AC drain pans: what they do and how they fail

What a drain pan does

In your air conditioner, the evaporator coil is the cold indoor coil that lowers the temperature of the air passing through it.

The Building America Solution Center, a U.S. Department of Energy (DOE) resource with content by Pacific Northwest National Laboratory, describes the process. As air passes through the cold coil, moisture condenses out and falls as liquid into a collector positioned under the coil. Manufacturers call that collector the primary condensate drain pan, or just the condensate pan.

The same guide says the pan's job is to thoroughly collect the water and direct it to a condensate drain. The U.S. Environmental Protection Agency (EPA) uses another common name for the part: drip pans.

The pan only works if it keeps emptying. The DOE guide says that as the pan fills, it must continually drain to prevent it from overflowing and damaging the equipment and its surroundings.

The primary pan and the auxiliary pan

The primary pan sits under the indoor cooling coil. Some homes also have an auxiliary pan, also called a secondary or emergency pan. It is a separate, larger tray set under the whole unit.

One manufacturer's air handler instructions say an auxiliary drain pan under the entire unit is required when the unit is installed in or above a living space.

The auxiliary pan is a backup, so it needs a way to warn you. The manufacturer's instructions say a separate drain line should run from the pan to a conspicuous point and serve as an alarm. Water dripping from that line means the primary drain is restricted. The DOE guide notes that some installers run this line out over a window for attic systems, so the drip is easy to notice.

The other option is a float switch, which is a water-level sensor that shuts off the equipment before the pan overflows. The DOE guide says the primary drain line should also have one. It adds that an alarm on the switch is not required by codes but can be added.

The DOE guide says auxiliary pans should extend beyond the air handler or coil box by 3 inches or more on all sides. A North Carolina State Fire Marshal interpretation of Section 307.2.3(1) of that state's mechanical code says the pan must be sized three inches larger than the unit in overall width and length; the code does not require three inches of clearance on each side. For a furnace and coil fastened together in a horizontal layout, that office said the pan goes under both. We could not retrieve Florida's own code text, so ask your contractor what your local building department requires.

How drain pans fail

The DOE guide says to check pans for stagnant water, blocked drain lines, overflowing pans, or corroded and leaking pans.

Standing water comes first. The DOE guide says poorly draining pans can collect stagnant water that can become a breeding ground for algae and bacteria. The EPA tells homeowners to keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly. The EPA also says to remove standing water under cooling coils by making sure drain pans slope toward the drain.

The DOE guide says older systems used steel pans, while today's major equipment makers use plastic to build smooth-surfaced, rust-proof pans. Many older units use galvanized steel pans under the coil, and the guide says these should be checked for pitting and corrosion. It also says metal drain pans can corrode over time, especially in humid environments.

The DOE guide warns that clogged drains and overflowing pans in attics, basements and closets can go unnoticed long enough to cause severe water damage in ceilings, floors and walls. For the full drain path and why it clogs, see AC leaking water.

Rusted-through pan or clogged drain?

The DOE guide says leakage under a pan could be a clogged drain line making the pan overflow, or a pan that has rusted through.

The difference matters. The same guide says a blocked drain can be unclogged and flushed out. It also says mistaking a rusted-out pan for a simple blocked drain can be costly.

The DOE guide says replacing the condensate pan often requires a total coil replacement. That makes it a job for a licensed contractor.

Keeping pans working in a Florida home

A UF/IFAS Extension publication puts it plainly: Florida is warm and humid. The DOE guide links metal pan corrosion to humid environments.

ENERGY STAR, the federal efficiency program, says a typical contractor maintenance check-up should include checking and inspecting the condensate drain (when in cooling mode). It warns that a plugged drain can cause water damage in the house and affect indoor humidity levels.

The manufacturer's instructions say to inspect the cooling coil, drain pan and condensate drain at the beginning of each cooling season for cleanliness. The EPA says to ask your service provider to clean cooling coils and drain pans during a maintenance visit. It describes yearly heating and cooling maintenance as including regular cleaning of drain pans and coils.

The primary pan sits under the indoor cooling coil. The manufacturer's instructions warn to disconnect all electrical power to the unit before any maintenance or service. They also say the work requires certain mechanical skills and tools. That puts opening the cabinet in a licensed contractor's hands.

What you can do is simpler. Change the air filter on schedule. If water drips from the auxiliary drain line, call for service.

For where indoor units sit and why that changes the risk, see air handler location. For a full seasonal list, see the Florida AC maintenance checklist.