How to Build a Condensate Trap for a Draw-Through Coil

Why this matters

On a draw-through air handler, the blower sits downstream of the coil and pulls air through it, so the drain pan lives in a cabinet under suction. A trap that is too shallow for that suction does not drain: the cabinet pulls air up the drain line, water stacks up in the pan, and on a humid afternoon it rides off the pan edge into the insulation, the duct or the closet floor. It usually works on install day with a clean filter and starts overflowing in August when the filter is loaded and the suction is higher. The callback gets blamed on the coil or a clog. It is the trap.

The code side of this (where traps are required, line size, slope, cleanouts, auxiliary protection) belongs to Condensate Drain Code and Trap Field Card. This article is the build: measure the suction, size the trap to the worst case, put it together, and prove it with water while the blower runs.

What the two dimensions do

A draw-through trap has two vertical dimensions that matter, both measured from the pan's primary outlet:

• The outlet drop, from the pan outlet down to the trap's outlet weir, the lip water spills over to leave. While the blower runs, water in the inlet leg stands higher than the weir by the cabinet's suction in inches of water column. If the outlet drop is smaller than the suction, the inlet column reaches back up into the pan and water stands in the pan to the difference. So the outlet drop has to be larger than the worst suction, with margin.
• The inlet leg depth, from the pan outlet down to the bottom of the trap. When the blower starts, suction pulls water out of the outlet leg into the inlet leg. If the trap is not deep enough below the weir, the outlet leg empties, the seal breaks, and air bubbles up through the drain into the pan. Deeper holds the seal through start-up.

The maker publishes both dimensions for their cabinet, usually tied to the cabinet static. Use their numbers. The physics above is what lets you check a trap someone else built.

Start state

• The air handler installed, level or pitched toward the primary outlet as the maker specifies, panels on, filter in place.
• A digital manometer and a length of tubing that fits snugly onto the pan's primary outlet fitting.
• The installation manual's trap drawing for this cabinet.
• The fall available from the pan outlet to the termination, checked with a level. The trap's outlet must still leave enough height for the run at the code slope of 1/8 inch per foot; see related: How to Run a Condensate Line at Slope to an Approved Termination.
• A closet with a gas appliance in it (water heater, furnace) gets its pilot or burner considered before any PVC cement comes out: primer and cement vapors are flammable. Ventilate the space, keep the cans capped between joints, and do the cementing outside the closet where you can.

Steps

1. Read the suction at the drain connection itself. With the trap not yet installed, push the manometer tube onto the primary outlet and seal it with your fingers. Run the blower on the speed with the highest airflow it will see, usually high cooling, with panels on and the filter in. That reading is exactly what the trap has to beat, measured where the trap will see it. A reading taken at a supply or return port is a different number. While you are set up, also read the return test port between the filter and the cabinet and write both down; step 9 needs the pair.

2. Add the worst case. The commissioning reading is the best day this cabinet will ever have. A loading filter adds resistance upstream of the coil and makes the pan more negative; so does a coil fouling over time. Measure the filter's pressure drop now, find the drop at which the shop or the owner changes it, and add the difference. If you cannot estimate it, design for at least the commissioning reading plus a few tenths of an inch.

3. Size the trap from the manual. Take the maker's outlet-drop and inlet-depth figures at your worst-case suction. Where the manual gives a formula rather than a drawing, run it, then round up to fittings you can actually buy.

4. Plug what the maker says to plug. A multi-position air handler has more drain ports than it uses. On a draw-through cabinet, an unused port left open is a hole in the suction side; air rushes in and water can be pulled out of the pan through it. Install the plugs supplied, hand tight plus the turn the manual allows. Plastic pan fittings crack when they are forced, and a crack in the pan fitting leaks into the cabinet.

5. Build and dry-fit. Assemble the trap from 3/4 inch PVC fittings or the maker's kit, with a tee on the outlet side for a cleanout cap or an open vent if the maker calls for one. That tee goes downstream of the trap. An open vent between the pan and the trap gives the cabinet a straight air path that bypasses the seal, and the trap stops working. Dry-fit, check both dimensions with a tape and a level, and only then prime and cement the joints. Connect to the pan fitting with the maker's adapter or a union so the trap can come off for cleaning without cutting.

6. Support it. A trap full of water hung only from the pan fitting levers on that fitting every time someone bumps it. Strap or block it so the pan fitting carries no weight.

7. Prime it. Fill the trap with water through the cleanout tee before the blower runs on cooling. A new trap is dry, and a dry trap is an open straw until the coil makes enough water to fill it, which on a mild day can be hours of air being sucked up the drain.

8. Test with water and the blower running. Start the blower on the high cooling speed. Pour water into the drain pan at a steady stream, about a quart over a minute. Watch three places: the trap (no gurgling or air bubbling back), the pan (water level falling to a thin film, not standing), and the termination (water arriving at about the rate you poured).

