IRC P2804 Water Heater Relief Valves: Settings and Discharge Piping

A storage water heater holds a lot of energy. If the thermostat sticks on, the tank keeps heating water it cannot use, and pressure inside climbs fast. Section P2804 of the International Residential Code keeps that from ending in a burst tank or a scald. It covers the relief valve itself and, just as much, the pipe that carries a release to a safe spot.

The two ways to relieve the tank

The code lets you protect the tank two ways: a single combination temperature-and-pressure valve, or a separate pressure valve paired with a separate temperature valve. Most new homes use the combination valve, often called a T&P valve, threaded into the top of the tank.

The settings are fixed. The pressure valve has to open at no less than 25 psi above the system pressure and at no more than 150 psi. The temperature valve has to open at no more than 210 degrees, and its sensing tip has to sit in the top 6 inches of the tank, where the water is hottest. Those two triggers cover the two ways a tank turns dangerous: too much pressure, or water hot enough to flash to steam.

Where the discharge pipe has to go

A relief valve that dumps scalding water at head height is its own hazard, so P2804 is strict about the pipe that carries the release away. The pipe drains by gravity to a safe end point and never has a shutoff or check valve in its path, because a closed valve would defeat the whole system. It ends over an air gap so nothing can siphon back:

Discharge through an air gap located in the same room as the water heater.

A few more rules round it out. The pipe is at least as wide as the valve outlet, which on a residential tank means three-quarters of an inch, and it is built from an approved material rated for hot water. It serves that one valve and nothing else. It ends no more than 6 inches above the floor or the drain it empties into, and at least two pipe-widths above that surface, so a release lands low and stays visible instead of spraying a person.

What this looks like on an Arizona build

For example, say your tank sits in the garage of a Gilbert or Surprise home. The plumber runs the T&P discharge line down the wall and ends it about 4 inches off the slab, pointed at the floor, with nothing capping it. If the valve ever lets go, you see water on the garage floor and know something is wrong.

Arizona adds two wrinkles. First, incoming water and garage air both run hot in summer, so the tank already sits closer to its limits. Second, many Valley homes have a pressure-reducing valve at the water service, which turns the plumbing into a closed system. In a closed system, heated water cannot push back toward the street, so pressure spikes land on the relief valve and it starts to weep. The fix is a thermal expansion tank, covered under water supply and pressure. A relief valve that drips is usually telling you that expansion tank is missing or waterlogged, not that the valve is bad.

Which edition applies to your build

The relief valve idea is stable across the 2018, 2021, and 2024 codes, though section numbers and small details shift between them. Arizona sets no statewide code, so each city and county adopts its own edition. Confirm which one your AHJ enforces in the building code editions by city reference before your plumber sets the tank.

The one thing to look at after move-in

A T&P line that stays dry is doing its job. A wet one is a message. Check the end of that pipe once a season for the first year, since a slow drip there is the cheapest early warning your plumbing gives you. P2804 is the detailed relief-valve rule inside the broader water heater installation requirements, which also cover the drain pan and seismic strapping. The valve and its pipe get checked during plumbing tests and inspection. Weighing a heat pump or solar tank to cut the running cost? That choice ties into whether solar is worth it on a new Arizona home. Chapter 28 of the 2021 IRC carries the relief valve text.