Outdoor Air Load Calculator
Calculate how much cooling it takes to bring outdoor ventilation air to indoor conditions — split into sensible and latent — or how much heat a reheat coil needs.
Method reviewed October 25, 2026. Version 1.0.0. How it works
Result
- Total cooling load of the outdoor air
- 11,054BTU/h
- Total load
- 0.92tons
200 CFM of outdoor air adds about 11,054 BTU/h (0.92 tons) of cooling load.
- Sensible
- 4,320BTU/h
- Latent (moisture)
- 6,734BTU/h
- Outdoor enthalpy
- 40.39BTU/lb
- Indoor enthalpy
- 28.11BTU/lb
Show the working
- Total = 4.5 × density ratio × CFM × Δh4.5 × 1 × 200 × (40.39 − 28.11) = 11,054 BTU/h
- Sensible = 1.08 × density ratio × CFM × ΔT4,320 BTU/h
- Latent = total − sensible6,734 BTU/h
Sensible, latent and total
Outdoor air brings in both heat and moisture. The total load is the airflow times the enthalpy difference between outdoor and indoor air: 4.5 × CFM × Δh, in BTU/h. The sensible part is the temperature change, 1.08 × CFM × ΔT; the latent part is the rest, the energy needed to condense out the extra moisture.
The constants 1.08 and 4.5 are for standard air at sea level, and the calculator scales them for air density at your elevation. Enthalpies come from the ASHRAE psychrometric equations.
Reheat mode
To dehumidify, a coil often cools air below the temperature the room wants, and the air is then reheated. Reheat mode calculates the sensible heat needed to warm the air from the coil-leaving temperature to the supply temperature you want: 1.08 × CFM × ΔT, density-corrected.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Calculate
- Outdoor (ventilation) air load
- Airflow
- 200 CFM
- Outdoor dry-bulb
- 95 °F
- Outdoor relative humidity
- 45 %
- Indoor dry-bulb
- 75 °F
- Indoor relative humidity
- 50 %
- Site elevation
- 0 ft
Result
- Total cooling load of the outdoor air
- 11,054 BTU/h
- Total load
- 0.92 tons
- Sensible
- 4,320 BTU/h
- Latent (moisture)
- 6,734 BTU/h
- Outdoor enthalpy
- 40.39 BTU/lb
- Indoor enthalpy
- 28.11 BTU/lb
200 CFM of outdoor air adds about 11,054 BTU/h (0.92 tons) of cooling load.
Assumptions
- Standard-air constants (1.08 sensible, 4.5 total), scaled by air density at your elevation; enthalpies from the ASHRAE 2017 equations (PsychroLib).
Limitations
- Steady conditions at one design point; use hourly weather data for annual energy.
Frequently asked questions
How much ventilation air does a home need?
ASHRAE Standard 62.2 sets the rate for dwellings from floor area and number of bedrooms, and many codes adopt it. This calculator takes the airflow as an input; it does not set it.
Why is the latent load so large in humid climates?
Removing moisture takes a lot of energy: about 1,000 BTU for each pound of water condensed. Humid outdoor air can carry far more moisture than indoor air, so the latent part can exceed the sensible part.
Does an ERV or HRV reduce this load?
Yes. An energy- or heat-recovery ventilator transfers part of the heat (and, for an ERV, moisture) between the exhaust and incoming air. Multiply the load by (1 − the unit’s rated effectiveness) for a rough estimate.
Sources
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
- PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
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