Some white piping is actually approved for T&P valve discharge use and will specifically state this on the pipe normally in red print. PVC pipe is white and will have “PVC” stamped on the pipe’s exterior for easy identification. Almost any other type of plumbing supply pipe may be used for the T&P valve’s discharge pipe, such as: copper, PEX, CPVC, and polybutylene (PB). PVC pipe is only cold water rated and can melt or burst if used for pressurized hot water. The T&P discharge pipe must be rated for hot water PVC pipe is not hot water rated and must not be used as a T&P discharge pipe. If the inspector can't verify this, it should be written up for review by a licensed plumber to ensure the proper T&P valve is installed. Some manufacturers do make T&P valves with a longer-than-normal sensor to allow for this situation the T&P valve's tag or model number may indicate if the T&P valve with the longer sensor is installed. Sometimes, the T&P valve is sticking out more than a few inches from the water heater. The lack of a proper discharge pipe can injure (such as 2 nd or 3 rd degree burns due to scalding) or kill someone standing near the water heater should the T&P valve open. Two common flaws found during home inspections are either: (1.) No installed discharge pipe, or (2.) the T&P discharge pipe is not hot water rated (i.e., PVC). Plumbing standards require a T&P valve on all water heaters these same standards also require a proper discharge pipe connected to the T&P valve to aim this potentially dangerous hot water to a safe location.
The T&P valve’s thermostat arm extends into the top 6” of the hot water tank and is exposed to the water at the top of the unit. If there is no place for this expansion to occur, such as within a thermal expansion tank, this pressure builds within the water heater tank and something must give. An expansion of almost a gallon of water may occur in a 50 gallon water heater. The T&P valve may be more prone to open if no thermal expansion tank is installed. In normal operation of the water heater, the valve should remain closed and discharge no water, however, in some cases, the valve will open for safety reasons. Several videos on the internet (including a demonstration done on the Discovery Channel show MythBusters) exist that demonstrate this extreme situation. Water exceeding this pressure or temperature (if not discharged through the T&P valve) may allow the water heater to shoot through your home like a missile hundreds of feet up into the air. By opening, amounts of water (small or large) will discharge out of this valve. T&P valves are typically designed to open at a pressure of 150 psi and at a temperature of 210° F. This device is a temperature and pressure relief valve, more commonly known in the home inspection and plumbing industries as a “T&P Valve”. Water heaters have the ability to become a pressure vessel should their internal pressure or temperature exceed safe limits, so water heaters have a device mounted either at the top or on the side (within 6” of the top) that helps prevent an unsafe condition.
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