HVAC Latent Load Calculator

1. Understanding latent load in HVAC design

Latent load is the cooling demand needed to remove water vapor from air. It rises when humid outdoor air enters the building or when indoor activities add moisture. In warm, humid weather, latent capacity can be a limiting factor for comfort and condensation control.

2. Humidity ratio and why it matters

Humidity ratio W measures moisture mass per dry air mass. Many workflows use g/kg, grains/lb, or lb/lb. Comfort targets often fall near 8–12 g/kg, while outdoor air can reach 14–20 g/kg. The difference in W is the driver behind latent load.

3. Airflow method and the 4840 constant

The airflow method estimates latent load from volume flow and humidity ratio change. This tool uses Q = 4840 × CFM × ΔW with ΔW in lb/lb. Example: 200 CFM with a 4 g/kg increase (ΔW = 0.004) adds about 3,872 Btu/hr.

4. Moisture generation sources in buildings

Common sources include occupants, cooking, showers, cleaning, and processes like drying or open water. If you can estimate or measure moisture generation, use Q = ṁ_w × h_fg. A 2 lb/hr moisture source with h_fg ≈ 1061 Btu/lb contributes roughly 2,122 Btu/hr.

5. Ventilation versus infiltration impacts

Ventilation is intentional outdoor air for air quality, while infiltration is uncontrolled leakage. Both can import humidity. When outdoor air is drier than the indoor target, the added latent load becomes zero in this model. When it is wetter, the penalty scales rapidly with airflow.

6. People latent gains and activity levels

People add moisture through respiration and perspiration, and the rate changes with activity. Presets cover seated, light work, and moderate activity, plus a custom entry for site data. For example, five seated occupants at 200 Btu/hr each add 1,000 Btu/hr of latent load.

7. Converting results to kW and tons

Reports may require kW or refrigeration tons. Conversions used here are kW = Btu/hr × 0.00029307 and tons = Btu/hr ÷ 12,000. A 7,744 Btu/hr latent load equals about 2.27 kW and 0.65 tons.

8. Interpreting numbers for equipment selection

Use the breakdown to see what drives moisture: coil removal, outdoor air, infiltration, people, or processes. If outdoor air dominates, consider energy recovery or dedicated dehumidification. If infiltration dominates, sealing and pressure control help. Apply a safety margin and verify with psychrometric checks.

1) What is latent load versus sensible load?

Latent load removes moisture from air, changing humidity. Sensible load changes dry-bulb temperature without changing moisture content. Total cooling capacity must cover both to maintain comfort and prevent condensation.

2) Which humidity unit should I use?

Use any unit you can source reliably. g/kg is common in psychrometric outputs, grains/lb is common in HVAC practice, and lb/lb is used in equations. The calculator converts them internally.

3) Why does the airflow method sometimes return zero?

If leaving humidity is higher than entering humidity, the model assumes no dehumidification is occurring and sets moisture removal to zero. Recheck sensor locations, units, and whether air is being humidified downstream.

4) What value should I use for h_fg?

For typical HVAC design, 1061 Btu/lb is a widely used approximation. If you need higher accuracy, use a value consistent with your temperature range and psychrometric reference method.

5) How do I estimate infiltration airflow?

You can use blower door data, air-change assumptions, or envelope leakage models to estimate an equivalent CFM. If you only have ACH, convert to CFM using room volume and divide by 60.

6) Should I add ventilation and infiltration together?

Yes, if both occur simultaneously, their latent loads add. Keep them separate in the calculator to see which dominates. This helps prioritize design actions like sealing, pressure control, or outdoor-air treatment.

7) How accurate are these results?

The calculations use standard HVAC approximations and depend on input quality. Airflow, humidity ratio, and moisture rate uncertainty can shift results materially. Use this as a design estimate and validate critical projects with detailed psychrometrics.