All water heaters must have a relief valve that is both temperature- and pressure-sensitive. A relief valve that responds to pressure only will not offer adequate protection for the homeowner. An excessively high pressure alone can cause the tank to rupture. Though this will cause water damage, it is not necessarily dangerous. However, if the tank should rupture as a result of a high-temperature failure, the superheated water contained therein will flash into steam, instantaneously releasing high energy in an explosion. A relief valve that is sensitive to both temperature and pressure can avoid this problem.
The relief valve should have an extension that will allow it to discharge into a bucket on the floor. Sometimes, a relief valve is found without this extension. (See FIG. 16-2.) This is a potential hazard because a person standing near the water heater when the relief valve discharges could be scalded. I have also found an extension on the relief valve that leads to a sink or floor drain located near the water heater or in the next room. (See FIG. 16-3.) This type of piping arrangement is usually installed to eliminate water damage if the relief valve discharges. Water discharging from the valve will flow directly down the drain rather than accumulate on the floor. The disadvantage of this installation is that the homeowner might never know a discharge condition had occurred. A discharging relief valve indicates an abnormal condition of some type that should be checked out. Obviously, if the homeowner does not know that there is a problem, he or she will not have it corrected. By having the relief valve discharge into a bucket, water damage can be eliminated, and the presence of a problem condition will be indicated.
Fig. 16-3. The relief valve has an extension that leads to a sink in the adjacent room. This is undesirable because the effluent will go down the drain, and the homeowner will not be aware of a discharge condition.
If you notice that the relief valve is dripping and the temperature of the hot water is not excessive, the problem will be either a defective relief valve or the result of thermal expansion. Many municipalities require a backflow preventer or a nonbypass pressure regulator on the inlet water service. This creates a closed system when a faucet is not drawing water. When the water is heated and expands, the resulting pressure buildup causes the relief valve to drip. Installing an expansion tank on the cold-water inlet to the water heater can control this condition. Record a dripping relief valve on your worksheet for later correction.
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 that it’s produced, resulting in an unsafe condition or possible explosion. Check the capacity of the relief valve by looking at the nameplate mounted on the valve. Specifically look for the AGA (American Gas Association) rating given in Btu/hour (see FIG. 16-4). This rating must exceed the Btu input to the water heater listed on the data plate mounted on the side or top of the tank casing.
To be effective, the relief valve should be located so that its sensing element is immersed in the top 6 inches of water in the tank rather than in the hot-water outlet pipe. 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 valve reaches 210° F.
During my inspections, I have often found the relief valve located on the cold-water inlet pipe. This location negates the effectiveness of the valve. If you find a relief valve on the cold-water inlet pipe, you should notify the homeowner of the potential hazard and have it relocated after you take possession of the house.
It is important that the diameter of the relief-valve extension pipe be the same as the outlet opening of the relief valve. If the extension pipe has a smaller diameter, it reduces the discharge capability of the valve and reduces its effectiveness as a safety device.