AC Condensate Production Calculator
Enter measured entering and leaving air conditions, airflow, altitude, runtime, and collection losses. The calculator estimates condensate flow, daily production, seasonal water, recoverable water, and a design drain allowance.
Condensate production rate (gal/hr)1.37
- Condensate per day (gal/day)
- 10.95
- Seasonal condensate produced (gal/year)
- 1,971.7
- Usable seasonal recovery (gal/year)
- 1,774.53
- Design drain or pump flow (gal/hr)
- 2.05
- Moisture removed (grains/lb dry air)
- 15.37
- Entering-air dew point (F)
- 59.71
- Condensation status
- Condensation predicted
Psychrometric details
| Quantity | Value |
|---|---|
| Site barometric pressure (psia) | 14.7 |
| Entering vapor pressure (psia) | 0.25 |
| Leaving vapor pressure (psia) | 0.2 |
| Entering humidity ratio (lb/lb dry air) | 0.01 |
| Leaving humidity ratio (lb/lb dry air) | 0.01 |
| Dry-air mass flow (lb/hr) | 5,200.79 |
| Condensed water mass rate (lb/hr) | 11.42 |
Assumes steady entering and leaving air states for the entered cooling runtime.
How to use this calculator
- Measure entering dry-bulb temperature and relative humidity just upstream of the cooling coil.
- Measure or estimate leaving dry-bulb temperature and relative humidity just downstream of the wet coil.
- Enter measured coil airflow, site altitude, daily cooling runtime, and cooling days for the period represented.
- Set collection efficiency and the design safety factor to size recovery and drainage capacity.
How AC condensate production is calculated
Condensate forms when the air leaving a cooling coil carries less water vapor than the air entering it. This calculator compares the entering and leaving humidity ratio, then multiplies that water removal by the dry-air mass flow through the coil.
Altitude is included because barometric pressure changes both humidity ratio and specific volume. The pressure model is P = 14.696 x (1 - 0.00000687535 x z)^5.2561, where z is altitude in feet and P is pressure in psia.
For each air state, the calculator evaluates the ASHRAE saturation vapor pressure at dry-bulb temperature, multiplies it by relative humidity, and converts vapor pressure to humidity ratio:
W = 0.621945 x pw / (P - pw)
The entering specific volume is calculated as v = 0.370486 x (tdb + 459.67) x (1 + 1.607858 x W) / P. Dry-air mass flow is mda = 60 x CFM / v. Water removal is then mw = mda x max(We - Wl, 0), so leaving air that is not drier than entering air produces zero condensate rather than a negative drain flow.
The headline flow is G = mw / 8.34 gallons per hour. Daily production is G x operating hours, seasonal production is daily production times cooling days, usable recovery is seasonal production times collection efficiency, and design drain or pump flow is the steady gallons per hour times the entered safety factor. Moisture removed is also shown as 7000 x max(We - Wl, 0) grains per pound of dry air. The entering-air dew point is found by solving for the temperature where saturation vapor pressure equals the entering vapor pressure.
What moves the answer most
The largest drivers are entering humidity, leaving coil condition, and airflow. A coil seeing humid return air and leaving near saturation at a much lower dry-bulb temperature can make substantial condensate. A coil with the same airflow in dry weather may remove little moisture. Runtime and cooling days do not change the instantaneous gallons per hour, but they strongly affect daily and seasonal volume.
What this calculator leaves out
The calculation assumes the entered air states are steady during the entered runtime. It does not model changing outdoor air fraction, equipment staging, fan-only re-evaporation, condensate held on the coil or pan, leakage, overflow, drain fouling, defrost, or water quality. Drainage and pump selection also depend on lift, pipe slope, fittings, local code, overflow protection, and manufacturer instructions.
Worked example
With the default inputs, entering air is 80 F and 50% RH, leaving air is 55 F and 95% RH, airflow is 1,200 CFM, and altitude is sea level. The calculated humidity ratios are about 0.01092 and 0.00873 lb water per lb dry air, so the moisture difference is 0.00220 lb/lb, or 15.37 grains per pound of dry air.
The entering specific volume is about 13.84 cubic feet per pound of dry air. At 1,200 CFM, dry-air mass flow is about 5,201 lb/hr, which removes 11.42 lb/hr of water. Dividing by 8.34 gives 1.37 gal/hr. At 8 hours per day for 180 cooling days, that is 10.95 gal/day and 1,971.70 gal/year. With 90% collection efficiency, usable seasonal recovery is 1,774.53 gallons, and a 150% design factor gives a 2.05 gal/hr drain or pump flow.
Common questions
How many gallons of condensate should an air conditioner produce per day?
It depends on humidity, coil leaving condition, airflow, and runtime. In the default example, a 1,200 CFM coil at 80 F and 50% entering RH produces about 11 gallons during an 8 hour cooling day. The same unit can produce much less in dry weather or much more with humid return air and long runtime.
Why does AC condensate output change with humidity?
Condensate is water vapor removed from the air, so the entering humidity ratio matters directly. Warmer, wetter return air contains more water per pound of dry air. If the leaving air is only slightly drier than the entering air, the calculated drain flow will be small even when the coil is cold.
What leaving-air relative humidity should I use if I cannot measure it?
Leaving air from a wet cooling coil is often close to saturation, so 90% to 100% is a reasonable planning range when no measurement is available. Use a measured leaving dry-bulb and RH when possible because bypass factor, airflow, coil cleanliness, and load all affect the leaving state.
Can condensate production be estimated from AC tonnage alone?
Tonnage alone is not enough because latent removal depends on moisture difference and airflow, not just total cooling capacity. Two systems with the same tonnage can produce different condensate rates if one is serving drier air, has different airflow, or leaves the coil at a different humidity ratio.
How does altitude affect condensate production?
Higher altitude lowers barometric pressure, which changes humidity ratio and specific volume. For the same entered dry-bulb, relative humidity, and CFM, the dry-air mass flow and pounds of water per pound of dry air shift together. The effect is usually modest but should be included for drainage or recovery estimates at elevation.
Can AC condensate be reused for irrigation?
Condensate can be part of a non-potable water strategy, but it is not drinking water. It may contain biological growth, metals, cleaning residues, or debris from the coil and drain pan. Reuse should follow local rules and include appropriate collection, treatment, storage, and overflow controls.