New water heater, no relief-valve drain line: why that fails code

The temperature and pressure relief valve, or T&P, is the last safeguard on a tank heater. IAPMO, which publishes the Uniform Plumbing Code, describes three layers: the thermostat, a high-limit switch that shuts off the fuel or power, and the T&P valve, which opens if both fail. The UPC requires every T&P valve to have a drain. A healthy system should never discharge through the valve, the International Code Council says, so any flow signals trouble.

Under IRC Section P2804.6.1 and IPC Section 504.6, as the ICC summarizes them, the discharge line must be at least as large as the valve outlet, drain by gravity with no traps or sags, and end through an air gap in the same room, no more than 6 inches above the floor or a receptor, in plain view. No threaded end, no valves, no tees. Pipe must be rated for 100 psi at 180°F. The model codes let the line end at the floor, the heater's pan, a receptor or outdoors; Washington state bans discharge into a pan. The manual from State Water Heaters says never to cap or plug the valve.

The pan depends on location. Codes require one where a tank leak would cause damage; a permit handout from Richmond Heights, Ohio, lists floors, subfloors, drywall and platforms and calls for a drain of at least 3/4 inch. State's manual advises against attic and upper-floor installs and, where they can't be avoided, calls for a pan no deeper than 1.5 inches. A tank on a bare slab by a floor drain may need none.

The expansion tank depends on the plumbing. UPC Section 608.3 requires one, or an equivalent device, on any system closed off by a check valve, a backflow device or anything similar that keeps pressure from bleeding back into the main. The IRC also requires a pressure-control device there, and State's manual says the T&P valve is not meant to relieve thermal expansion over and over. Richmond Heights makes every home install a pressure regulator, so every new water heater there gets an expansion tank, and a permit.