I hope your holidays were restful and relaxing.

January’s newsletter is about a safety device found on all the hot water heaters, and steam and hot water boilers found in the homes and businesses across America.  I don’t feel this safety valve receives the attention it is do and because of this many injuries and burns, which could have been avoided occur.  This is the Temperature and Pressure Relief valve which is found on both hot water and steam boilers, and hot water heaters. This device is truly a safety device installed for your protection. 

 I remember watching films at the Heating Institute in Cromwell, CT on hot water heaters installed without “TPR’s” that would shoot 300 feet into the air.  The TV show The Myth Busters also performed many of the same tests with the same results.  A hot water heater, without a Temperature and Pressure Relief Valve is a ticking bomb which can shoot vertically through the framing of a two level home like a rocket.  An improperly installed TPR can also cause significant burns to anyone coming in contact with the steam or hot water discharge the boiler or hot water heater is producing.

Temperature/Pressure Relief Valves

The following are five titles of newspaper articles.

Avon High School Hot Water Heater Explosion    It was determined that a five gallon electric hot water heater - catastrophically failed causing severe damage.

Four Hurt as Water Heater Explodes    The steam-powered tank rockets out of a Burien store and crosses over 6 lanes of traffic landing 439 feet away.

A November 1, 2007 article detailed a water heater that landed 70 feet away from its starting point and caused an estimated $20,000 worth of damage to the home from where it was launched. 

A Hot Water Heater in School Kills Seven  A malfunction of an 85 gallon hot water heater occurred during a lunch period at Star Elementary.

Explosion in Union County Pennsylvania October 22, 2007  A water heater is to blame for an explosion that destroyed an apartment.

When a water heater explodes the force generated is equal to that caused by the explosion of a stick of dynamite.  Can this kind of an explosion happen in your home?  All of the above explosions were caused by a very simple, but often overlooked, portion of a hot water heater or boiler, a failed or missing Temperature Pressure Relief valve. This safety valve releases water, and in this process relieves pressure in a boiler or hot water heater when the temperature or pressure within the unit gets too high. Over 21,000 accidents occur annually in the United States from failed TPR’s.

Temperature and pressure relief valves are included on all closed-system equipment (hot water heaters and boilers) that are designed to produce hot water. The purpose of a relief valve is to provide protection from

Temperature Pressure Release Valve

the build up of excessive temperature and/or pressure that can cause an explosion or scalding water caused by the water in the vessel turning to steam. Like any mechanical device, though, a TPR is subject to failure, especially when tampered with or improperly installed, or some times do to advanced age.  If the valve can’t operate as it is designed the water in the tank will become superheated, and surpass the boiling point, which in a contained vessel is 210 degrees.  The two main reasons that cause pressure vessels to explode are excessive water temperature, which also adds internal pressure, or a failure in the tank itself. As the temperature climbs and surpasses the boiling point the tank is placed under ever increasing atmospheric pressure which causes the risk of explosion.

A Temperature Pressure Relief valve responds to the presence of excessive water temperature (boiling water) and the related build up of atmospheric pressure which goes with it. This build up of water temperature, inside the tank, leads to the expansion of the water present, which increases atmospheric pressure. Because of this expansion of water pressure, an internal thermostat present in the TPR physically lifts a disc off its seat and allows for the discharge of the superheated water, from the tank, into the atmosphere at the exterior of the tank. The lifting of the disc allows for the draining or release of the hot boiling water that is present in the tank.  This draining of the boiling water from inside the tank also allows for the replacement of the existing water with a new cooler supply of water. This all results in the water present in the tank being cooled thus relieving the dangerous atmospheric conditions. 

Proper Discharge Pipe Conditions:

  1. A properly installed temperature and pressure relief valve should be installed within the top six inches of the hot water tank, or boiler.  This is important because as we all the top six inches of the hot water tank, or boiler.  This is important because as we all the TPR located where the greatest amount of hot water is located.

  2. The discharge pipe should be carried to a safe distance above the floor. Manufacturers and the plumbing codes prefer 6”s +/-.  Remember what condition is happening here.  The water in the tank is superheated and has turned to steam.  When the valve blows and a homeowner enters the utility room to determine what is wrong, all they will see is steam. This steam will burn skin.  So it is vital to direct this steam to where it can have the least physical impact: towards your feet.

Temperature Pressure Release Valve
  1.  The discharge pipe shall be the same size as the outlet valve.  The steam we are talking about is under pressure, by reducing the pipe size to smaller than the valve it is constricting, or allowing for the build up of pressure, in the pipe. (Valve sizes are ¾ of an inch). 

  2.  The discharge pipe shall be galvanized steel or copper.  CPVC is acceptable it some municipalities but I always read the temperature range printed by the manufacturer on  the pipe labeling.  If the high temperature does not exceed the boiling point then I personally don’t approve of it.  (If steam is coming out of it I don’t want it to melt.)

  3.  No kinks or restrictions in the pipe.

  4.  The end of the discharge pipe must be visible and not threaded.  (Threaded pipes may allow for someone to cap-off the discharge pipe, making it a bomb).

  5.  If you find leaks or corrosion at the end of the discharge pipe this is an indicator of a condition with the valve. Conditions such as the build up of pressure in the tank may exist.  More commonly it is a sign of something wrong with the valve, or the advancing age of the valve itself.  Any of these reasons should lead to the valves replacement.

In summary, a TPR is a simple and easy fix or installation, but one that can have life saving implications.

To learn more about plumbing, visit our website's Anatomy of a Home section

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