What is condensate load?

Condensate load is the rate at which liquid water drains from a cooling coil. When warm, humid air passes over a coil surface cooled below its dew point, water vapor condenses. The condensate load depends on the airflow rate and the difference in moisture content (humidity ratio) between the entering and leaving air.

How is humidity ratio calculated from dry-bulb and relative humidity?

Saturation vapor pressure is estimated using the Magnus formula. Actual vapor pressure equals saturation pressure multiplied by relative humidity. Humidity ratio (W) is then: W = 0.62198 × Pv / (P_atm − Pv), where pressures are in the same units. This is the standard ASHRAE calculation at sea-level pressure.

What entering conditions should I use?

Use the mixed-air conditions entering the cooling coil — typically a blend of outdoor air and return air. Entering dry-bulb and relative humidity (or wet-bulb) from a psychrometer or building management system are the most common inputs.

What leaving conditions should I use?

Leaving conditions are the air state off the coil. The leaving dry-bulb is usually the supply air temperature (e.g. 55 °F / 13 °C). Leaving relative humidity is typically 90–98 % for a well-performing coil (near saturation). Use coil submittals or measured data when available.

What drain pipe size does this result suggest?

ASHRAE Handbook and most mechanical codes require the drain pan connection to handle at least the calculated condensate flow with appropriate slope (1/8 in/ft minimum). A common rule of thumb is 3/4-in pipe for up to about 2 gal/hr, 1-in pipe up to 4 gal/hr. Always verify against the applicable code for your jurisdiction.

Does altitude affect the result?

Yes. At higher altitudes atmospheric pressure is lower, which raises the humidity ratio slightly for the same dry-bulb and relative humidity. This calculator assumes standard sea-level pressure (101.325 kPa / 14.696 psia). For sites above ~1,000 m (3,300 ft), results may underestimate condensate by a few percent; use altitude-corrected psychrometric software for precision work.

Condensate Load Calculator

Name: Condensate Load Calculator
Availability: InStock
Author: Nham Vu

Enter airflow, entering and leaving humidity conditions, and unit system to calculate condensate drainage rate from an HVAC cooling coil.

Inputs

Unit System

Airflow Rate (CFM)

Entering Air (into coil)

Dry-Bulb (°F)

Relative Humidity (%)

Leaving Air (off coil)

Dry-Bulb (°F)

Relative Humidity (%)

Condensate Flow Rate

Gallons per Hour

—

gal/hr

Liters per Hour

—

L/hr

Moisture Removal Details

Entering humidity ratio (W₁) —

Leaving humidity ratio (W₂) —

ΔW (moisture removed) —

Dry-air mass flow —

Moisture removal rate —

Typical Drain Pipe Guidance

Flow Rate	Min Pipe Size	Notes
Up to 2 gal/hr	¾ in (DN20)	Fan coils, small AHUs
2–8 gal/hr	1 in (DN25)	Mid-size AHUs
8–20 gal/hr	1¼ in (DN32)	Large AHUs, RTUs
20+ gal/hr	1½ in (DN40)+	Central plant / chilled water

Minimum slope 1/8 in per foot (1%). Verify pipe size with applicable mechanical code for your jurisdiction.

Summary

Enter airflow, entering and leaving humidity conditions, and unit system to calculate condensate drainage rate from an HVAC cooling coil.

How it works

Enter the supply airflow rate in CFM (cubic feet per minute) or L/s.
Input the entering air conditions: dry-bulb temperature and relative humidity (or wet-bulb temperature).
Input the leaving air conditions off the coil (typically near the apparatus dew point).
The calculator derives the humidity ratio (lb water / lb dry air) for both states using standard psychrometric equations.
It multiplies the humidity-ratio difference by the dry-air mass flow rate to get moisture removal in lb/hr or kg/hr.
The result is converted to condensate flow rate in gal/hr or L/hr for drain sizing.

Use cases

Size condensate drain pipes and trap depths for air-handling units.
Verify condensate pump lift-station capacity for above-ceiling fan coils.
Estimate drain neutralizer sizing for high-efficiency furnace condensate.
Check drain line slope and diameter compliance with mechanical codes.
Calculate seasonal condensate volume for water reuse or disposal planning.
Validate equipment submittals and coil performance data.
Assist energy auditors in quantifying latent load removal.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu