Water heater drain pan: when you need one and how it should be set up

Why a pan matters

A. O. Smith's installation manual is blunt about it: because of the normal corrosive action of water, the tank will eventually leak. It says to install the heater where a leak from the tank or connections won't damage the area next to it or the floors below. Where that's not possible, it calls for a metal drain pan piped to a drain, to catch condensation and leaks from the tank or piping connections.

A pan is sized for drips and slow leaks. It buys time and sends the water somewhere harmless, but it won't hold a tank's worth of water if it splits. Our guide to a water heater leaking from the bottom helps you tell condensation from a failing tank.

When the code requires one

Under section P2801.6 of the 2018 International Residential Code, a tank-type water heater installed where leakage would cause damage must sit in a pan. The code's pan rules:

• Depth: at least 1-1/2 inches, big enough to catch drips or condensation from the tank.
• Drain: an indirect waste pipe at least 3/4 inch in diameter, running full size to a suitable drain, or outside to end 6 to 24 inches above the ground.
• Gas heaters: a plastic pan under a gas-fired heater must have a flame spread index of 25 or less and a smoke-developed index of 450 or less. The International Plumbing Code goes further and doesn't allow a plastic pan under a gas heater at all.
• Replacements: where no pan drain was installed before, a replacement water heater doesn't require one.

States and towns adopt different editions with their own amendments, so the local inspector has the final word. A. O. Smith's own instructions ask for a metal pan at least 2 inches wider than the heater that limits water to 1-3/4 inches deep, and warn that the pan must not block combustion air to a gas burner.

The Massachusetts rule

Massachusetts writes its own plumbing code, 248 CMR. Section 10.12 calls them safe waste pans, and requires one under any water heater or storage tank of 6 gallons or more that sits above an occupied space. Its rules:

• At least 2 inches deep, with at least 2 inches of clearance around the base of the heater.
• A waste pipe of at least 1 inch, piped indirectly to a trapped and vented fixture, or to within 12 inches of the lowest floor.
• When a replacement makes the drain piping impractical, the state allows a pan without the drain if a product-accepted automatic water heater shutoff device is installed and the relief valve discharges into the pan.

This matters most in apartment and condo buildings. About 40% of Quincy's homes are in buildings with five or more units (see our Quincy page). In Hingham, about 16% are in buildings with 50 or more units, where the heater may belong to the condo association (see our Hingham page).

A pan with no drain

Plenty of basements have a pan with no drain line, or a heater sitting on the floor next to a floor drain. A pan without a drain still catches drips, but it fills and overflows on a steady leak. If there's nowhere to pipe it, a leak alarm or a shutoff valve that closes the cold supply when it senses water gives you warning, and in Massachusetts that combination is the state's accepted alternative for replacements.

Check the pan when you flush the tank. Water sitting in it, rust stains or a corroded pan are clues worth following up.

Adding one to an existing heater

A pan goes under the heater, so adding one later means draining and disconnecting the tank, lifting it into the pan and reconnecting it. That's why it's usually done at replacement time. If your heater is near the end of its life anyway, see how old your water heater is and our replacement page.

Local pages mentioned

• Quincy, MA
• Hingham, MA