A water heater tank should be installed inside a pan in locations in a dwelling where a leak from a conventional hot water tank could cause damage to the structure or property. The pan is intended to catch water leaks from the tank or associated connections, or condensate from the tank.
The pan should be galvanized steel or other listed material for that use, with a minimum thickness of 24-gauge (0.016 inch) (0.4 mm). Prefabricated aluminum and plastic pans are common and widely used. Aluminum and plastic pans may not be allowed by the authority having jurisdiction (AHJ) or code official, because they are not galvanized steel. Some tank manufacturers require the use of a metal pan only.
A relief-valve pipe terminating into a water leak catch pan is not permitted, because the pan is not an indirect waste receptor. Most pans have only a ¾ inch-diameter (19 mm) drain outlet, which is not capable of gravity draining the pressurized discharge of the relief valve at full flow.
The pan should not be less than 1.5 inches (38 mm) deep. The pan should be of sufficient size and shape to catch all dripping water or condensate leaks. The pan should be drained by an indirect waste pipe having a minimum diameter of ¾ inch (19 mm). The pan drain must not be reduced in size over its entire length, because a reduction will act as a restriction and will impede the discharge.
The pan must not connect directly to the drainage system. The pan should terminate over a suitably located indirect waste receptor or floor drain or extend to the exterior. An air gap must be provided to prevent backflow when the pan drain terminates into an indirect waster receptor or a floor drain.
When the pan terminates to the exterior of the dwelling, it should terminate at least 6 inches (152 mm) and at most 24 inches (610 mm) above the adjacent ground surface. This makes the pan low enough not to be a nuisance and high enough to prevent the pan drain from becoming blocked by vegetation, snow, and ice.
“How to Properly Inspect Hot Water Tanks” – An online training video for property inspectors. https://www.nachi.org/advancedcourses.htm