Question: I just installed a new water heater and the T&P (temperature and pressure relief) valve is releasing hot water. What could be causing this? Answer: The temperature and pressure relief valve is a safety device installed on a water heater. It is designed to open if the temperature or pressure inside the tank increases beyond the design parameters of the T&P valve. There are several conditions, which might cause the valve to open. If the thermostats are set at their maximum upper limit, the water could be so hot the T&P valve opens. This is a very dangerous situation and the thermostats should be adjusted to a lower setting immediately. Some applications like dairy barns, however, do require hotter water. If the T&P on a heater used in a dairy barn drips or weeps, making certain the top thermostat is set 10 to 15 degrees lower than the bottom thermostat can help control the condition. This enables the bottom thermostat to accurately control the final water temperature and prevents the heater from overheating the water at the top of the tank. Another condition that can cause the T&P to open is high pressure inside the plumbing system. This condition is usually associated with thermal expansion. Remembering what we learned in basic physics, water expands when it is heated. To realize how much it expands, consider this: When an MR85245 Marathon is filled with water at 50 degrees F, it takes 85 gallons to fill the tank. As the water is heated to 120 degrees F, the volume of the water increases by a full gallon. So what happens inside the plumbing system? The pressure inside the tank and the plumbing lines increases dramatically. The pressure may increase to the point that water is pushed beyond the faucet seals, causing them to drip. If there is not a back flow prevention valve on the water system, the expanding water will move back into the well tank, or back into the water main on a municipal or rural water system. However, many plumbing systems require backflow prevention (check valves), making them "closed" system. If a home does have a check valve on the water supply and pressure inside the plumbing system increases, the T&P valve is doing its job if it opens. It is preventing a dangerous condition. If a water heater is installed in a "closed" system, an expansion tank should be installed on the incoming cold supply line to compensate for the thermal expansion. Factors which might point to thermal expansion as the cause of the T&P releasing: The home has a "closed" system. (A backflow/check valve is installed.) A new heater is installed of larger volume than the old heater. The T&P only releases at certain times of the day, after a large draw of hot water.