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.

Can the calculated flow determine the required drain pipe size?

Not by itself. Mechanical codes and manufacturers may size drains by equipment capacity, connection size, slope, trap geometry and local requirements. The comparison table is only an early planning reference; verify the final drain and trap design with the equipment instructions and authority having 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

Calculate HVAC condensate in gal/hr, L/hr, gal/day and L/day, plus the estimated latent load in BTU/hr, tons and kW.

Inputs

Unit System

Daily Operating Hours

Used only for the daily production estimate.

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

Daily Condensate Production

Gallons per Day

—

gal/day

Liters per Day

—

L/day

Moisture Removal Details

Entering humidity ratio (W₁) —

Leaving humidity ratio (W₂) —

ΔW (moisture removed) —

Dry-air mass flow —

Moisture removal rate —

Latent cooling load —

Drain Flow Planning Reference

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

Planning comparison only—not code sizing. Final pipe and trap design depends on equipment capacity, connection size, pressure, slope, manufacturer instructions and local code.

Summary

Calculate HVAC condensate in gal/hr, L/hr, gal/day and L/day, plus the estimated latent load in BTU/hr, tons and kW.

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.
Size condensate lines for dedicated outdoor air systems (DOAS) handling high latent ventilation loads.
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: 2026-09-22 · Reviewed by Nham Vu