Is a Secondary Drain Pan Required Under an Air Handler

Yes, wherever an overflow from the coil would damage the building, but two of the four ways to meet the rule, methods 2 and 4, need no auxiliary pan. Section M1411.3.1 of the 2021 International Residential Code (IRC) requires a secondary drain or an auxiliary pan for "each cooling or evaporator coil where damage to any building components will occur," and a pan you pick has to be at least 1.5 in. deep and 3 in. larger than the unit in width and length.

For a 3-ton Daikin AMST36CU14 air handler lying horizontal in an attic, that's a pan of at least 52 x 24-1/8 in., which holds about 8 gallons at 1.5 in. The fill-time numbers below use the example coil in the free condensate calculator, 1.57 gallons per hour.

When does the IRC require a secondary drain pan?

When an overflow from the equipment pan or a stopped primary line would damage any building component. The rule is M1411.3.1 in the 2021 IRC and M1411.9.1 in the 2024 IRC, with the same wording, and it asks for one of four methods, two of which need no auxiliary pan. A manufacturer's instructions can still require the pan outright.

M1411.3.1 applies to every cooling or evaporator coil where overflow would reach something that can be damaged, whether it sits in an attic, a closet or over a finished ceiling. Manufacturers can ask for more, and M1401.1 makes their instructions part of the install.

Goodman's installation instructions for its R-32 AMST air handlers, the U1300 models, document IO-4011A from November 2023, cover horizontal units in section 9.5. "If installed above a finished living space, a secondary drain pan (as required by many building codes), must be installed under the entire unit," with its line routed where the user will see the water come out. Section 12 says the unit's own pan has a secondary outlet. Pipe it on its own and end it where you will see the discharge, if your code requires that line. It also says that for units above ceilings, or anywhere overflow may do damage, a field-made auxiliary pan under the coil cabinet is "MANDATORY." The AMST36CU14 sized below, from Daikin's SS-DAMST sheet, is an R-410A model; the R-32 AMST36CU1300 is one of the models IO-4011A covers.

A condensing furnace adds its own line. Section M1411.5 requires an auxiliary pan under a Category IV appliance where a stoppage would damage the building, unless the appliance shuts itself down when its condensate line stops.

Which of the four overflow methods fits the job?

Each method either carries the overflow somewhere you'll see it or shuts the unit down. Method 1 is a pan with its own outlet pipe, method 2 is an overflow line from the unit's own pan, method 3 is a pan with a water-level switch, and method 4 is a switch in the primary line or pan. The switches in methods 3 and 4 have to conform to UL 508.

The pipes in methods 1 and 2 follow the same section, sloped at least 1/8 in. per foot and at least 3/4 in. nominal. Those are the code minimums. IO-4011A asks for 1/4 in. per foot, so follow the unit's manual. Both end at a conspicuous point of disposal, and the code says why, "to alert occupants in the event of a stoppage of the primary drain."

What size does the auxiliary pan have to be?

At least 1.5 in. deep and at least 3 in. larger than the unit or coil in both width and length, in corrosion-resistant material. Per M1411.3.1 item 1, galvanized steel needs 0.0236 in., No. 24 gage, and nonmetallic material needs 0.0625 in. For a Daikin AMST36CU14 lying horizontal, 49 x 21-1/8 in., that's a pan of at least 52 x 24-1/8 in.

The cabinet numbers come from Daikin's SS-DAMST spec sheet, dated April 2025. The AMST36CU14 is 49 in. tall, 21-1/8 in. wide and 21 in. deep. Lying horizontal it covers 49 in. by 21 or 21-1/8 in., depending on which side is down, so the pan below uses the 21-1/8 in. side.

2021 IRC § M1411.3.1, item 1

"The pan shall have a minimum depth of 1.5 inches (38 mm), shall be not less than 3 inches (76 mm) larger than the unit or the coil dimensions in width and length and shall be constructed of corrosion-resistant material." - International Code Council, codes.iccsafe.org

DOE's Building America Solution Center tells installers that pans "should extend beyond the air handler or coil box by 3 inches or more on all sides." That's 6 in. larger in each direction, 55 x 27-1/8 in. for this cabinet. That is DOE guidance, and a pan built to it also meets the 3 in. code minimum.

Section M1411.3.1.2 puts the parts of the equipment and insulation that water would damage above the rim, on supports in the pan that are water resistant and approved.

How long does a 1.5 inch pan hold the water?

About 5 hours for that 8.15 gallon minimum pan at 1.57 gallons per hour. That rate is our condensate calculator's example coil, 1,200 CFM with air entering at 80°F dry bulb and 67°F wet bulb and leaving at 55/54. It holds at most 8.15 gallons, so divide 8.15 by your own coil's rate for its hours.

