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Question - I have been considering buying an older home. The building inspector has told me that the water heater in the house that I like the best is improperly protected. Does this mean that it will blow up or otherwise create a dangerous condition? How do I find out just what is wrong with it and what do I do to correct it?

Answer - One problem that building appraisers run into is an improperly protected domestic hot-water heater. To answer your question, your concern about a water heater explosion is correct. Live steam from the explosion of a tank-type hot-water heater could cause severe damage or, if someone happened to be nearby at the time, serious injury or death.

All tank-type water heaters, whether they are electrically heated, gas fired or oil fired, must have a relief valve that is both temperature and pressure sensitive. Unfortunately, many relief valves are pressure sensitive only and this will not provide you with the needed protection against an explosion.

Should the water heater's thermostat malfunction, the water in the tank could become superheated, meaning it's heated beyond the temperature of 212 degrees Fahrenheit at atmospheric pressure. As the boiling point of water increases, its pressure increases and both combined can weaken a tank and cause it to rupture, or simply exceed its yield strength.

If this happens, the water pressure immediately drops to atmospheric pressure. The super-heated water instantly flashes into steam, increases its volume and liberates an amount of energy that can be greater than a pound of nitroglycerin. If the water had not been heated above 212 F, it would not flash into steam, and an explosion would be impossible.

Consequently, pressure- and temperature-sensitive relief valves are designed to prevent the water temperature from exceeding 210 F. It does this by discharging the overheated water and allowing cooler water to enter the tank, safely lowering the temperature.

It's also important that the BTU-capacity of the relief valve exceed the BTU input of the water heater. If the capacity is less, the relief valve will not be able to discharge the overheated water at the same rate it's produced, resulting in an unsafe condition or possible explosion. Check the capacity of the relief valve on the nameplate mounted on the valve. Specifically, look for the AGA (American Gas Association) rating given in Btu/hours. This rating must exceed the Btu input to the water heater listed on the plate on the tank casing.

Also, when the relief valve is installed, its temperature-sensing element should be immersed in the top 6 inches of the water in the tank. This is important because there is a temperature difference between the tank and the hot-water outlet pipe. For instance, when the relief valve is installed in the hot-water outlet pipe 5 inches away from the tank, the water in the tank could be raised to 250 F before the temperature at the relief valve reaches 210 F.

So, you can see that installing a correct relief valve is important, but it should not be too difficult or expensive. Once this is done, you don't have to worry about a hot-water heater tank explosion and you can decide on the basis of the home's other features. But, if you buy the house, be sure to have a new relief valve installed.