💧 HVAC

Free Condensate Drain Pipe Sizing Calculator (ASHRAE / IMC)

Size AHU and FCU condensate drain pipes from cooling capacity. Condensate flow rate, drain size per IMC Table 307.2.2 and minimum slope. Free tool.

📐 Standard: ASHRAE / IMC 307
✅ Free to use
📄 PDF export
📱 Mobile friendly

ℹ️ About This Calculator

A cooling coil pulls moisture out of the air, and that condensate has to leave the drain pan reliably. Undersized or flat drain lines back water into the pan, which overflows into ceilings and grows microbial slime. This calculator gives the condensate flow rate from cooling capacity, the drain pipe size per IMC Table 307.2.2, and the fall required at your chosen slope.

Condensate rate is driven by latent load, not just tonnage - a humid coastal AHU can shed three times the water of the same unit in a dry climate, which is why the humidity factor matters. Beyond size and slope, two things cause most drain failures: no P-trap (or the wrong trap depth) on a draw-through unit, and no air break or vent, which makes the line airlock and drain in slugs. Always trap the pan outlet, slope continuously at 1% or more, and provide a cleanout - condensate lines block with biofilm more often than any other pipe in the building.

📐 Condensate Flow & Drain Size (ASHRAE / IMC 307)

ASHRAE / IMC 307

Condensate flow:
  Q (L/hr) ≈ TR × humidity factor × 1.2
  humidity factor: dry 0.5 | moderate 1.0 | humid/coastal 1.5

Drain pipe size (IMC Table 307.2.2, by equipment capacity):
  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:
  Minimum 1% (approx 10 mm fall per metre)
  Fall per 10 m = slope% × 100 mm

❓ Frequently Asked Questions

How do you size a condensate drain pipe? +
Take the cooling capacity in tons and read the drain size from IMC Table 307.2.2 - up to 3 TR needs 20 mm (3/4 inch), 4-20 TR needs 25 mm (1 inch), 21-90 TR needs 32 mm (1-1/4 inch). Then confirm the line slopes at least 1% continuously to the discharge point.
How much condensate does an AHU produce? +
Roughly 0.6 to 1.8 litres per hour per ton of refrigeration depending on humidity - dry climates sit near the low end, humid or coastal installations near the high end because the latent load is much greater.
What slope is required for a condensate drain? +
A minimum of 1%, which is about 10 mm of fall per metre of run (100 mm per 10 m). Flatter runs allow standing water, biofilm growth and blockage. More slope is always better where headroom allows.
Why does a condensate drain need a P-trap? +
On a draw-through AHU the drain pan sits at negative pressure, so without a trap the fan pulls air up the drain instead of letting water flow down, and the pan overflows. The trap seal must be deeper than the fan static pressure.
Can condensate drain into a sanitary sewer? +
Only through an air gap or air break, never a direct connection, so sewer gas cannot enter the drain pan and the airstream. Local codes usually require an indirect connection to a floor drain or hub drain.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

💧 Condensate Drain Pipe Calculator
Reference: ASHRAE / IMC 307
Enter cooling capacity in TR, select the climate humidity, and set the drain slope. Results give condensate flow rate, the required drain pipe size per IMC 307.2.2 and the fall per 10 metres.