The calculator gets 1.57 gallons per hour from the moisture balance in the ASHRAE Handbook of Fundamentals psychrometrics chapter: pounds of dry air through the coil times the drop in humidity ratio. Our own check with the chapter's equations gave 0.01117 lb of water per lb of dry air entering and 0.00863 leaving, 5,199 lb of dry air an hour, and 13.2 lb of water an hour, which is 1.58 gallons. The calculator shows 13.09 lb and 1.57 gallons, within 1 percent.

So 8.15 gallons fill in about 5.2 hours at the calculator's example rate. Supports and anything else sitting in the pan take up some of that volume, so the real figure is shorter.

Fill time matters most with method 3. With method 1, the pan's own pipe carries the water to a spot where someone sees it, and the clock only matters if that pipe clogs too. With method 3 there's no pipe, so the UL 508 switch has to shut the unit down before the rim, and a pan that fills in an afternoon gives a stuck float switch no slack.

What about downflow units and coils with no secondary port?

The switch goes inside the primary pan. M1411.3.1.1 covers downflow units and any other coil without a secondary outlet or room for an auxiliary pan, and requires a water-level device inside the primary pan that shuts the equipment off when the primary line restricts. It also says "Devices shall not be installed in the drain line."

For those units it rules out the drain-line location that method 4 otherwise allows. The same wording sits in M1411.9.1.1 of the 2024 IRC.

Does an attic air handler need both a pan and a float switch?

M1411.3.1 accepts any one of the four methods for an attic unit, but the unit's manual can require the pan (M1401.1). A pan plus a switch is method 3, and a pan piped to a visible spot is method 1. An attic condensate pump also has its own shutoff rule in M1411.4. If an insurer asks for both a pan and a switch, as it did in one Reddit thread, that is the carrier's condition.

A pan with a float switch is method 3, and methods 1, 2 and 4 each satisfy M1411.3.1 on their own. In the Reddit thread, a Goodman system in a third-floor attic of a 2020 house was flagged by the poster's insurer, which would not renew without a pan and a shutoff. Ask the carrier what it wants before you pick a method.

A commenter in the thread suggested a pump wired to shut off the condenser as the method 4 device. Method 4 puts the switch above the primary connection and below the pan's overflow rim, which a pump's own switch may not meet. M1411.4 requires a pump in an attic or crawl space to stop the equipment when the pump fails, whatever overflow method the coil uses. Whether the pump's switch counts as the method 4 device is the inspector's call, so ask before you count on it.

The thread's house is in Massachusetts, and the poster pasted M1411.3.1 as the state's code. That wording matches the 2021 model IRC, but state amendments weren't checked here, so check the wording your jurisdiction adopted.

Checking the water rate with BuildSolver

Enter the airflow and the entering and leaving dry bulb and wet bulb readings in the condensate calculator. It returns gallons per hour and the 3/4 in. minimum line size, and you divide the pan's gallons by that rate for its hours. The calculator doesn't size auxiliary pans, secondary lines, traps or float switches.

1. Measure the CFM through the coil, or read it off the blower table at the measured static pressure, plus dry bulb and wet bulb in the return and the supply.
2. Enter them in the calculator and note the gallons per hour.
3. Size the pan from the cabinet dimensions in the spec sheet, 3 in. larger each way and 1.5 in. deep at minimum, and work its gallons as length x width x depth / 231.
4. Pick the method, then build the primary line to the unit's manual. The seal depths from six manufacturer manuals are in our guide to the primary line's trap.

Sources

• International Code Council, 2021 International Residential Code, Chapter 14, Sections M1401.1, M1411.3, M1411.3.1, M1411.3.1.1, M1411.3.1.2, M1411.4 and M1411.5: codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Chapter 14, Sections M1411.9.1 and M1411.9.1.1: codes.iccsafe.org
• Goodman (Daikin Comfort Technologies), AMST U1300 R-32 air handlers installation and operating instructions IO-4011A, November 2023, Horizontal Applications and Condensate Drain Lines: daikincomfort.com
• Daikin, AMST series product specifications SS-DAMST, April 2025, Dimensions: daikinac.com
• U.S. Department of Energy, Building America Solution Center, HVAC has Corrosion-Resistant Condensate Drain Pan: basc.pnnl.gov
• ASHRAE Handbook, Fundamentals, Psychrometrics chapter, humidity ratio and the moisture balance across a cooling coil: ashrae.org
• Source question, r/hvacadvice, "Does my HVAC in attic need an auxiliary drain pan?": reddit.com

For estimation purposes only. Install to the equipment manufacturer's instructions and to the code and amendments your jurisdiction has adopted.