides an extra level of safety in helping to eliminate the steam explosion hazard. Temperature controls (limit switches, gas valves, heating element thermostats, etc.) need to be regularly checked and properly maintained.
But pressure-temperature relief valves can back up temperature limit controls in the event of control malfunction, runaway firing conditions, corrosion and pressure transients.

Regardless of the safety valve type, they need to be installed correctly and tested annually. With a little bit of attention to this equipment, we can all look forward to an uneventful and pleasurable hot shower- especially critical for those of us who need reviving some mornings!

How can I tell if my water heater is at risk of exploding?

As we mentioned, most water heaters operate safely 24 hours a day, 365 days a year. Here are some simple guidelines and warnings for water heater safety:

If it's been more than a year since you Temperature - Pressure relief valve has been tested you should test it. T & P valves are mechanical devices which means that they can fail or clog.

This device is your last line of defense in case of a thermostat failure resulting in overheating of your tank.

It does not matter how old your water heater is. The tank in our video was purchased new just for this demonstration.

Sputtering and spewing from your faucets as you turn them on could be a sign of your water heater overheating. This is especially true if the water heater has been sitting idle for a while.
The spewing/sputtering can occur on either the hot or cold side of your faucet.

Excessively hot water is a sign of a thermostat malfunction, a precursor to excessive pressure build up.

A dripping or trickling T&P valve can mean that your water heater is overheating or over pressure.

What made the water heater explode in the video?

We used a new, 12 gallon water heater (typically, water heaters in homes are at about 4 times this size)

For heating, we used a standard, 4500 watt heating element.
We began with 0 (zero) pounds of pressure.
Could also have used a Natural Gas water heater . It does not matter what heats the water; gas, L.P. gas, electric or oil.

Typical homes have between 40 and 80 pounds of pressure.
As water heats, it expands which results in a pressure increase.

In our demonstration, it took about 1 1/2 hours to rise from 0 to 410 psi which is the pressure at which the tank exploded.
The water temperature was about 400 degrees.

When the tank ruptured, the water erupted into a steam explosion.

These are the same conditions that could exist if your thermostat became stuck "on" and your T&P was faulty.

Is there a bomb in your home?

Because of built in safety devices most water heaters safely operate day in, day out without any major problems. But don't let the excellent safety record of water heaters lull you into forgetting about the explosive potential of these marvels of convenience. When a water heater explodes, it releases a tremendous blast force which can easily demolish a building.

Click here to watch the video of the water heater blast.

Water heater blows house apart! November 7, 2007
KANNAPOLIS, N.C. -- A house was left in pieces Thursday after a hot water heater exploded inside. Firefighters say the explosion scattered debris hundreds of feet from the site.

The entire bottom left corner of the building is just gone. Firefighters say there were no flames from the explosion because it was all caused by water.
It's hard to imagine that a foundation, a wall, and an entire house were all destroyed by a three foot tall electric hot water heater.
"It sounded like a bomb went off. It didn't sound like a gun or anything like that, it was just a big explosion that went off," said neighbor Kevin Reynolds.

Cinder blocks shot straight across a neighbor's back yard and into her back window, her glass patio table and the window of her truck. And the sound echoed for more than a mile.
"I mean it blew stuff all the way across the street. There's people a mile and a half two miles away that heard this," said Reynolds.

Firefighters say the home's hot water heater was switched on a few days ago, but the unit was so old it did not have a pressure release valve. Investigators believe a broken thermostat caused the heater to remain on for at least two straight days until the pressure became too much to contain and it shot through the roof of the house like a rocket.

"The hot water heater ended up like 200 feet from the house where it went through the roof," sid Reynolds.
Firefighters say most hot water heaters now have pressure release valves built in.
"It'll be on the side, usually up toward the top," says District Chief Randall Faggart with the Kannapolis Fire Department.
Firefighters say that became standard more than 15 years ago after this kind of explosion happened in other homes across the country.
If you want to check for your pressure release valve, firefighters recommend that you not touch it because it may leak.
As for this house, the owner says it's a total loss and will have to be torn down in the next 48 hours.