T and P discharge piping must direct relief valve flow safely away from occupants and finished spaces.

Follow the thumb rule that the discharge pipe should be the same nominal diameter as the temperature and pressure relief valve outlet and run downward with a continuous slope to either within six inches of the floor or to an approved drain, never terminate higher than the valve, and never be trapped or reduced in size. Use rigid or listed flexible piping rated for the system temperature and pressure and avoid copper sweating or torching on the valve during installation because heat can damage the valve seat and gasket. Leave the valve and the first few inches of pipe accessible for manual testing and visual inspection, and secure the pipe at close intervals to prevent movement or strain on the valve outlet. If routing through framing leave adequate clearance and protect the pipe with plates where it passes near studs to avoid future nail or screw damage. Install a drip pan and route the pan drain to an approved location if the heater is above finished space and tie the pan drain into the same approved drain when local code allows. Test the installation by operating the valve lever to confirm clear, unobstructed flow and that the pipe directs water to the intended safe discharge point.

Three common mistakes are reducing pipe diameter which can restrict flow and cause pressure buildup, routing the discharge to a finished area where hot scalding water can injure occupants, and soldering or torching near the valve which can soften or ruin internal seals. If the pipe is undersized replace it with the same nominal diameter as the valve outlet to restore correct flow. If the discharge ends in a finished space reroute the pipe to within six inches of the floor or to an approved drain and add a drip pan when needed. If torching was used on or near the valve replace the valve and the adjacent piping and avoid open flame by using mechanical fittings or a safe work method.

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