Compare outdoor-air states and flows with target conditions to calculate cooling, heating, and moisture duties.
Concepts behind this calculation
Compare sensible, moisture and total outdoor-air treatment duty across entered weather states using either one shared actual airflow or a separate airflow for each state.
Outdoor-air conditioning load compares the entering outdoor state with a selected target air state. Actual outdoor CFM is converted to dry-air mass using the specific volume of each outdoor state. Humidity ratio and enthalpy then describe the moisture and energy changes; identical actual CFM across cases need not carry identical dry-air mass.
Sensible and latent components describe the stated process convention, while their selected total governs the air-stream balance. Opposing temperature and moisture demands can cancel in a small net total. Liquid-drain correction additionally accounts for discharged water energy and is restricted to compatible above-freezing water removal. Alternative weather cases are separate possibilities, not simultaneous loads.
How to use this calculator
Document Weather/design condition reference and select Outdoor airflow across cases.
Enter Absolute barometric pressure, Target air dry-bulb, and Target air relative humidity.
Use Outdoor actual airflow per scenario with Outdoor design-state rows, or complete Per-case airflow and outdoor-state rows with coincident conditions.
Select Energy convention and, if applicable, supply Drained liquid-water specific enthalpy; inspect each row’s signed components before comparing maxima.
p/pw = total/vapor pressure (psia); W = humidity ratio (lb water/lb dry air); h = enthalpy (BTU/lb dry air); v = specific volume (ft³/lb dry air); mda = dry-air mass flow (lb/h). Thermal Q terms are BTU/h, whereas Qout,actual is outdoor ACFM.
Input definitions and units
Weather/design condition reference
Required: record location/station, data source/edition, design percentile, condition pairing and row meanings. For measured cases record sensor/date/basis. No station or percentile is inferred.
Outdoor airflow across cases
Use one measured/design actual flow for all weather alternatives, or enter the actual delivered ventilation flow separately for each operating state. This is not an annual energy schedule. Choices: One shared airflow; Airflow varies by case.
Outdoor actual airflow per scenario (ACFM)
Used in shared-flow mode only. Actual volume at each entered outdoor state, not SCFM or target-room CFM; dry-air mass changes with state-specific specific volume. Applies to Outdoor airflow across cases: One shared airflow.
Absolute barometric pressure (psia)
Same site pressure for outdoor and target states. Do not enter gauge/static duct pressure. No station elevation or pressure is inferred.
Target air dry-bulb (°F)
The desired ventilation-air state, which may be room-equivalent or a supply condition. Not automatically a coil leaving condition.
Target air relative humidity (%)
Use RH matching target dry-bulb and pressure. Source convention uses saturation over ice below the triple point; check subfreezing weather RH conventions.
Energy convention
Air-stream duty compares air enthalpies. Liquid-drain correction additionally subtracts discharged water enthalpy; use only for above-freezing dehumidification, not humidification. Choices: Air-stream enthalpy duty; Include liquid condensate drainage.
Drained liquid-water specific enthalpy (BTU/lb water)
Use a liquid-water property at the drain temperature on the same ASHRAE enthalpy reference (approximately zero at 32°F). 18.07 is the Handbook example at 50°F, not a default for every target. Applies to Energy convention: Include liquid condensate drainage.
Outdoor design-state rows (°F, RH %)
Shared-flow mode: enter 1–20 rows of temperature and RH separated by comma; the same ACFM applies to every row. Document case order and use coincident pairs from each chosen design category; do not combine independent maxima. Applies to Outdoor airflow across cases: One shared airflow.
Per-case airflow and outdoor-state rows (ACFM, °F, RH %)
Per-state schedule mode: enter 1–20 rows of actual outdoor ACFM, dry-bulb temperature and RH. Enter zero ACFM for an explicit off case. These are discrete operating scenarios, not annual energy bins. Applies to Outdoor airflow across cases: Airflow varies by case.
How to interpret the result
Positive removal represents cooling/dehumidification duty; negative total represents heating/humidification demand. Case maxima apply only to supplied alternatives and do not constitute a whole-building equipment load.
Worked examples
One shared airflow
Inputs: Shared-flow mode: 1,000 outdoor ACFM at 14.696 psia; target 75°F / 50% RH. Outdoor rows: 95°F / 60%, 82°F / 90%, 100°F / 40%. All conditions are constructed examples, not a weather-station dataset.
