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Formula Used
Occupant heat gain is split into sensible and latent components, then adjusted.
- Sensible Load (W) = N × qs × D × S
- Latent Load (W) = N × ql × D × S
- Total Load (W) = Sensible + Latent
- BTU/h = W × 3.412
- Energy (kWh) = (Total W × hours) ÷ 1000
Here, N is occupants, q is heat rate per person, D is diversity, and S is safety factor.
How to Use This Calculator
- Enter the expected number of occupants for the zone.
- Select a preset activity level, or choose custom rates.
- Set diversity for partial attendance or shifting schedules.
- Add a safety factor to cover uncertainty and variation.
- Press Calculate to view loads above the form.
- Use CSV or PDF buttons for reports and submittals.
Example Data Table
| Scenario | Occupants | Activity | Diversity | Safety | Sensible (W) | Latent (W) | Total (BTU/h) |
|---|---|---|---|---|---|---|---|
| Small office | 12 | Office work | 1.00 | 1.10 | 990.0 | 726.0 | 5856.2 |
| Classroom | 30 | Standing, light | 0.90 | 1.15 | 2794.5 | 2014.9 | 16430.2 |
| Light workshop | 18 | Light work | 1.10 | 1.20 | 2613.6 | 1782.0 | 15000.7 |
Example outputs assume preset heat rates and standard conversions.
Occupant Heat Gain in Cooling Load Planning
1) Why occupant heat matters
People add both sensible heat (temperature rise) and latent heat (moisture). In occupied zones, this gain can rival lighting and plug loads, so it must be counted when sizing ventilation, cooling capacity, and dehumidification. Accurate people loads also improve diffuser selection, noise control, and comfort complaints during peak use. In renovation work, confirming occupant density prevents costly change orders.
2) Typical heat rates and what they mean
Design references commonly place sedentary occupants near 65–85 W sensible and 45–60 W latent per person. Light standing activity often rises toward 90 W sensible and 65 W latent, while moderate work can exceed 140 W sensible and 90 W latent. These ranges vary by clothing, air temperature, and air movement.
3) Sensible versus latent split
Sensible heat influences air temperature and coil sensible capacity. Latent heat increases humidity ratio and drives condensation load on cooling coils. A space with many occupants may require more latent capacity than expected, especially when outdoor air rates are high and relative humidity targets are tight.
4) Diversity factor for real schedules
Diversity reflects that not everyone is present at the same time. For offices with staggered attendance, 0.7–0.9 is common; for classrooms, meeting rooms, or peak events, 0.9–1.0 is typical. Using diversity helps avoid oversizing while still modeling realistic peak periods.
5) Safety factor for uncertainty
Safety adds margin for unknowns such as higher activity, denser seating, or temporary crowding. Many designers use 1.05–1.15 for routine zones and may increase it for critical comfort areas. Apply safety after diversity so the margin protects the final expected peak.
6) Unit conversions and reporting
This calculator reports watts and BTU/h. The conversion is 1 W ≈ 3.412 BTU/h, so a 1,000 W internal gain equals about 3,412 BTU/h. Clear reporting helps coordinate with equipment schedules, submittals, and commissioning checklists.
7) Energy over time for operations
Energy is calculated as (total watts × hours) ÷ 1000 to produce kWh. For example, a 1.7 kW combined occupant load over 8 hours contributes roughly 13.6 kWh. Pairing this estimate with occupancy schedules supports control sequences, setback strategies, and demand planning.
8) Good practice for project use
Use presets during early design, then switch to custom rates if your specification lists people heat values. Keep assumptions documented: occupants, activity, diversity, safety, and duration. When space usage changes, update the inputs and export a fresh CSV or PDF to maintain traceable calculations.
FAQs
1) What is the difference between sensible and latent heat?
Sensible heat raises air temperature. Latent heat adds moisture, increasing humidity and dehumidification demand. Both must be included to size coils, airflow, and comfort targets correctly.
2) When should I use custom heat rates?
Use custom rates when your project standards, codes, or consultant guidelines specify people heat values. It is also useful for unusual clothing levels, elevated temperatures, or specialized process areas.
3) What diversity factor should I choose?
Choose diversity based on realistic simultaneous occupancy. Offices often use 0.7–0.9, while classrooms and meetings trend 0.9–1.0. If the peak is uncertain, stay conservative and document the assumption.
4) Why include a safety factor?
A safety factor covers uncertainty from behavior, schedule variation, or short-term crowding. A modest margin like 1.05–1.15 helps protect comfort without pushing equipment into excessive oversizing.
5) How do watts relate to BTU per hour?
Multiply watts by 3.412 to get BTU/h. For example, 500 W is about 1,706 BTU/h. The calculator reports both to match metric and imperial project documentation.
6) Does occupant heat affect ventilation sizing?
Indirectly, yes. Higher occupant count increases outdoor air requirements, which can raise latent load depending on climate. Always coordinate people loads with ventilation rates and humidity targets.
7) Is the kWh result the same as HVAC energy use?
No. It is the internal heat added by occupants over time. Actual HVAC energy depends on system efficiency, controls, outdoor conditions, and whether heat gains reduce heating needs or increase cooling demand.
Estimate occupant heat quickly for better cooling and comfort.