Water Heater Drain Pan: Sizing and Installation

Is a water heater drain pan necessary?

Often it's required, and almost always it's a good idea. A.O. Smith's manual says most codes require, and the company recommends, installing the heater in a metal drain pan piped to an adequate drain. Boise, Idaho, as one example, requires a pan on any floor surface other than concrete.

Rules vary by town and by where the heater sits, so check with your local building department. Attic and upper-floor installs are where inspectors care most, because that's where a leak goes through the ceiling.

What a pan won't do: it won't save you from a tank that ruptures. It catches drips and slow leaks. Its real value is that you see the water before the subfloor does.

Sizing a drain pan

Two numbers matter: diameter and depth.

Diameter. Measure the water heater at its widest point, usually the jacket. A.O. Smith says the pan must be at least two inches wider than the heater's diameter. Boise's guide says never less than one inch of clearance between the heater and the pan all the way around. Pans are sold by inside diameter, so a heater that measures just under a stock size may need the next size up.

Depth. A.O. Smith says to install the pan so the water level would be limited to a maximum depth of 1-3/4 inches. In practice that means the drain fitting sits low in the pan wall.

Airflow on gas heaters. A.O. Smith says the pan must not restrict air flow to the burner. On many gas heaters, air enters through the bottom, so don't choose a pan so tall or tight that it chokes the intake.

Materials: metal or plastic

A.O. Smith specifies a metal pan for its atmospheric gas heaters. Aluminum and galvanized steel are common. Plastic pans are cheaper and don't rust, and they show up under electric heaters and tankless units, but check your heater's manual and local code before you use one, especially near a burner.

Where the drain pan drains to

The pan needs a drain line, run to a place you can see or that can take the water. Boise lists an approved drain, the garage floor, or the outside of the building, with the pipe sloping away from the pan. Other common options are a floor drain or a laundry standpipe with an air gap, where local code allows.

Things to get right:

• Slope the line downhill the whole way. Water won't climb.
• Don't connect it straight into a sewer line without the trap and air gap your code calls for.
• If it runs outside, end it where it won't freeze or turn a walkway into ice.

Can the T&P relief valve drain into the pan?

It depends where you live. The International Plumbing Code allows the relief valve discharge to end in the water heater's drain pan, among other options. Boise forbids it. Ask your building department. Either way, the IPC has the relief line end no more than 6 inches above the floor or the rim of the receptor it drains into. More on that on our water heater T&P relief valve page.

How to install a drain pan under an existing water heater

Tools and parts

• A pan sized from your measurement.
• PVC or CPVC pipe and fittings sized to the pan's drain outlet, plus primer and cement.
• A garden hose for draining the tank.
• Pipe wrenches or a tubing cutter, depending on your connections.
• A helper. Even an empty tank is awkward.

Steps

This is the job most people hand off. It's easy to add a pan during a water heater replacement, when the old tank is out anyway.

Tankless water heater pans

Tankless heaters hang on the wall, so a floor pan doesn't work. Wall-mounted pans that sit under the unit exist, and they catch drips from fittings and the pressure relief valve. Condensing tankless units also make acidic condensate that needs its own line to a drain, which is a separate requirement in the manual. Follow your tankless manual for both.

Water in the drain pan: normal or not?

A little water can be normal. A.O. Smith says the pan is there to catch condensation, which can form on a cold tank, especially when the heater starts up with cold incoming water. That moisture should dry up once the tank is hot.

Steady water is a leak. Wipe the pan dry and check back in an hour. Common sources:

• The T&P valve or its discharge line dripping.
• A fitting on top of the tank dripping down the side.
• The drain valve at the bottom.
• The tank itself, which means it's time to replace the heater.

A.O. Smith says to inspect the pan, pipes and the area around the heater regularly and fix any leaks you find. If you have water in the pan and no hot water, that's usually a tank failure or a leak that put out the burner. Start with water heater leaking and leaking from the bottom.