Calculator Inputs
Formula Used
IP air sensible formula: Q = 1.08 × CFM × ΔT
SI air sensible formula: Q = 0.33 × m3/h × ΔT
Total internal sensible gains: Occupants + Lighting + Equipment + Envelope/Solar
Subtotal sensible load: Air sensible load + Internal sensible gains
Final sensible load: Subtotal × (1 + Safety Factor ÷ 100)
This calculator uses the absolute temperature difference for quick design checks. For cooling, the room condition is normally warmer than the supply condition.
How to Use This Calculator
- Select the unit system you want to use.
- Enter airflow, room temperature, and supply air temperature.
- Add people, lighting, equipment, and envelope sensible gains.
- Set a safety factor for design margin.
- Click the calculate button to show the result above the form.
- Review the graph, compare components, and export the report as CSV or PDF.
Example Data Table
| Scenario | Airflow | Room Temp | Supply Temp | Internal Gains | Estimated Sensible Load |
|---|---|---|---|---|---|
| Small office | 1200 CFM | 75 °F | 56 °F | 7,200 BTU/h | 31,824 BTU/h |
| Retail zone | 1800 CFM | 76 °F | 55 °F | 12,000 BTU/h | 52,824 BTU/h |
| Classroom | 2400 CFM | 75 °F | 57 °F | 15,500 BTU/h | 62,156 BTU/h |
These examples are for comparison only. Actual projects should use your measured airflow and realistic sensible gain assumptions.
Frequently Asked Questions
1) What is sensible load in HVAC?
Sensible load is the portion of cooling or heating that changes dry-bulb air temperature without changing moisture content. It comes from airflow temperature difference and internal heat sources such as lights, people, equipment, and solar gains.
2) Why does airflow matter so much?
Airflow directly controls how much heat the system can move. With a larger airflow rate and the same temperature difference, the air stream carries more sensible energy, so the sensible load handled by the system increases.
3) Why is there a 1.08 constant in IP units?
The 1.08 factor combines air density, specific heat, and minutes-to-hours conversion for typical standard air conditions. It gives a convenient estimate of sensible heat transfer in BTU per hour when airflow is entered in CFM.
4) Why is there a 0.33 constant in SI units?
The 0.33 constant is a practical shortcut for sensible heat when airflow is entered in cubic meters per hour and temperature difference is in degrees Celsius. It produces an approximate result in watts for quick design checks.
5) Should I use room temperature minus supply temperature?
Yes. For sensible cooling checks, the room air is usually warmer than the supply air. This calculator uses the absolute difference so the result stays positive for quick planning, summaries, and comparison between options.
6) Are occupants always a big part of the load?
Not always. Occupants can dominate in classrooms, meeting rooms, and gyms, but lighting, plug loads, or solar gains may be larger in offices, stores, or perimeter zones. The chart helps you see the strongest contributor.
7) What safety factor should I choose?
A modest safety factor, such as 5% to 10%, is common for early sizing. Oversizing too much can reduce efficiency and comfort, so it is better to use realistic inputs instead of relying on a large margin.
8) Does this replace a full HVAC design?
No. It is a quick sensible load tool for screening and planning. A full design should also consider latent load, ventilation rules, duct pressure losses, zoning, schedules, diversity, equipment selection, and local design conditions.