AHU Condensate Rate: Calculate Drain Volume from Humidity Ratio

An AHU condensate estimate is useful when planning a drain-collection test, checking a moisture-removal claim or screening a possible water-recovery project. The useful output is a rate at stated operating conditions, followed by a volume over a stated period. Cooling capacity alone cannot provide that answer because the sensible and latent portions change with the entering air and coil operation.

1. Start with the water mass balance

For a steady wet cooling coil, with no unmeasured air leakage or liquid carryover, condensate production is m_cond = m_da × (W_in − W_out). Here m_cond is kilograms of liquid water per second, m_da is kilograms of dry air per second, and W is kilograms of water vapor per kilogram of dry air. A negative humidity-ratio difference would indicate net moisture addition, not condensate production. The corresponding moisture-removal relationship is presented in ASHRAE’s cooling-coil chapter.

Humidity ratio and relative humidity are different quantities. A difference of 20 percentage points of relative humidity cannot be inserted into this equation. Convert measured dry-bulb temperature, humidity and absolute pressure into W using a consistent psychrometric method. ASHRAE’s Psychrometrics chapter defines the dry-air property basis and the moist-air relationships.

When airflow is an actual volume measured at the entering state, m_da = V_in/v_in, with v_in in m³/kg dry air. Keep the airflow station and property state together. An actual supply-air volume after cooling cannot be paired with entering-air specific volume. Document whether a meter reports actual volume or a density-normalized volume before calculating mass.

2. Calculate a complete illustrative duty

Assume a coil receives 3.20 m³/s of actual air at 28°C and 101.325 kPa, with W_in = 0.0140 kg/kg. The leaving condition is 13°C with W_out = 0.0090 kg/kg. These are invented steady-state test conditions, not a product rating. Assume complete drainage, no carryover, no additional moisture source and constant dry-air mass flow through the boundary.

For this example use the ideal-gas approximation v = 0.287042(T + 273.15)(1 + 1.607858W)/p, where T is °C and absolute p is kPa. The following calculation retains extra digits only to make the arithmetic reproducible; field results should be rounded to match measurement quality.

Illustrative steady moisture balance. At the assumed 0.999 kg/L liquid density, 66.03 kg/h corresponds to 22.03 L collected over 20 minutes.

1. Entering specific volume is 0.872327 m³/kg dry air. Dry-air mass flow is 3.20/0.872327 = 3.66835 kg/s.

2. The humidity-ratio reduction is 0.0140 − 0.0090 = 0.0050 kg/kg, equal to 5.0 g/kg. Convert grams to kilograms before multiplying.

3. Condensate production is 3.66835 × 0.0050 = 0.018342 kg/s, or 66.03 kg/h.

4. Using an assumed liquid density of 0.999 kg/L, the volume rate is 66.10 L/h. A 20-minute stable collection interval should yield approximately 22.03 L.

If these same conditions continued for eight hours, the calculated volume would be about 529 L. That conditional total is useful for planning a test container or short trial. It is not an annual yield estimate: actual humidity, outdoor-air fraction, airflow, coil duty and operating hours must be integrated over time.

3. Distinguish production from water reaching the outlet

The steady equation predicts moisture removed from the air. During startup, some liquid remains on fins and in the pan. The collection bottle can therefore receive less water than the airside calculation predicts while stored liquid is increasing. After shutdown, drainage can continue while moisture production has already fallen.

A more general boundary statement is drain mass rate = net airside moisture-removal rate − liquid-storage rate − liquid-carryover rate. Include any other water entry or exit explicitly. Re-evaporation is already reflected in a properly measured net airside humidity-ratio change; do not subtract it a second time.

Wet-coil transients are real. The EnergyPlus Engineering Reference describes moisture retention and off-cycle evaporation in its cooling-coil latent-degradation model. That model has a defined equipment scope; it is evidence for the physical mechanism, not a universal correction factor for every AHU.

4. Run a useful collection check

1. Choose a stable operating period and trend airflow, entering and leaving air conditions, cooling command and fan status. Start the timed comparison after coil and pan behavior have settled.

2. Verify the collection route serves only the coil being evaluated. Exclude cleaning water, rain entry and other connected drains. Preserve safe drainage and the operating trap arrangement.

3. Prefer weighing collected water when practical. Record empty and full container masses, elapsed time and any pump cycling. A collection boundary upstream of a storage sump avoids confusing sump inventory with coil production.

4. Compare integrated moisture removal over the entire interval with collected mass. If conditions vary, sum each interval’s m_da × ΔW × elapsed time rather than multiplying one snapshot by the total duration.

A 20-minute collection of 22.0 L would be close to this example’s prediction. A much smaller collection should trigger checks of humidity readings, airflow basis, storage changes and unmeasured discharge paths. The observation alone does not identify a blocked drain or a defective coil. Keep the drainage safety inspection separate from the calculation.

5. Check sensitivity before declaring a discrepancy

Suppose dry-air mass could differ by ±5%, and each humidity ratio could differ by ±0.0004 kg/kg. Treat these as assumed bounding errors, not statistical uncertainties. Opposite humidity errors produce ΔW from 0.0042 to 0.0058. Combining those endpoints with the flow bounds gives approximately 52.69 to 80.42 kg/h, compared with the nominal 66.03 kg/h.

That deliberately conservative range shows why small humidity differences deserve careful measurement. At a nominal ΔW of only 0.0010 kg/kg, the same two humidity bounds can shift the difference by 80%. Improve sampling representativeness and instrument checks before treating a modest drain-volume mismatch as a performance failure.

6. Use the result for the right decision

Record the measurement boundary, property method, instrument details, timestamps, operating conditions, assumptions and observed collection. Keep condensate quantity separate from trap geometry, pipe capacity and code review; those require their own design checks. For reuse, include seasonal availability, storage and water quality. DOE’s captured-condensate guidance warns that storage can support bacterial growth and contact with equipment can introduce metals. Collected condensate should not be presumed suitable for drinking.

For broader AHU design reference, review the paid Air Handling Unit (AHU) Selection & Fault Diagnosis Handbook. Use the documented operating case and field measurements to make the project-specific decision.

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