To check if a water heater is full, first locate the pressure relief valve (PRV) near the top of the tank. Lift the metal lever on the valve to release pressure; if water flows out, the tank is full. If no water releases, the valve may be stuck or the tank is not full. Next, inspect the temperature and pressure relief valve (TPR) for damage or blockages. A full tank maintains proper pressure and temperature, ensuring efficient heating. If the tank is not full, water may exit the TPR valve during heating cycles, leading to energy waste and potential damage.

The process relies on maintaining optimal water levels to prevent internal pressure fluctuations. According to the U.S. Department of Energy (DOE), 90-95% of water heaters operate at peak efficiency when properly filled, as excess water reduces heat transfer and increases energy consumption. A 2021 study by the American Water Works Association (AWWA) found that 30% of water heater failures involve improper water levels, with overfilled tanks prone to TPR valve malfunctions. Overfilling by 1.5% above the recommended capacity can reduce energy efficiency by 10%, as noted in DOE guidelines, due to increased thermal stress and standby losses. Additionally, AWWA data indicates that 15% of residential water heater leaks stem from faulty PRVs, often caused by blocked or stuck valves when the tank is overfilled. Regular checks ensure compliance with manufacturer specifications, typically 80-85% of tank capacity, to balance safety and performance. Neglecting this step may lead to costly repairs or replacements, with the DOE estimating that 20% of U.S. households experience water heater issues annually due to maintenance oversights.