Quick answer
The best condensate drain pan for an air handler or furnace coil in an attic or closet is a secondary pan at least 1.5 in deep and 3 in larger than the cabinet in width and length, the sizing in the model codes, piped to its own drain or fitted with a float switch that shuts the AC off. For a square upflow cabinet up to about 23 in across, buy the DiversiTech 6-2424L A/C Secondary Condensate Drain Pan, rated 4.4 stars across 772 Amazon ratings. A horizontal attic unit needs a long pan such as the DiversiTech 6-2748L.
Buyer's Guide
What to know before you buy.
Primary Pan, Secondary Pan, and Why the Second One Exists
Every air conditioner coil makes water. A primary pan inside the cabinet catches it, and a primary drain line carries it away. That line clogs with algae, dust and sludge over time, and the primary pan can crack or rust through. When either happens in a basement, the water reaches a concrete floor. When it happens in an attic or a closet above finished rooms, it soaks the ceiling. A secondary pan, also called an auxiliary or emergency pan, sits under the whole unit and catches that overflow. EPA's moisture guidance advises keeping air conditioning drip pans clean and the drain lines unobstructed and flowing properly, because a working drain is still the first defense and the secondary pan is only the backup.
Sizing: The 3 Inch Rule, Worked Out
The model codes many US areas adopt (IRC M1411.3.1 for homes, IMC 307.2.3 in the mechanical code) call for a secondary pan at least 1.5 in deep and at least 3 in larger than the unit or coil in width and length. North Carolina's fire marshal has read that as 3 in larger overall, not 3 in on every side. The math is simple. Measure the cabinet's width and depth, add 3 in to each, and buy a pan at least that size. A cabinet 21 x 22 in needs a pan of at least 24 x 25 in. A horizontal unit 21 x 55 in needs at least 24 x 58 in. Measure the pan's usable inside size, since a rolled rim takes up room, and check your local amendments, which can differ.
Drain Line, Float Switch, or Both
A secondary pan has to tell you it caught water, or the leak just moves upstream. The codes give two main routes. One is a separate drain line from the pan to a conspicuous spot, often above a window or soffit, where dripping water warns you the primary drain is blocked. The other is a water-level switch, usually a float, that shuts the unit off before the pan overflows; a pan with no drain line needs one. In an attic, fitting both gives two layers of warning. None of the picks here include a float switch, so buy one made for your pan type and have it wired to cut the cooling call, then test it by lifting the float with the system running.
Plastic or Galvanized Steel
Plastic pans, like the two DiversiTech models, are molded in one piece, so there are no seams to leak and nothing to rust. They are light and easy to slide under a unit. The trade-off is brittleness: buyers of both report cracks, and a plastic pan can split if someone steps in it during attic work. Galvanized steel pans, like F&L's, are rigid and come in many more sizes, but their corners are folded and sealed, and buyers report pinholes there. Steel can also corrode where water sits for long periods. Whichever you choose, fill it with water before installing it. If the pan will sit under a gas furnace, check the furnace maker's clearance instructions before choosing plastic.
Water in the Secondary Pan Means the Primary Drain Failed
Treat any water in the secondary pan as a warning, not a chore. It means the primary drain is blocked or the primary pan is leaking, and next time the backup may not be enough. Turn the cooling off at the thermostat. Clear the primary line, often with a wet/dry vacuum at the outdoor end, and check that the air filter is clean, since a clogged filter can freeze the coil and flood the pan when it thaws. Dry the pan and watch it for a day of normal running. If it fills again, the float switch keeps tripping, or a ceiling below shows a stain, call an HVAC technician, and dry any wet drywall within 24 to 48 hours, as EPA advises, to prevent mold.
Who Does Not Need a Secondary Pan
The codes ask for a secondary pan where overflow would damage the building. A furnace and coil sitting on a basement slab next to a floor drain may not need one, and an outdoor package unit drains outside anyway. Check with your local building department if you are unsure. A portable air conditioner needs only a floor tray, such as the VANPET, and a window unit drains outside. If the problem is that condensate has nowhere to drain by gravity, a pan does not solve it; a condensate pump does. And if water is already coming through a ceiling now, a new pan is the second step: switch the AC off, catch the water, and get the drain cleared first.
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