Inputs
Formula used
This calculator uses a sensible heat balance on the air stream:
Q = ρ · Cp · V̇ · (Tsupply − Troom) (heating)
Rearranged for supply air temperature:
Tsupply = Troom + Q / (ρ · Cp · V̇) (heating)
Where Q is sensible load, ρ is air density, Cp is specific heat, and V̇ is volumetric airflow.
How to use this calculator
- Choose unit system and mode (cooling or heating).
- Enter room temperature, sensible load, and airflow.
- Adjust air density and Cp for your site assumptions.
- Optionally enter return/outdoor temperatures and outdoor air fraction.
- Press Calculate to show results above the form.
- Use CSV/PDF buttons to export the computed report.
Example data table
| Mode | Room/Zone Temp (°C) | Sensible Load (kW) | Airflow (m³/s) | Density (kg/m³) | Cp (kJ/kg·K) |
|---|---|---|---|---|---|
| Cooling | 24 | 18 | 1.40 | 1.20 | 1.006 |
| Cooling | 23 | 9 | 0.80 | 1.18 | 1.006 |
| Heating | 21 | 12 | 1.10 | 1.22 | 1.006 |
These sample rows demonstrate typical scenarios for quick validation.
Design intent and typical ranges
Supply air temperature is usually selected to meet sensible load without excessive airflow. For comfort cooling, many systems target 12–15°C supply to maintain 22–25°C zones, depending on diffuser performance and mixing. Heating applications often deliver 30–45°C supply, constrained by occupant comfort and terminal equipment limits.
Load sensitivity to airflow
The sensible balance shows ΔT is inversely proportional to volumetric flow. At ρ=1.20 kg/m³ and Cp=1.006 kJ/kg·K, a 10 kW zone load with 1.0 m³/s requires about 8.3°C of temperature drop, while 1.5 m³/s reduces the drop to about 5.5°C. This explains why fan turndown can quickly increase required supply temperature in cooling.
Impact of air properties
Air density varies with altitude and temperature; a 10% decrease in ρ increases ΔT by roughly 11% for the same load and airflow. Moist air Cp is slightly higher than dry air, so using Cp≈1.02 kJ/kg·K can reduce ΔT by about 1–2% in humid conditions. For high-elevation sites, updating ρ materially improves setpoint estimates.
Mixed air and coil implications
When return and outdoor air are mixed, entering temperature shifts with outdoor fraction. For example, Treturn=26°C, Tout=35°C, and OA=0.20 gives Tmix=27.8°C. If the computed supply target is 14°C, the coil sensible duty becomes ρ·Cp·V̇·(27.8−14), which is about 16.6 kW at 1.0 m³/s.
Operational checks and constraints
Low supply temperatures can drive condensation risk and require adequate drainage, insulation, and control of coil leaving conditions. High supply temperatures in heating can cause stratification and local discomfort near diffusers. Practical checks include minimum and maximum limits, plus confirming that cooling mode yields Tsupply below the zone temperature.
Using outputs for documentation
The exported CSV and PDF capture inputs, unit assumptions, and computed results for design packages and commissioning logs. Record the load basis (peak sensible vs diversity), the airflow reference (design vs VAV minimum), and any mixing assumptions. Consistent documentation supports troubleshooting when measured ΔT differs from calculated values. For VAV systems, compare calculated Tsupply at minimum flow to reheat setpoints, and verify that the resulting discharge temperature stays within diffuser ratings. When auditing data, note sensor locations and averaging intervals; coil leaving sensors can differ from zone supply sensors by 0.5–1.5°C. Include fan heat if it materially changes coil leaving temperature.
Frequently asked questions
1) Does this include latent cooling?
No. It is based on sensible heat only. For humid climates, size coils and supply conditions using psychrometrics and verify moisture removal and dewpoint targets.
2) What airflow should I enter for VAV systems?
Use the airflow associated with the operating point you are checking, such as peak cooling, minimum ventilation, or a commissioning test condition.
3) Why does my measured supply temperature differ from the calculation?
Common causes include sensor location, fan heat, duct heat gain, mixing at terminals, and loads that vary from the assumed sensible value.
4) How should I choose air density and Cp?
Use site altitude and expected supply conditions. If you do not have project values, ρ≈1.20 kg/m³ and Cp≈1.006 kJ/kg·K are practical starting points.
5) What does the mixed air option change?
It estimates entering temperature from return and outdoor air, then computes an approximate coil sensible load needed to reach the calculated supply temperature.
6) Can I use this for heating supply setpoints?
Yes. Select heating mode and enter sensible heating load. Confirm diffuser limits and comfort, especially if the discharge temperature is high.