Quick answer: Size a condensate drain from the equipment cooling capacity using IMC Table 307.2.2 — up to 3 TR needs 20 mm (3/4"), 4–20 TR needs 25 mm (1"), 21–90 TR needs 32 mm (1-1/4"). Condensate flow is roughly 0.6–1.8 litres per hour per ton depending on humidity, and the line must fall at least 1% (10 mm per metre) all the way to discharge.
How much condensate does a coil produce?
Condensate comes from the latent load, not the total tonnage, which is why the same AHU sheds very different amounts of water in Chennai versus Delhi in winter. A practical design figure:
Q (L/hr) ≈ TR × humidity factor × 1.2
Dry / low RH factor ≈ 0.5
Moderate factor ≈ 1.0
Humid / coastal factor ≈ 1.5Drain pipe size (IMC Table 307.2.2)
| Equipment capacity | Minimum drain size |
|---|---|
| Up to 3 TR | 20 mm (3/4") |
| 4 – 20 TR | 25 mm (1") |
| 21 – 90 TR | 32 mm (1-1/4") |
| 91 – 125 TR | 40 mm (1-1/2") |
| 126 – 250 TR | 50 mm (2") |
Slope, trapping and venting
- Slope: minimum 1% — about 10 mm fall per metre, or 100 mm per 10 m run.
- Trap: every draw-through pan needs a P-trap deeper than the fan static, or it will not drain at all.
- Vent / air break: without one the line airlocks and drains in slugs; never connect directly to a sanitary sewer.
- Cleanout: condensate lines block with biofilm more than any other pipe in the building.
Worked example
A 20 TR AHU in a moderate climate: Q = 20 × 1.0 × 1.2 = 24 L/hr of condensate. From the table, 20 TR falls in the 4–20 TR band → 25 mm (1") drain. At 1% slope over a 10 m run, the line drops 100 mm. Trap depth comes from the fan static — see the trap calculation below.
Flow rate, pipe size and slope from cooling capacity. Free, no sign-up.
Open the Condensate Drain Calculator →Frequently Asked Questions
How do you size a condensate drain pipe?
Read the minimum drain size from IMC Table 307.2.2 based on equipment tonnage - 20 mm up to 3 TR, 25 mm for 4-20 TR, 32 mm for 21-90 TR. Then confirm the line falls at least 1% continuously to the discharge point.
How much condensate does an AHU produce per ton?
Roughly 0.6 to 1.8 litres per hour per ton, depending on humidity. Dry climates sit near the low end and humid or coastal installations near the high end, because condensate tracks the latent load.
What slope does a condensate drain need?
A minimum of 1%, which is about 10 mm of fall per metre. Flatter runs hold standing water, grow biofilm and eventually block.
Can a condensate drain connect straight to the sewer?
No. It must discharge through an air gap or air break to an indirect waste, so sewer gas cannot be drawn back into the drain pan and the supply airstream.