Condensate Drain Code and Trap Field Card
Why this matters
A cooling coil makes water every hour it runs, and on a humid day a 3-ton system can put a few gallons into the pan. The drain that carries it away is the cheapest part of the install and the source of some of the most expensive callbacks in residential HVAC: a stained ceiling under an attic air handler, a pan that spits water past a coil because the trap is shallower than the cabinet's suction, a line that drops slope halfway along a truss and holds a belly of algae. This card owns the code rules and trap depth for the shelf. The step-by-step builds are in the sibling HowTos it lists at the end.
Which code, and which edition
Condensate disposal is in Section 307 of the International Mechanical Code and, for one- and two-family dwellings built under the residential code, in Section M1411.3 of the International Residential Code, which carries substantially the same rules. Both reach a job only as adopted by your jurisdiction, often with local amendments. The section numbers below are from the 2021 IMC; the 2024 edition keeps the same structure, but check the edition your building department enforces before you quote a number to an inspector.
The rules, in the order the water meets them
Disposal (IMC 307.2.1). Condensate goes to an approved place of disposal and may not be discharged where it creates a nuisance, such as onto a walkway, a driveway or a neighbor's lot. What counts as approved is the local official's call: an indirect connection to a drain, a lavatory tailpiece fitting made for the purpose, a downspout in some places, grade away from the foundation in others. Some jurisdictions ban discharge to grade near a foundation or into a sanitary drain without an air gap. Ask once and write the answer in the shop's job notes for that town.
Size and slope (IMC 307.2.2). The drain is not less than 3/4 inch internal diameter and does not shrink from the pan connection to the point of disposal. Where several units manifold into one line, the combined line is sized from Table 307.2.2 by total tonnage. Horizontal runs are installed at a uniform slope of not less than 1/8 inch per foot (one percent). For a single residential system, 3/4 inch at 1/8 inch per foot is the floor; more slope is better wherever it can be had, because a line laid at exactly the minimum develops low spots as framing moves.
Traps (IMC 307.2.4). Condensate drains are trapped as the equipment or appliance manufacturer requires. The code does not set a trap depth; the maker does. That is why trap depth below is tied to the coil's static pressure and the maker's instructions, not to a fixed number.
Ductless units (IMC 307.2.4.1). Mini-split equipment that produces condensate gets an in-line check valve or a trap in its drain line, per the 2021 text.
Maintenance access (IMC 307.2.5). The line is configured or fitted with a cleanout so it can be cleared without being cut. A capped tee at the trap is the usual answer.
Auxiliary protection (IMC 307.2.3). Where overflow from the primary drain could damage building components, one of four methods is required for each coil or condensate-producing appliance:
- An auxiliary drain pan under the coil or unit, with its own drain line, discharging where occupants will notice it. The pan is not less than 1-1/2 inches deep and not less than 3 inches larger than the unit or coil in both width and length, and made of corrosion-resistant material.
- A separate overflow drain line from the secondary connection on the equipment's own pan, connected higher than the primary connection, discharging where occupants will notice it.
- An auxiliary pan as in item 1 without its own drain line, fitted with a water-level detection device that shuts the equipment off before the pan overflows.
- A water-level detection device in the primary drain line, the overflow drain line, or the equipment pan, located above the primary drain connection and below the pan's overflow rim, that shuts the equipment off.
Fuel-fired appliances that shut themselves down automatically when their condensate drain stops are excepted. Coils with no secondary outlet and no room for an auxiliary pan, such as many downflow installations, get a water-level device inside the primary pan under IMC 307.2.3.1.
"Where damage could occur" is the trigger, and it is the attic air handler over a finished ceiling, the closet air handler on a wood floor, the furnace and coil above a finished basement. A unit on a concrete basement floor beside a floor drain may not need one under the rule, though most shops fit a float switch anyway.
Trap depth on a draw-through coil
On most residential air handlers the blower is downstream of the coil and pulls air through it. That puts the coil compartment, and the drain pan, under negative static pressure. The drain line is then a straw: without a trap seal deeper than that negative pressure, air is drawn up the drain into the cabinet, and the water cannot leave the pan. It sits there, or gets blown off the pan edge into the duct or the insulation.
The trap works by holding a column of water taller than the suction can lift. A cabinet at -0.6 in w.c. can lift a 0.6 inch column of water in the trap's inlet leg. So the inlet side of the trap needs a seal depth greater than the most negative static the compartment will see, and the outlet side sits lower than the inlet so water spills out before it is pulled back. Makers publish their own trap dimensions, usually as a seal depth and an outlet drop tied to the cabinet static; use them.
