HVAC Latent Load Calculator

Calculator Inputs

This calculator estimates moisture-driven cooling load from ventilation, infiltration, and people. Results appear above this form after calculation.

Project Name

Unit System

Site Altitude (ft)

Used to estimate local barometric pressure.

Ventilation Air (CFM)

Infiltration Air (CFM)

Occupants

Latent Gain Per Person (Btu/h-person)

Outdoor Dry Bulb (°F)

Outdoor RH (%)

Indoor Dry Bulb (°F)

Indoor RH (%)

Safety Factor (%)

Operating Hours Per Day

Operating Days Per Month

Moisture Removal COP

Used for monthly energy estimation.

Electric Rate (per kWh)

Example Data Table

Case	Vent Air	Infiltration	Outdoor Condition	Indoor Condition	People	Latent Load
Small Office	500 CFM	70 CFM	92°F, 58% RH	75°F, 50% RH	10	17,960 Btu/h
Retail Space	900 CFM	160 CFM	95°F, 62% RH	74°F, 50% RH	22	34,810 Btu/h
Conference Room	700 CFM	90 CFM	90°F, 55% RH	73°F, 48% RH	30	27,420 Btu/h

Formula Used

1) Saturation vapor pressure:
P_ws = 0.61078 × exp[(17.2694 × T) / (T + 237.3)]

2) Humidity ratio:
W = 0.62198 × P_v / (P - P_v)

3) Humidity difference:
ΔW = max(W_outdoor - W_indoor, 0)

4) Outdoor-air latent load:
Q_latent = 0.68 × CFM × Δgrains

5) Occupant latent load:
Q_people = Number of occupants × latent gain per person

6) Total latent load:
Q_total = (Ventilation + Infiltration + People) × (1 + Safety Factor)

The constant 0.68 is commonly used for standard air-density HVAC latent-load estimates in IP units.

How to Use This Calculator

Choose IP or SI units.
Enter project name and site altitude.
Enter ventilation and infiltration airflow rates.
Enter outdoor and indoor dry-bulb temperature and relative humidity.
Add occupant count and latent gain per person.
Set safety factor, operating hours, days, COP, and electric rate.
Click Calculate Latent Load.
Review the result block above the form, graph, and export options.

Frequently Asked Questions

1) What is HVAC latent load?

Latent load is the cooling needed to remove moisture from air. It does not lower dry-bulb temperature directly. Instead, it condenses water vapor and helps maintain indoor humidity within the design target.

2) Why does humidity ratio matter more than temperature here?

Latent load depends on moisture content difference between outdoor and indoor air. Humidity ratio measures actual water vapor in the air, so it is more useful than temperature alone for moisture calculations.

3) Why are ventilation and infiltration separated?

Ventilation is intentional outdoor air brought in by design. Infiltration is uncontrolled leakage through cracks and doors. Splitting them helps show which source is adding more moisture burden to the HVAC system.

4) What happens if outdoor air is drier than indoor air?

This tool limits outdoor-air latent load to zero when outdoor humidity ratio is lower than indoor design humidity ratio. In that case, outside air does not add moisture load to the system.

5) What latent gain per person should I enter?

Use a value that matches occupancy activity. Seated office occupants are lower, while gyms or active spaces are higher. If you have a design handbook value, use that directly for better project accuracy.

6) Why include a safety factor?

A safety factor covers uncertainty from varying occupancy, weather swings, door openings, and real operating conditions. It should stay reasonable, because a very high factor can push equipment selection beyond true design need.

7) What does moisture removal in liters per day mean?

It estimates how much water the system must condense during the entered daily runtime. This is useful for understanding condensate production and comparing latent load severity across different operating schedules.

8) Can I use SI values instead of IP values?

Yes. Switch to SI mode and enter airflow in L/s, temperatures in °C, altitude in meters, and latent gain per person in watts. The calculator converts values internally and still reports detailed results.