9. Test again at the worst case, then put it back. Once the trap is on, the drain connection can no longer be read directly, so use the return test port from step 1. Everything between that port and the pan is unchanged, so suction added at the port by restricting the filter shows up at the pan, slightly less of it because airflow and the coil's own drop fall a little as the filter closes off. With the compressor off and the blower in fan-only mode, cover part of the return filter with cardboard until the return port reads more negative than its step 1 value by the worst-case allowance from step 2. The blower has to run at the same speed as in step 1: if fan-only runs a lower speed, use a cooling call with the outdoor unit's disconnect pulled instead, and afterward put the disconnect back and confirm the condenser starts on the next call. Pour water again. Keep this test short, and never run it with the compressor on or on a heating call, because a starved coil can freeze and a starved furnace runs hot. Remove the cardboard, confirm the return port reading comes back to its step 1 value, and cap the cleanout.

Worked example: two traps on one closet air handler

A 3-ton upflow air handler in a hall closet over a hardwood floor, draw-through. The previous installer used a short trap built from a P-trap fitting: measured outlet drop 0.75 inch, inlet depth about 1.25 inches. The homeowner reports water on the floor a few times each summer.

Step 1. Manometer on the primary outlet, high cooling speed, panels on, a new 1 inch pleated filter in: -0.85 in w.c. Return test port between filter and cabinet at the same time: -0.40 in w.c.

Step 2. Filter drop when clean, read across the filter: 0.12 in w.c. The owner changes it when a neighbor reminds them; the tech uses 0.35 in w.c. as a realistic end-of-life drop. Added suction: 0.35 - 0.12 = 0.23 in w.c. Worst case: -0.85 - 0.23 = -1.08 in w.c., designed for as -1.1.

The old trap against those numbers. Water stands in the pan by about the suction minus the outlet drop. On commissioning day: 0.85 - 0.75 = 0.10 inch of water standing in the pan. With a loaded filter: 1.08 - 0.75 = 0.33 inch. A third of an inch of water in a shallow pan, with air moving across it at coil face velocity, is the water on the floor. And with an inlet depth of 1.25 inches, the seal sits only 0.5 inch below the weir, little reserve against the surge of suction when the blower starts.

Step 3, sized. Say the manual's drawing for this cabinet asks for an outlet drop larger than the worst-case suction and an inlet depth at least 1 inch more than the worst-case suction (illustrative; use your maker's figures). Worst case 1.1 in w.c.: outlet drop over 1.1 inches, inlet depth at least 1.1 + 1.0 = 2.1 inches. Built with fittings: 1.5 inch outlet drop and 2.5 inch inlet depth, so the trap bottom sits 1.0 inch below the weir.

       coil compartment, -1.1 in w.c. worst case
 +-------------------------------------+
 |  coil over primary drain pan        |
 +--+----------------------------------+
    | pan outlet ...................... A
    |                                  :
    | inlet                  1.5 in drop
    | leg                    A down to C
    |                                  :
    |                   tee: cleanout  :
    |                   cap, downstream:
    |                         |        :
    |      C ..... +----------+--------+--> run at
    |              | outlet               1/8 in/ft
    |              | leg                  min
    +--------------+
     trap bottom B, 2.5 in below A

Margin check. At the worst case the inlet column stands 1.1 inches above the weir, which is 1.5 - 1.1 = 0.4 inch below the pan outlet: the pan drains with 0.4 inch to spare. On commissioning day, 1.5 - 0.85 = 0.65 inch to spare.

Fall. The termination is a floor-level connection 12 feet away. 12 x 0.125 = 1.5 inches of fall needed from C to the termination. The weir at C sits 9 inches above the floor connection, plenty. Had the closet floor been the limit, the cabinet would have gone up on a stand rather than the trap getting shallower.

Steps 7 to 9. Primed through the cleanout. At -0.85 in w.c., a quart poured over a minute arrived at the termination steadily, no gurgle, pan back to a film inside a minute. At the simulated worst case, cardboard over part of the filter in fan-only mode until the return port read -0.63 in w.c. (-0.40 - 0.23), which puts the pan at a little under -1.08: same result. Cardboard out, return port back to -0.40, cap on.

Finish test

The trap passes when, at the design worst-case suction with the blower running, there is no air gurgling back through the trap, no standing water in the pan beyond a film, and water poured into the pan arrives at the termination. Record the commissioning suction, the worst case designed for, the outlet drop and the inlet depth as built. The next tech who finds water on the floor can then check the static against the number the trap was built for, instead of rebuilding a trap that was never the problem.

References

• Air handler installation instructions: trap drawing, outlet-drop and inlet-depth dimensions, plug locations
• See related: Condensate Drain Code and Trap Field Card (code basis, slope, cleanout, auxiliary protection)
• See related: How to Run a Condensate Line at Slope to an Approved Termination
• See related: Static Pressure Budget Across a Residential System; How to Install a Secondary Drain Pan and Float Switch