In 1921, Chetwood Smith had this to say in the American Water Works Association Journal, “I have seen over 35 explosions from overheating water. The force of these explosions — no one can realize who has not seen them. A 30-gallon range boiler raised to a temperature of 290° F has as much energy as 1.5 pounds of nitroglycerine when the explosion occurs, and I think very few people realize the force of the energy they are storing up.”

In the early 1900s, water heaters and boilers were exploding on a weekly basis, often taking lives and causing extensive property damage. Just one year later, Smith patented the first temperature and pressure relief valve:

At the time, water heaters were equipped with pressure-only relief valves and Smith understood the correlation between superheated water (water heated above 212°), and the end result when water under pressure was heated beyond its saturation temperature or a sudden release of pressure – boom! The instantaneous — and unstoppable — change of state from liquid to steam released enormous amounts of energy. I clearly remember a water heater explosion in 1972, in a nearby town, where the water heater traveled up through three floors and became lodged halfway through the home’s roof. The owners were asleep in bed when the water heater blew through their bedroom, catching the corner of their bed, which slammed them into the bedroom wall. Fortunately for them, being pinned against the wall by the mattress, protected them from being scalded by steam. 

Pretend we’ve turned back the hands of time to the late 1920s, and that you are Burchard E. Horne. You own the Watts Regulator Co., a relatively small company that primarily manufactures products for the power plant industry. You are the past national double-blade canoeing champion and a fierce competitor; and you’ve just met with a gentleman named Chetwood Smith. Your chief engineer, Wendell Dillion, is at your side as Smith hands you the drawings to his patented invention, a temperature and pressure relief valve. Smith has just asked you, Burchard E. Horne, to produce his patented temperature and pressure relief valve because you are the head of a small manufacturing plant whose reputation for producing the highest quality goods has brought Smith to your door. In spite of the fact that pressure relief valves have been in production for a long time, water heaters have continued to explode at inopportune moments, causing property damage and loss of life. Is Smith onto something here, or is he just another dreamer?

Water normally boils at 212°. However, when it is subjected to increased pressure, the boiling point is raised. In a sealed vessel, such as a water heater, that water can become superheated. That is, it can be stored at temperatures well above its 212° boiling point, yet remain in liquid form. If the pressure suppressing the water’s natural inclination to change state from liquid to vapor is suddenly released, an extremely violent reaction will take place as the water expands some 1,700 times in volume! The higher the temperature and corresponding levels of superheating, the more potentially violent that reaction can become due to an ever increasing number of Btu. In the early 1900s, an average of one to two sealed pressure vessels were exploding every week in the USA. Watts, under the direction of Burchard E. Horne, began production of the T&P valve.

Unique sales strategy

By 1936, George Horne had taken over the responsibility for sales of these new temperature and pressure relief valves from his father. George became the head of marketing for Watts immediately following college, and he set out across America initiating a heretofore-unprecedented growth for Watts. In 1935, the American Gas Association sponsored the Z21-22 standard for T&P valves, which recognized the Watts valve as its foundation. With this solid boost from a nationally recognized code body, George knew he had the right product at the right time. He still had to convince the suppliers and plumbers of the need to use this valve in promoting public safety. Knowing the marketplace was full of skeptics, George came up with the best sales tool anyone has ever brought to our trades. He simply blew up a water heater everywhere he went! From a comparatively safe distance, of course. Sales, instead of water heaters, went through the roof and Watts soon found itself working around the clock in order to keep up with production.

You see, up until that day, the belief had been that water heaters exploded because of excess pressure. All they needed was an adequate pressure relief valve, right? Wrong! Relieving pressure from superheated water simply converted the water heaters into steam-powered bombs. By graphically demonstrating the awesome power of steam expansion, George drove home a sales point in a way not-to-be-forgotten and sure to be discussed by those witnesses the remainder of their lives. This sales promotion was so successful that Watts then produced the film “Explosion Danger Lurks.” It is as powerful to watch today as it was in that era. It’s available through Watts via their website ( and should be mandatory viewing for anyone in the trades. Watch the video here:

Disaster conditions

There are three basic conditions that will lead to the destruction of a water heater in a catastrophic fashion: Excessive temperature; pressure; or vacuum. Of those three, excessive temperature presents the most serious hazard. The Watts T&P valve eliminated this problem — when installed correctly — by opening at 210° and/or its pressure rating. As a result, it was no longer possible to superheat water contained in these vessels. At the time of this invention, tanks were not typically provided with a separate tapping for the T&P valve in the top 6 inches of the storage tank. It became necessary to install a T&P valve with a longer than normal (by today’s standards) probe, and proper training was required so that it would be installed on the hot outlet side of the tank. Why the hot water outlet? If installed on the cold water inlet, it would be possible to superheat the tank’s water if a slight trickle existed somewhere in the system, which would bathe the probe in cooler water and mask the true temperature.

If the T&P valve has been tampered with because some lunkhead saw it dripping and seeing a threaded fitting has installed a plug, they’re gambling with their very lives. I’ve seen relief valves plugged or capped by homeowners because they did not understand the need for a thermal expansion tank. Typically, by the time we see the water heater, its flue tube has collapsed from the excessive pressure and the products of combustion are spewing into the home.

National plumbing codes state that all water heaters will be protected with a T&P valve that meets or exceeds the rated capacity of the equipment served and conforms to ANSI Z21.22, the same standard adopted in 1935! When a plugged relief valve is ignored and an overheating condition occurs, it’s a whole different ball game. 

Let’s examine what happens in a sealed system of a 40 gallon water heater with a plugged relief valve and a defective thermostat. As the temperature climbs, so does the pressure from thermal expansion and therefore, so does the critical temperature wherein water will flash to steam. At 150 psi, the critical temperature of water becomes an astounding 358.42° F. But that’s only the standard relief valve setting and this one is plugged. Water heaters are tested to failure at the factory to withstand twice the rated working pressure, to at least 300 psi. At 300 psi, you’re now dealing with a critical temperature of 429° and enough stored energy to match several pounds of high explosives! All that’s needed to set the violent chain reaction in motion for an instantaneous change from water to steam, is the release of pressure from anywhere within the system, a tank rupture or continued heating beyond the water’s saturation temperature which will produce steam and a rapid escalation of pressure beyond the bursting point. That 40 gallons will now forcefully take up 67,000 gallons of space in less than the blink of an eye!

Temperature and pressure relief valves remain one of the least expensive and most reliable safety devices available in the marketplace today. They are also largely ignored during the lifetime of the water heater. Many are installed without a drip leg or with ones using plastic piping not rated to withstand 210° temperatures. A simple test can be performed by first ensuring the water temperature is below 212° and then simply lifting the test lever to ensure the valve is not clogged.