UPC Water Heater Drain Pan: When One Is Required and How It Drains

Code requires a pan where a leak would cause damage, and it requires that pan to actually drain. The requirement is stated plainly:

Where a water heater is located in an attic, in or on an attic ceiling assembly, floor-ceiling assembly or floor-subfloor assembly where damage may result from a leaking water heater, a pan shall be installed under the water heater.

Three parts follow from that. First, a pan goes under the heater, made of a corrosion-resistant material and large enough to catch drips and overflow. Second, the pan has a drain line, a pipe from the pan that runs to an approved discharge point, so a slow leak flows away instead of filling the pan and spilling over. Third, that drain must terminate at a safe, visible location, such as outside, a floor drain, or another approved spot, where escaping water will be noticed and cause no harm. The pan is not for the big burst, which the temperature and pressure relief valve and shutoff handle. It is for the slow, silent leak that would otherwise rot a ceiling for weeks unnoticed.

This governs where a heater is placed and what has to go with it. Picture a water heater installed in a second-floor closet above a living room: code requires a pan and a pan drain, because a leaking tank there would ruin the ceiling below. A heater sitting on a garage slab that drains to the exterior often does not need a pan, since a leak causes no structural damage. An inspector checks for the pan and a properly routed drain wherever the location calls for it.

If your water heater is in an attic, upstairs, or over finished space, it should have a pan with a working drain, and that drain should terminate where you would actually see water coming out. If a pan is overflowing, that is a sign of an active leak that needs attention, not just a full pan. See water heater drain pan overflow for what that means. A tank leaking from the bottom is the usual reason a pan starts filling, and replacing the heater is work for a licensed water heater installer. A pan is cheap insurance against an expensive ceiling repair.

The UPC water-heater pan rules are part of the Uniform Plumbing Code, published and copyrighted by IAPMO, and parallel the residential code's P2801.6. The excerpt above reflects the requirement as adopted; Phoenix enforces the 2024 UPC with local amendments, so verify the current pan and drain rules and section numbers. Read Chapter 5 on UpCodes, or confirm local amendments through the City of Phoenix: phoenix.gov/pdd.