Three things push the real negative static above the commissioning number: a filter loading up, a coil fouling, and a return that has been undersized from day one. Size the trap against the worst static the system will plausibly run at between filter changes, not the reading on a clean filter. And prime it. A new trap is dry, and a dry trap is an open straw until the coil makes enough water to fill it.
On a blow-through arrangement (a furnace with the coil on top, the blower pushing air up through it), the pan is under positive pressure. Air pushes out through the drain rather than in, and the trap's job is mostly to stop conditioned air and odor escaping. Follow the maker, but the depth stakes are lower.
The layout
horizontal air handler, attic, draw-through
+-----------------------------------------+
| coil over primary drain pan 2nd outlet: plug
+---+-------------------------------------+
| primary outlet, 3/4 in
+------+
| capped cleanout tee,
| in outlet side
| leg |
| +------+---> to termination,
| | out 1/8 in per ft min
| | leg
+------+
trap seal
===========================================
| auxiliary pan 1-1/2 in deep, 3 in larger |
| than unit each way; float switch opens |
| the 24 V cooling call before overflow |
===========================================
The tee on the outlet side gives a place to flush and to see standing water. On a draw-through unit, an open vent tee upstream of the trap defeats the trap, because the cabinet draws air through the tee instead of through the seal; that is why it sits downstream.
Worked example: an attic air handler over a finished ceiling
A 3-ton horizontal air handler in an attic over a finished bedroom ceiling. Static at the coil compartment on commissioning, clean filter: -0.55 in w.c. The drain runs 28 feet across the attic to an exterior wall, where the local official allows discharge to grade with a splash block, at least the distance from the foundation the town wants.
Size. One system, 3 tons: 3/4 inch, unchanged in size the whole way. Table 307.2.2 only matters if a second unit joins the line.
Slope. 28 feet at the 1/8 inch per foot minimum needs 28 x 0.125 = 3.5 inches of fall. Check that the 3.5 inches is there before cutting pipe: the outlet of the trap has to sit at least 3.5 inches above the point where the line leaves the building. If it does not, either the air handler goes up on a stand or the line goes to a condensate pump. A line run "close to level" because the fall was not available is a line that holds water.
Trap depth. The commissioning static is -0.55 in w.c. The filter is a 1 inch pleated filter in a return grille that the owner changes when they remember. Say the static at the coil compartment can reach -1.0 in w.c. at the end of a filter's life. The maker's instructions for this unit call for a trap seal greater than the cabinet's negative static plus their own margin; a seal of 2 inches clears -1.0 in w.c. with room to spare. A shallow trap built from a P-trap fitting and a short elbow, with an inlet column of about 3/4 inch, would work on commissioning day at -0.55 and start spitting water by August at -1.0. That is the callback that gets blamed on the coil.
Auxiliary protection. Damage could occur: the ceiling below is finished. Method 3, an auxiliary pan with a float switch, is chosen. The unit footprint is 21 inches by 52 inches; the pan must be at least 24 by 55 inches, so a stock 30 by 60 inch pan is used, 2 inches deep. The float switch is wired to open the 24 V cooling call, so a full pan stops the compressor and the indoor unit stops making water.
Secondary outlet. The equipment pan's secondary connection is plugged, because method 3 does not use it. Piping it to a conspicuous point (method 2) would have been equally compliant. Leaving it open, uncapped and unpiped is not.
Cleanout. A capped tee on the outlet side of the trap.
What changes the answer
- The unit is in a basement on a slab with a floor drain and nothing finished below: auxiliary protection may not be required under the rule's trigger, though the float switch is cheap insurance.
- The line must run uphill: a condensate pump, with its own overflow switch wired to stop the equipment.
- A high-static commercial-style air handler at -2 in w.c. or more: the maker's trap gets deep enough that the trap height and the fall have to be planned together before the unit is set.
References
- 2021 International Mechanical Code, Section 307, as adopted by your jurisdiction; check the current local edition
- 2021 International Residential Code, Section M1411.3, as adopted, for one- and two-family dwellings
- Air handler and furnace installation instructions: trap dimensions and secondary connection notes
- See related: How to Build a Condensate Trap for a Draw-Through Coil; How to Install a Secondary Drain Pan and Float Switch; How to Run a Condensate Line at Slope to an Approved Termination