Advanced Infiltration Heat Gain Calculator

Calculator Inputs

The page uses one main column, while the form below switches to three columns on large screens, two on small screens, and one on mobile.

Airflow input method

Volume input style

Operating hours per day

Room length (ft)

Room width (ft)

Room height (ft)

Direct room volume (ft³)

Air changes per hour (ACH)

Direct infiltration airflow (CFM)

Indoor dry-bulb temperature (°F)

Indoor relative humidity (%)

Outdoor dry-bulb temperature (°F)

Outdoor relative humidity (%)

Barometric pressure (psia)

Custom air density (lb/ft³)

Leakage / opening multiplier

Safety factor (%)

Reset

Formula Used

1) Room volume

Volume = Length × Width × Height

2) Infiltration airflow from ACH

CFM = (ACH × Volume) ÷ 60

3) Humidity ratio from dry-bulb temperature and RH

W = 0.62198 × Pv ÷ (P − Pv)

Pv is vapor pressure. P is barometric pressure. The page estimates vapor pressure from dry-bulb temperature and RH.

4) Moist air enthalpy

h = 0.24T + W(1061 + 0.444T)

T is in °F. h is in Btu per pound of dry air.

5) Total infiltration load

Total Load = ṁ_{dry air} × (h_out − h_in)

6) Sensible infiltration load

Sensible Load = ṁ_{dry air} × 0.24 × (T_out − T_in)

7) Latent infiltration load

Latent Load = Total Load − Sensible Load

How to Use This Calculator

Select whether you want to enter infiltration by ACH or direct CFM.
Choose whether room volume will come from dimensions or direct volume.
Enter indoor and outdoor temperature and relative humidity values.
Enter pressure for site correction, or leave density blank for auto-estimation.
Apply a leakage multiplier and optional safety factor for design allowance.
Press the calculate button to show results above the form.
Review the graph, result table, and export options for reporting.

Example Data Table

Project Type	Space Size	Ceiling Height	ACH / CFM Basis	Indoor Condition	Outdoor Condition	Typical Use Note
Small office	1,200 ft²	10 ft	0.35 ACH	75°F / 50% RH	92°F / 60% RH	Light envelope leakage and normal occupancy.
Retail entry zone	2,000 ft²	12 ft	650 CFM	74°F / 50% RH	95°F / 65% RH	Frequent door opening increases infiltration.
Warehouse support room	3,600 ft²	16 ft	0.80 ACH	78°F / 55% RH	100°F / 55% RH	Dock traffic and pressure imbalance matter.
Lobby with vestibule	1,500 ft²	14 ft	0.50 ACH	75°F / 50% RH	90°F / 70% RH	Vestibule reduces direct outdoor mixing.

FAQs

1) What does infiltration heat gain mean?

It is the cooling load caused when unwanted outdoor air leaks into a conditioned building. That incoming air changes room temperature and moisture content, increasing equipment load.

2) When should I use ACH instead of CFM?

Use ACH when leakage is estimated from building size and air change assumptions. Use direct CFM when blower-door, balancing, or field measurements already provide airflow.

3) Why are both sensible and latent loads shown?

Sensible load reflects temperature change. Latent load reflects moisture added or removed. Both matter because cooling equipment must handle dry-bulb control and humidity control together.

4) Why does pressure affect the result?

Pressure changes air density. Density changes mass flow, and mass flow changes heat transfer. This matters more at high elevations or unusual weather conditions.

5) Can the result ever be negative?

Yes. If outdoor air is cooler or less energetic than indoor air, infiltration can reduce cooling demand. The tool still reports the signed value so you can see relief instead of gain.

6) What does the leakage multiplier do?

It scales the base infiltration airflow. Use it to represent door opening frequency, envelope quality, pressure imbalance, or conservative design adjustments during early estimating.

7) Why add a safety factor?

A safety factor increases design airflow beyond the raw estimate. It helps account for uncertainty, future operation changes, and incomplete field information during preliminary sizing.

8) Is this suitable for final engineered design?

It is useful for advanced estimating and quick checks. Final design should still confirm envelope leakage, psychrometric assumptions, schedules, and code requirements with project-specific engineering review.