Condensate flow (rule-of-thumb or exact latent load), minimum IMC drain pipe size, gravity capacity check, P-trap depth, slope, annual water recovery and condensate-pump sizing — with a trap diagram, updated live.
About the AC Condensate Drain Pro Calculator
This professional tool fully details a cooling-coil condensate drain: how much condensate forms, the minimum code pipe size, whether that pipe has ample gravity capacity, the P-trap depth for the coil's pressure, the slope, the water you can reclaim each year, and — when gravity won't work — the condensate pump duty.
It suits HVAC and MEP engineers, installers and inspectors, and sustainability teams. Every result is based on standard IMC, ASHRAE and open-channel-flow relationships, shown below.
Condensate flow — two methods
By capacity: GPH = Tons × Rate (0.1 dry · 0.2 avg · 0.3–0.4 humid)
By latent load: GPH = Latent (BTU/hr) ÷ 8,840 (exact)
The latent-load method is preferred when you have the load split from a cooling-load calculation; the rule-of-thumb rate is fine for quick sizing.
Minimum drain pipe — IMC §307
| Equipment capacity | Min. drain diameter |
|---|
| Up to 20 tons | 3/4 in (DN20) |
| 21 – 40 tons | 1 in (DN25) |
| 41 – 90 tons | 1-1/4 in (DN32) |
| 91 – 125 tons | 1-1/2 in (DN40) |
| 126 – 250 tons | 2 in (DN50) |
Gravity capacity check (Manning)
The capacity bar compares the condensate flow with the full-bore gravity capacity of the chosen pipe at your slope, using the Manning equation:
Q = (1.486 ÷ n) × A × R^(2/3) × S^(1/2) (n ≈ 0.011 for PVC)
Condensate is typically only a few percent of capacity — confirming the IMC size is governed by fouling and priming, not by flow. A high percentage would flag an unusually wet or under-sloped line to review.
P-trap, slope, recovery & pump
Seal depth (A) = Static pressure (in w.c.) + 1 in (min 2 in draw-through)
Outlet drop (B) = A ÷ 2
Annual water = GPH × hours/day × cooling days/yr
Pump head ≈ Lift + friction; Pump rating ≥ 2 × condensate GPH
Trap dimensions follow common manufacturer/ASHRAE practice (the unit submittal governs). Recovered condensate is distilled-quality and well suited to cooling-tower makeup or irrigation. A condensate pump is only needed where there is no continuous downhill path; select it from the manufacturer's curve at your design lift.
Assumptions & limitations
- The GPH/ton rate is a rule of thumb; the latent-load method is more accurate where the split is known.
- Pipe sizing follows the common IMC table — verify against your adopted code and local amendments.
- Capacity uses nominal diameter and Manning n ≈ 0.011; partially-full drains carry less than full-bore but still far exceed condensate flow.
- Trap and pump figures are preliminary — confirm with the equipment submittal and pump curve.
Frequently asked questions
How much condensate does an air conditioner produce?
It depends on the latent (moisture) load. As a rule of thumb a coil makes about 0.1 GPH per ton in dry climates, 0.2 in average conditions and 0.3–0.4 in humid or tropical climates. Exactly, condensate (GPH) ≈ latent load (BTU/hr) ÷ 8,840, since each gallon removed carries roughly 8,840 BTU of latent heat. The Pro tool lets you use either method.
Is the condensate drain pipe ever too small for the flow?
Almost never on a hydraulic basis — even a 3/4 in pipe at minimum slope can carry tens of gallons per hour, far more than a small coil produces. The capacity check shows condensate is usually only a few percent of the pipe's gravity capacity. The IMC sizes the drain by tonnage to resist fouling and keep the line primed, not because of flow volume.
What size should the condensate drain pipe be?
Per IMC §307: up to 20 tons needs 3/4 in, 21–40 tons 1 in, 41–90 tons 1-1/4 in, 91–125 tons 1-1/2 in and 126–250 tons 2 in. The calculator returns the minimum for your tonnage; confirm against your adopted code edition.
How deep should the condensate trap be?
For a draw-through (negative-pressure) coil the water seal should be at least the drain-pan static pressure plus about 1 inch, with the outlet roughly half the seal depth below the inlet. These are common manufacturer/ASHRAE values; the equipment installation instructions govern.
When do I need a condensate pump?
When the drain point is above the unit or there is no continuous downhill path to an approved termination — common with basement air handlers, high-efficiency furnaces and ductless mini-splits. The Pro tool estimates the total lift (head) and a recommended pump capacity; always select from the manufacturer's pump curve at your design lift.
How much condensate water can I recover in a year?
Multiply the hourly condensate by run hours and cooling days per year. A modest commercial unit can yield thousands of gallons a season — essentially distilled water suitable for cooling-tower makeup or irrigation. The recovery panel estimates the annual volume and the water-bill saving.
What slope does a condensate drain need?
A minimum of 1/8 inch per foot (≈1%); 1/4 inch per foot is better. Run continuously downhill with no sags, and provide a cleanout.
Do I need a secondary drain or float switch?
Codes generally require overflow protection — a secondary drain, an auxiliary pan, or a float switch that shuts the unit off on high water. This is essential for any unit above a finished space.
Last updated: 2026-01-03 · Preliminary design only — verify against the manufacturer's instructions and local code.