Ventilation Moisture Load: How Outdoor Air Changes AC Demand

ByMuzaffer Caner Ankaralı

Outdoor air delivered for ventilation must be brought toward the indoor condition. On a humid day, that can require both a dry-bulb temperature change and water-vapor removal. The moisture rate depends on dry-air mass flow and the difference between outdoor and indoor humidity ratio—not on relative humidity alone.

Ventilation is not a defect simply because it adds conditioning load. It serves an indoor-air purpose and must be included in the design, schedule, distribution, pressure balance, and moisture-control strategy. ASHRAE Handbook—Fundamentals, Chapter 16 explains that conditioning outdoor air can be a significant part of the space-conditioning load.

Humidity ratio puts outdoor and indoor moisture on the same basis

Relative humidity changes with temperature. Designers therefore compare humidity ratio W, the mass of water vapor divided by the mass of dry air. Under the ideal-gas approximation:

Humidity ratio: W = 0.621945 × pw / (p − pw)

Here, W is kg water/kg dry air or lb water/lb dry air, pw is water-vapor partial pressure, and p is total barometric pressure. If dry-bulb temperature and RH are inputs, pw = RH × pws(Tdb), with RH expressed as a fraction. These relationships are documented in ASHRAE Handbook—Fundamentals, Chapter 1: Psychrometrics. A weather-app RH percentage cannot be multiplied directly by CFM to obtain moisture or latent Btu/h.

Moisture load begins with a dry-air mass balance

For a steady outdoor-air stream with no recovery device in the calculation boundary:

Water-vapor rate: ṁw = ṁda × (Wo − Wi)

Dry-air mass flow from actual volume: ṁda = V̇actual / vo

Water mass over a duration: mw = ṁw × t

  • ṁw = net water-vapor mass rate, kg water/h
  • ṁda = dry-air mass flow, kg dry air/h
  • Wo, Wi = outdoor and indoor humidity ratio, kg water/kg dry air
  • V̇actual = actual volumetric airflow at the stated air condition, m³/h
  • vo = outdoor moist-air specific volume, m³/kg dry air
  • t = operating duration, h

If Wo − Wi is positive, the outdoor stream adds moisture relative to the indoor target. If it is negative, the stream has a drying potential. Schedule or duty cycle enters through the duration; nominal CFM alone does not determine daily water mass.

Actual CFM and standard CFM are not interchangeable

ACFM is the volumetric flow at the actual measurement condition. It must be converted using specific volume at that condition: ṁda = V̇actual/vo. SCFM represents a mass-flow equivalent at a declared standard temperature, pressure, and moisture convention. A calculation must state that standard and must not apply an actual-density correction a second time. ASHRAE Chapter 16 notes that air density changes with pressure, temperature, and humidity ratio rather than remaining equal to a universal standard value.

Reproducible 100 ACFM moisture example

Inputs and assumptions: steady 100 ACFM outdoor airflow measured at the outdoor state; 101.325 kPa barometric pressure; outdoor air at 32.0°C dry bulb and 60% RH; indoor target at 24.0°C dry bulb and 50% RH; 24 hours of operation; no ERV, leakage, cycling, storage, or condensate behavior inside the calculation boundary.

  1. Outdoor saturation vapor pressure: pws,o = 4.75853 kPa; outdoor vapor pressure: pw,o = 0.60 × 4.75853 = 2.85512 kPa.
  2. Wo = 0.621945 × 2.85512 / (101.325 − 2.85512) = 0.018033 kg/kgda.
  3. Indoor saturation vapor pressure: pws,i = 2.98513 kPa; indoor vapor pressure: pw,i = 1.49256 kPa.
  4. Wi = 0.621945 × 1.49256 / (101.325 − 1.49256) = 0.009299 kg/kgda.
  5. ΔW = Wo − Wi = 0.008735 kg/kgda.
  6. 100 ACFM = 0.0471947 m³/s. Outdoor specific volume is vo = 0.88952 m³/kgda.
  7. ṁda = 0.0471947 / 0.88952 = 0.053056 kgda/s = 191.003 kgda/h.
  8. ṁw = 191.003 × 0.008735 = 1.668 kg/h = 3.678 lb/h.
  9. mw = 1.668 × 24 = 40.041 kg/day ≈ 40.0 L/day, using approximately 1 kg/L for liquid water.

Interpretation: 40.0 L/day is the theoretical net water-vapor mass entering with the ventilation air relative to the stated indoor target and calculation boundary. It is not condensate production, dehumidifier capacity, or ERV downstream load. Actual condensate depends on equipment runtime and latent capacity, coil bypass, retained water, drainage, air mixing, and other moisture sources. A dehumidifier rating uses specified test conditions. An ERV changes the entering state according to its certified performance, leakage, balance, airflow, controls, frost operation, and outdoor conditions.

Exhaust, recovery, and controls change the real system boundary

Bath, kitchen, dryer, and other exhaust removes indoor air. Replacement air may enter through planned inlets, another ventilation device, or enclosure leakage, so measured flow and pressure balance are needed. Exhaust can reduce a strong local source while still creating an outdoor-air conditioning demand.

Balanced supply and exhaust do not eliminate conditioning load. An energy-recovery ventilator does not have one universal moisture-removal percentage; use certified sensible, latent, or total effectiveness at the applicable airflow and conditions. The ventilation rate should not be disabled as a general humidity fix. Coordinate required airflow, treatment, dew-point control, and the actual equipment sequence.

Measure the stream before changing cooling capacity

Commissioning should record ventilation airflow, duty cycle, outdoor and indoor dry-bulb and moisture conditions, pressure balance, intake location, equipment operation, and indoor dew-point trend. The intended ventilation assumptions should appear in the Manual J load calculation without counting the same stream again as infiltration.

Increasing AC tonnage can shorten cooling runtime and does not directly correct excessive ventilation, return leakage, or an untreated outdoor-air stream. Compare the calculated moisture rate with actual airflow, controls, and manufacturer performance. The whole-house dehumidifier versus AC guide explains the difference between temperature-driven cooling and independent moisture control; the return-air design guide covers distribution and room-pressure effects.

Frequently Asked Questions

Does lower outdoor RH always mean drier outdoor air?

No. Temperature and pressure must be included. Warmer air at a lower RH can have a higher humidity ratio or dew point.

Can an ERV remove all ventilation moisture?

No universal result applies. Use certified performance, airflow balance, operating mode, and actual outdoor and indoor conditions.

Should ventilation be turned off during humid weather?

Not as a general fix. Required or intended ventilation needs an approved control and conditioning strategy; disabling it can create indoor-air and code problems.

Revision history

  • August 21, 2026 — Technical evidence update: Added dry-air moisture mass-balance equations, actual-versus-standard airflow guidance, a reproducible 100 ACFM calculation, unit conversions, limitations, and claim-level ASHRAE references.

Sources and verification

Final ventilation rate, sizing, airflow, electrical, refrigerant, and control decisions must follow the exact equipment instructions and applicable local requirements.

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