Water Heater Over-Temperature Protection Issues

Did you know that a water heater can explode with the same force as dynamite? Home inspectors refer to the safety device to prevent an explosion as the TPR valve or Temperature & Pressure Relief Valve. Problems with this valve or the discharge piping coming from the valve can result in injury or even death.

Take a look at the following:

What causes a water heater or hot water storage tank to explode?

Explosions result primarily from overheating or excessive temperature. Contributing physical causes are corrosion and system weaknesses. The resultant heat rupture exposes its pressurized contents to the atmosphere. This causes the superheated water to immediately flash into steam. This combination of factors creates the explosion. To illustrate what "heat rupture" means, let us suppose that a water heater operating at a normal temperature of 120°F can satisfactorily withstand water pressure of, say, 75 psi. However, the same tank, due to corrosion and other factors, when overheated may only be able to safely handle a pressure of 50 psi.

Why is it temperature and not pressure that causes a hot water explosion?

Water stores energy when it is heated. The boiling point of water rises when water is under pressure. Explosions resulting from the release of this stored energy can occur at normal operating pressures. The force and energy of the explosion derives from steam pressure resulting from the superheated water flashing into steam under the atmospheric pressure condition. It is not water pressure that causes an explosion, it is the stored energy present in heated water contained in a pressurized vessel. Tank pressure can be built up to over 500 psi in an unheated tank, if a rupture occurs under this condition nothing happens except a squirt of high pressure water.

How must a Temperature & Pressure Relief Valve and overflow piping system be installed?

Combination temperature and pressure relief valves with extension thermostats must be installed so that the temperature-sensing element is immersed in the water within the top 6" of the water heater tank. They must be installed either in the hot outlet service line or directly in a tank tapping. To avoid water damage or scalding due to valve operation, a discharge line must be installed and run to a safe place of disposal. Discharge line must be as short as possible and be the same size as the valve discharge connection throughout its entire length. Discharge line must pitch downward from the valve and terminate at least 6" above the location where a discharge will be clearly visible. The discharge line must terminate plain, not threaded. Discharge line material must conform to local plumbing codes. Excessive length over 30', or use of more than four elbows or reducing discharge line size is not allowable as it will cause a restriction and reduce the discharge capacity of the valve.