A plumber is servicing a residential hot water heater and finds the temperature and pressure (T&P) relief valve weeping continuously. The system's pressure gauge reads 690 kPa (100 psi) when cold and rises to 1035 kPa (150 psi) when the water is hot. The T&P relief valve is rated for 1035 kPa. What is the most likely cause of the weeping T&P valve?

Question: A plumber is servicing a residential hot water heater and finds the temperature and pressure (T&P) relief valve weeping continuously. The system's pressure gauge reads 690 kPa (100 psi) when cold and rises to 1035 kPa (150 psi) when the water is hot. The T&P relief valve is rated for 1035 kPa. What is the most likely cause of the weeping T&P valve?

Answer options: ✅ A failed or undersized thermal expansion tank.

• The hot water heater thermostat is set too high.
• Excessive municipal water supply pressure.
• A partially clogged drain line connected to the T&P valve.

Correct answer: A failed or undersized thermal expansion tank.

Explanation: A continuous weeping T&P valve, especially with significant pressure fluctuation between cold and hot, strongly indicates an issue with thermal expansion, typically an undersized or failed expansion tank. The tank's purpose is to absorb this expansion. While high municipal pressure can contribute to overall system pressure, the significant rise from cold to hot points more directly to uncontrolled thermal expansion. A high thermostat setting contributes to expansion but the problem lies in the system's inability to accommodate that expansion. A clogged drain line would exacerbate the issue once the valve opens, but wouldn't cause the primary weeping from thermal expansion.