Discharge Tube for the Hot Water Heater Is Leaking Water

T&P Valve Operation

• A water heater temperature and pressure relief valve is a safety device to ensure that temperature and pressure extremes inside the heater caused by a malfunction do not cause an explosion. Most residential T&P valves are engineered to open when either the internal pressure reaches 150 pounds per square inch gauge (psig) or the water temperature reaches 210 degrees Fahrenheit. At that point, the spring-loaded valve opens and releases water.

Why It May Be Dangerous

• When water continuously flows from the discharge tube, the safest course of action is to assume that it is the result of excessive pressure or heat in the water heater and call a plumber at once. The most hazardous malfunction would be either a thermostat that has been inadvertently turned to the maximum heat setting or a defective thermostat that is allowing the burner to run continuously and overheating water in the tank. Because of potentially dangerous temperatures and high pressure, diagnosis and repair should be left to a professional plumber.

Minor Issues

• Routine water heater maintenance calls for the temperature and pressure relief valve to be manually operated annually by lifting the spring-loaded lever that opens the valve, allowing a small quantity of water to flow through the discharge tube, then releasing the lever to close the valve. The purpose of this procedure is to ensure that the valve operates freely. Occasionally, accumulated mineral deposits prevent the valve from sealing properly after it is manually operated. The result is a dribbling T&P valve. Minor leakage due to improper sealing calls for replacement of the valve by a qualified plumber. Valves are not expensive and replacement is normally uncomplicated and quick for a plumbing professional.

Thermal Expansion

• A plumber may determine that leakage from a temperature and pressure valve is not a malfunction of the valve or heater. When a typical 40-gallon tank of water is heated to 120 degrees, the volume of water expands by approximately 1/2 gallon. Assuming all faucets are closed, water can’t expand into the hot water outlet pipe. Therefore, thermal expansion must push backward out of the tank into the cold water pipe from the municipal water line. If a pressure regulator valve is installed at the meter, however, a one-way check valve incorporated in the meter obstructs backward water flow out of the heater. With no place to go, thermal expansion in the tank increases pressure until the T&P valve opens. In this scenario, your plumber can install a small expansion tank in the line to accommodate the half-gallon thermal expansion without causing the valve to actuate.