Infiltration and Ventilation Load Calculator
Outdoor air that leaks in, or that a fan brings in, has to be heated or cooled and dried. Enter the airflow and your design conditions to see how much load it adds.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Heating load of the air
- 7,776BTU/h
- Total cooling load of the air
- 5,387BTU/h
133 CFM of outdoor air adds about 7,776 BTU/h of heating load and 5,387 BTU/h of cooling load at these design conditions.
- Sensible cooling
- 2,304BTU/h
- Latent (moisture) cooling
- 3,083BTU/h
- Cooling load
- 0.45tons
- Airflow used
- 133CFM
Show the working
- CFM = ACH × volume ÷ 600.5 × 16,000 ÷ 60 = 133.3 CFM
- Heating = 1.08 × CFM × ΔT1.08 × 133.3 × 54 = 7,776 BTU/h
- Sensible cooling = 1.08 × CFM × ΔT1.08 × 133.3 × 16 = 2,304 BTU/h
- Latent cooling = 0.68 × CFM × Δgrains0.68 × 133.3 × 34 = 3,083 BTU/h
The formulas
For standard air, the heat needed to warm or cool one CFM by one degree is 1.08 BTU/h, and the latent load of removing one grain of moisture per pound of air is 0.68 BTU/h per CFM:
- Heating or sensible cooling = 1.08 × CFM × temperature difference
- Latent cooling = 0.68 × CFM × moisture difference in grains per pound
The constants come from the density and specific heat of air at sea level. At altitude the air is thinner, so the tool scales them by the air density at your elevation.
Three ways to enter the airflow
If you know the CFM — a ventilation fan, an outdoor-air damper setting or a measured leakage rate — enter it directly.
From air changes per hour: CFM = ACH × volume ÷ 60. Use natural air changes, not the ACH50 from a blower-door test.
From a blower-door test: the average natural leakage is roughly CFM50 divided by an N-factor that depends on your climate zone, building height and wind shielding (the LBNL N-factor method). Look up the N-factor for your house rather than relying on the example value.
Reading the result
Air leakage is often one of the largest single parts of a house’s heat loss, and in humid climates its latent load matters for cooling equipment selection. Sealing leaks reduces both. Ventilation air is needed for indoor air quality, so it is better delivered on purpose — and through a heat or energy recovery ventilator in cold or humid climates — than left to random leaks.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Airflow from
- Air changes per hour and volume
- Natural air changes per hour
- 0.5
- Conditioned volume
- 16,000 ft³
- Indoor heating set point
- 70 °F
- Outdoor heating design temperature
- 16 °F
- Indoor cooling set point
- 75 °F
- Outdoor cooling design temperature
- 91 °F
- Moisture difference, outdoor − indoor
- 34 gr/lb
- Site elevation
- 0 ft
Result
- Heating load of the air
- 7,776 BTU/h
- Total cooling load of the air
- 5,387 BTU/h
- Sensible cooling
- 2,304 BTU/h
- Latent (moisture) cooling
- 3,083 BTU/h
- Cooling load
- 0.45 tons
- Airflow used
- 133 CFM
133 CFM of outdoor air adds about 7,776 BTU/h of heating load and 5,387 BTU/h of cooling load at these design conditions.
Assumptions
- Standard-air constants: sensible 1.08 BTU/h per CFM·°F and latent 0.68 BTU/h per CFM·gr/lb, scaled by air density at your elevation.
- Natural CFM from a blower test = CFM50 ÷ N (LBNL N-factor method); from air changes, CFM = ACH × volume ÷ 60.
Limitations
- Leakage changes with wind and temperature hour by hour; one airflow is used for both seasons. Summer leakage is usually lower than winter leakage.
- Ventilation through a heat or energy recovery ventilator carries only part of this load; use the ERV/HRV calculator for that.
Frequently asked questions
Why does the tool count no latent load in dry climates?
Latent load is the work of removing moisture. When the outdoor air is drier than the indoor air, leakage dries the house instead, so no latent cooling load is counted.
Is summer leakage the same as winter leakage?
Usually not. Stack effect is stronger when it is cold outside, so winter leakage tends to be higher. The tool uses one airflow for both seasons; enter a lower figure if you want a summer-only estimate.
What is the 0.018 factor I see in some heat-loss sheets?
It is the same physics per cubic foot per hour: 0.018 = 1.08 ÷ 60. Heat loss = 0.018 × ACH × volume × ΔT gives the same answer as 1.08 × CFM × ΔT.
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.
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
- Infiltration Guidance for Buildings at Design Conditions — NYS Clean Heat (NYSERDA and utilities). LBNL N-factor method: natural ACH = ACH50 ÷ N and CFMn = CFM50 ÷ N (example 10,112 ÷ 13 = 778), with N by climate zone, height and shielding.
- Air Change Rates in Rooms — The Engineering ToolBox. ACH = CFM × 60 / room volume.
- 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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