Calculation: For each state, calculate dry-air mass from the shared 1,000 ACFM and that state's specific volume; compute total enthalpy removal and sensible load. Latent is the selected total balance minus sensible.
Result: Maximum cooling removal 76,099.493 BTU/h in Case 1. Separate sensible/latent maxima must not be added.
Airflow varies by case
Inputs: Per-state airflow mode: 1,000 ACFM at 95°F / 60% RH; 500 ACFM at 82°F / 90% RH; 2,000 ACFM at 100°F / 40% RH; target 75°F / 50% RH at 14.696 psia. Constructed cases only.
Calculation: Each row uses its own ACFM and outdoor specific volume to calculate dry-air mass. Total removal scales with that row's mass flow; rows are evaluated independently.
Result: Case 1: 76,099.493 BTU/h; Case 2: 31,826.375 BTU/h; Case 3: 117,219.292 BTU/h. The controlling cooling case is Case 3.
Assumptions and limits
Calculates outdoor ventilation air only. It does not determine required ventilation flow, whole-building loads, duct heat gain, supply-fan heat, mixing, reheat, equipment capacity, recovery efficiency or code compliance.
Dry-air mass is calculated separately for each state as 60 ×ACFM/v, not an assumed4.5×CFM. Shared-flow mode applies one entered actual CFM to every state; per-state mode uses the corresponding row's actual CFM. Standard-volume, room-volume or fixed-mass input is not automatically converted.
Named psychrometric basis: ASHRAE 2017 perfect-gas humidity ratio, enthalpy and volume relations with SI Hyland-Wexler saturation coefficients and a triple-point branch. These engineering approximations are not real-gas IAPWS properties or a complete psychrometric tool.
Supported temperatures −40 to 140°F; RH 0–100%. Below the water triple point, the saturation relation is over ice. Many weather RH records use a liquid-water reference below freezing; convert such data before entry rather than mixing conventions. Vapor partial pressure must remain below total pressure.
Sensible/latent separation follows the stated temperature change at fixed OUTDOOR humidity ratio, then moisture change at target dry-bulb. Another process convention can allocate components differently; their selected total balance remains the governing result.
Positive removal means cooling/dehumidification; negative means heating/humidification demand. A net enthalpy near zero can still require separate sensible cooling/heating and moisture treatment. Air-stream duty excludes condensate and water/steam-side enthalpy.
Liquid-drain mode subtracts discharged liquid enthalpy and is restricted to above-freezing water removal for every scenario. It does not model frosting or a humidifier. Drain enthalpy must match its actual temperature and reference; do not use18.07 universally.
No weather database, station autofill or design-percentile selection is embedded. The reference text is user supplied and not authenticated. Use documented coincident pairs, including appropriate dehumidification/enthalpy conditions, not only the hottest dry bulb.
Case maxima are across alternatives, not simultaneous loads. Per-state mode evaluates discrete rows only and does not integrate runtime, annual energy or control transitions. Approximate property equations and rounded source chart benchmarks have stated verification limits.
FAQs
Can I add the largest sensible and latent loads from different rows?
That creates a case not represented by the entered conditions. Review each complete coincident state and carry the controlling scenario into further design.
Does a near-zero total mean the outdoor air needs no treatment?
No. Sensible cooling can offset a moisture-related heating contribution, or the reverse. Inspect the signed components and required humidity change even when net enthalpy duty is small.
Sources
ASHRAE 2017 Fundamentals, IP Chapter 1: PsychrometricsOriginal Equations 20, 25, 26, 30, 41, and 43 directly inspected. Example 3 supplies a rounded cooling/dehumidification case and liquid-drain energy balance; source chart rounding is kept distinct from the continuous-equation calculation.
ASHRAE 2017 Fundamentals, SI Chapter 1Hyland-Wexler saturation coefficients directly inspected in Batch14; reused here. Water/ice branch convention documented in the method.
CaptiveAire: Outdoor Air Load CalculatorDirect same-intent peer: distinguishes cooling, dehumidification and enthalpy design scenarios. Its weather dataset and fixed-coefficient formulas are not copied; this page requires your own traceable design states.
