Condensate Drain Pipe Sizing Calculator - ASHRAE / IMC

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

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Condensate Drain Pipe Calculator Calculator

Reference: ASHRAE / IMC 307

💧 HVAC

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.

Inputs

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.

Cooling Capacity (TR)

Climate / Humidity

Drain Slope (%)

Results

Calculation confidence: Planning-levelSuitable for early design, estimating and feasibility checks. Verify the final design against the governing code and a licensed engineer before construction or procurement.

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ℹ 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

Condensate Drain Sizing Reference (IMC 307 / ASHRAE)

Drain size comes from cooling capacity, not condensate flow. IMC Table 307.2.2 fixes the minimum pipe diameter by equipment tonnage, while the condensate flow rate (driven by the latent load and humidity) tells you how much water the trap and slope must actually carry.

Minimum Drain Size by Cooling Capacity (IMC Table 307.2.2)

Equipment capacity	Min. drain (metric)	Min. drain (inch)
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"

So a typical 5 TR split or AHU needs a 25 mm (1") drain, a 2 TR unit needs 20 mm (3/4"). Never reduce below the drain-pan outlet size, and always round up at a band boundary.

Condensate Flow — Humidity Factor

Climate / coil condition	Factor	Approx. flow
Dry / low RH (inland winter)	0.5	≈ 0.6 L/hr per TR
Moderate	1.0	≈ 1.2 L/hr per TR
Humid / coastal (monsoon)	1.5	≈ 1.8 L/hr per TR

Flow ≈ TR × factor × 1.2 L/hr. A 10 TR AHU on a humid coastal day can shed ~18 L/hr — the same unit inland in winter, barely 6 L/hr.

Slope & Trap

Slope the line continuously at ≥ 1% (10 mm fall per metre; 1/8" per foot). Every draw-through coil (pan under negative pressure) needs a P-trap deep enough to hold against the fan static — trap seal ≈ fan static pressure in mm w.g. plus a margin — or the fan pulls the seal dry and air-locks the line. Add a cleanout and an air break at discharge.

Drain sizes follow IMC Table 307.2.2; local codes (ASHRAE, UPC) and the equipment maker's pan-outlet size govern the final selection.

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.

What size condensate drain does a 5 ton AC need? +

A 5 TR (5 ton) unit falls in the 4–20 TR band of IMC Table 307.2.2, so the minimum condensate drain is 25 mm (1"). A 2–3 TR unit needs 20 mm (3/4"); a 25–90 TR AHU needs 32 mm (1-1/4"). Never go below the drain-pan outlet size.

Can two AHUs share one condensate drain line? +

Yes — if the common line is sized for the combined tonnage per IMC 307.2.2 and each unit keeps its own P-trap before joining the header. Tie the branches into the top or side of the main with a continuous fall so one unit cannot back water into another's pan.

How do I stop a condensate drain from clogging? +

Slope the line at 1% or more, fit a cleanout at the trap, and flush the pan and line with diluted vinegar or a biocide tablet each service. Biofilm and algae — not pipe size — cause the majority of condensate blockages and ceiling leaks.