Calculator Inputs
Example Data Table
| Scenario | Airflow | Outdoor / Indoor | Humidity Difference | Typical Use |
|---|---|---|---|---|
| Small site office | 250 CFM | 92°F / 75°F | 34 gr/lb | Temporary cooling check |
| Retail shell | 900 CFM | 95°F / 74°F | 46 gr/lb | Make up air sizing |
| Classroom zone | 1,250 CFM | 93°F / 75°F | 42 gr/lb | Outdoor air unit review |
Formula Used
- People plus area airflow: CFM = people × CFM/person + floor area × CFM/ft².
- Air changes airflow: CFM = room volume × ACH ÷ 60.
- Sensible load: BTU/h = 1.08 × CFM × ΔT × density factor × (1 − sensible recovery).
- Latent load: BTU/h = 0.68 × CFM × Δgrains × density factor × (1 − latent recovery).
- Total load: BTU/h = (sensible + latent) × (1 + safety factor).
- Cooling tons: tons = total BTU/h ÷ 12,000.
- Cooling kW: kW = total BTU/h ÷ 3,412.142.
The constants 1.08 and 0.68 are common IP design factors. They assume standard air properties. The density factor lets you adjust results for elevation or unusual air conditions.
How to Use This Calculator
- Select the airflow method that matches your available data.
- Enter design outdoor and target indoor dry bulb temperatures.
- Enter outdoor and indoor humidity ratios in grains per pound.
- Add recovery percentages when a verified recovery device is included.
- Add a safety factor that matches your design stage.
- Press the calculate button and review the result above the form.
- Use the CSV or PDF buttons to save the calculation summary.
Fresh Air Cooling Load Design Guide
Why Fresh Air Load Matters
Fresh air improves comfort and indoor air quality. It also adds heat and moisture. A designer must size cooling equipment for that added load. Outdoor air may look small beside room load, yet it can dominate humid climates. This calculator separates sensible and latent parts, so the result is easier to review.
Key Design Inputs
The first input is airflow. You can enter direct CFM, liters per second, people plus floor area, or air changes per hour. The next inputs compare outdoor and indoor dry bulb temperatures. That difference drives the sensible load. Humidity ratio in grains per pound drives the latent load. Recovery wheel effectiveness reduces each portion before the safety factor is added.
Reading the Results
The result shows fresh air flow, sensible load, latent load, total BTU per hour, tons of refrigeration, and cooling kilowatts. It also estimates seasonal energy. Use the split between sensible and latent loads to decide whether coil capacity, dehumidification, or energy recovery needs more attention.
Construction Use Cases
During construction planning, the tool helps compare temporary ventilation, make up air, and permanent HVAC assumptions. It is useful for site offices, retail shells, apartments, clinics, schools, and mechanical rooms. It can also check whether a proposed energy recovery unit has enough benefit to justify its pressure drop and maintenance needs.
Good Practice Notes
Use local design weather data whenever possible. Enter indoor conditions that match the project criteria. Keep humidity values realistic. Do not hide high latent load with a large safety factor. Review code ventilation rates separately. Final equipment selection should consider coil bypass factor, fan heat, duct leakage, filtration, diversity, controls, and part load performance.
Common Design Checks
Compare the calculated tons against available coil capacity. Then review the latent share. A high latent share can require lower coil temperatures, reheat, or dedicated outdoor air equipment. For temporary work, confirm power limits and condensate removal. For final work, coordinate controls with occupancy schedules. Small errors can grow during peak weather.
Interpreting Recovery
Sensible recovery lowers temperature load. Latent recovery lowers moisture load. Enter each value only when the selected device can deliver it at design conditions. Fouled filters, frost limits, wheel purge, and bypass dampers can reduce real performance. Conservative inputs usually produce safer early budgets.
FAQs
What is fresh air cooling load?
It is the cooling capacity needed to condition outdoor ventilation air before it enters the occupied space. It includes both temperature load and moisture load.
Why are sensible and latent loads separated?
Sensible load comes from temperature difference. Latent load comes from moisture difference. Separating them helps designers check coil capacity and dehumidification needs more clearly.
What airflow method should I use?
Use direct CFM when known. Use people plus area for code style ventilation checks. Use ACH for rough space exchange estimates. Use L/s when metric airflow is available.
What is humidity ratio in grains per pound?
It expresses moisture mass in air. One pound of dry air carries a certain number of grains of water vapor. Higher grain difference increases latent cooling load.
Can this calculator size final equipment?
It can support early sizing and design checks. Final selection should include local code, detailed psychrometrics, coil data, fan heat, duct leakage, controls, and engineer review.
What does heat recovery do here?
Heat recovery reduces the outdoor air load before it reaches the cooling coil. Sensible recovery affects temperature load. Latent recovery affects moisture load.
Why is a safety factor included?
A safety factor covers early design uncertainty. It should not replace accurate weather data, real ventilation rates, or proper equipment performance checks.
What does the density factor mean?
It adjusts the standard load constants for air density changes. Use 1.00 for normal early estimates. Reduce it at higher elevations when appropriate data is available.
Why is the load shown in tons?
Cooling equipment is often described in tons. One ton equals 12,000 BTU per hour. The calculator also shows BTU per hour and cooling kilowatts.
Does the tool include fan heat?
No. The main result focuses on conditioning outdoor air. Add fan heat separately when fan power, motor location, and system arrangement are known.
Can I use this for temporary construction cooling?
Yes. It helps estimate outdoor air impact for site offices, temporary make up air, and phased occupancy. Verify power limits and condensate handling before installation.