Water Heaters and Temperature Control
Water heater installation
Water heaters must be listed and installed per the manufacturer, with clearance to combustibles and access for service. A temperature and pressure relief valve is mandatory, sized to the heater's rating, and installed in the tank's designated opening. The discharge pipe must be full size, gravity-drained, terminate not more than 6 inches above the floor or outside, and must not be threaded, trapped, or reduced. A drain pan is required where leakage would cause damage; it must be at least 1 1/2 inches deep and drained to an approved location.
Temperature control
Hot water at fixtures used for bathing and washing must be limited to prevent scalding. The code generally limits domestic hot water at fixture fittings to 120 degrees F, with stricter limits for facilities serving children, the elderly, or persons with disabilities. Tempering valves, mixing valves, and thermostatic controls must be listed and set so that the delivered temperature is controlled. A thermal expansion tank is required on a closed system where thermal expansion cannot be relieved.
Worked example
A 50-gallon heater has a 3/4-inch relief valve, but the installer runs a 1/2-inch discharge pipe to the floor. The reduction is a violation: the discharge must remain full size. The correct fix is to replace the pipe with 3/4-inch material and terminate it 6 inches above the floor.
Traps
Do not cap, plug, or thread the relief discharge; do not run it through a freeze-prone exterior wall without protection; do not omit the pan under a heater installed above a ceiling. Do not assume a tank's factory setting is safe at every fixture.