Supply Air Temperature Calculator
On this page
Calculate
Target or current indoor dry-bulb temperature
Total sensible cooling or heating load for the space
Volume airflow rate of supply air
Standard air density is 1.202 kg/m³. Adjust for altitude or temperature if needed.
Overview
The Supply Air Temperature Calculator helps HVAC engineers determine the required supply air temperature to offset a given sensible heat load. Whether sizing an air handling unit (AHU), fan coil unit (FCU), or rooftop unit (RTU), knowing the correct supply air temperature is essential for maintaining indoor comfort and energy efficiency.
This calculator uses the standard HVAC sensible heat equation to compute the supply air temperature from room temperature, airflow rate, and total sensible heat load. It supports both heating and cooling modes and provides results in metric (°C) and imperial (°F) units.
Accurate supply air temperature estimation prevents overcooling, overheating, and occupant discomfort while optimizing coil selection and energy consumption.
How to Use This Calculator
Select operating mode — choose from Cooling, Heating.
Enter room temperature — in °C or °F.
Enter sensible heat load — in W or BTU/hr.
Enter supply airflow — in m³/h or CFM.
Enter air density (optional) — in kg/m³.
Click "Calculate" — get supply air temperature, temperature difference (ΔT), and mass flow rate.
Use the supply temperature for coil selection; subtract fan heat gain (1–3°C) to get the required coil leaving temperature, and check condensation risk below 12°C.
Inputs & Outputs
Inputs
- •Operating Mode — Options: Cooling (supply air below room temp), Heating (supply air above room temp)
- •Room Temperature (°C / °F)
- •Sensible Heat Load (W / BTU/hr)
- •Supply Airflow (m³/h / CFM)
- •Air Density (optional) (kg/m³)
Outputs
- •Supply Air Temperature (°C / °F)
- •Temperature Difference (ΔT) (°C / °F)
- •Mass Flow Rate (kg/s / lb/hr)
- •Sensible Load (kW / BTU/hr)
Formula
Sensible Heat Formula
ṁ = ρ × V̇
ΔT = Q / (ṁ × Cp)
T_supply = T_room − ΔT [cooling]
T_supply = T_room + ΔT [heating]
where:
- Cooling mode: supply air is below room temperature
- Heating mode: supply air is above room temperature
Variable Reference
| Variable | Meaning | Units |
|---|---|---|
| Q | Sensible heat load | W or BTU/hr |
| ṁ | Mass airflow rate | kg/s or lb/hr |
| Cp | Specific heat of air | 1,005 J/(kg·K) or 0.24 BTU/(lb·°F) |
| ΔT | Supply-to-room temperature difference | °C / °F |
| T_room | Room air temperature | °C / °F |
| T_supply | Supply air temperature | °C / °F |
| ρ | Air density | kg/m³ (default 1.2) |
| V̇ | Volume airflow rate | m³/s or CFM |
What is Supply Air Temperature
Supply air temperature is the dry-bulb temperature of the conditioned air delivered to a space by an HVAC system. In cooling mode, supply air is cooler than the room temperature to absorb heat from the space. In heating mode, supply air is warmer than the room temperature to add heat to the space.
The supply air temperature is one of the most important design parameters in HVAC engineering. It directly affects occupant comfort, energy consumption, coil sizing, and duct design. Too-cold supply air can cause drafts and condensation, while too-warm supply air in cooling mode fails to remove enough heat.
How Supply Air Temperature is Determined
The required supply air temperature depends on three primary factors:
- Sensible heat load — the amount of heat that must be added or removed from the space
- Airflow rate — the volume of air delivered by the supply fan or AHU
- Room temperature — the target indoor dry-bulb temperature
Using the sensible heat equation Q = ṁ × Cp × ΔT, engineers can calculate the temperature difference between supply and return air, then determine the exact supply air temperature needed.
Typical Supply Air Temperature Ranges
| Application | Cooling Supply Temp | Heating Supply Temp |
|---|---|---|
| Office / Commercial | 12–16°C (54–61°F) | 30–40°C (86–104°F) |
| Residential | 13–18°C (55–65°F) | 35–55°C (95–131°F) |
| Data Center | 12–18°C (54–65°F) | N/A |
| Hospital / Healthcare | 13–16°C (55–61°F) | 28–35°C (82–95°F) |
| Industrial / Warehouse | 14–20°C (57–68°F) | 35–50°C (95–122°F) |
HVAC Unit Conversions
| Unit | Equivalent |
|---|---|
| 1 W | 3.412 BTU/hr |
| 1 kW | 3,412 BTU/hr |
| 1 m³/h | 0.5886 CFM |
| 1 CFM | 1.699 m³/h |
| 1 kg/s | 7,936.6 lb/hr |
| °C to °F | °F = °C × 9/5 + 32 |
| ΔT °C to ΔT °F | ΔT °F = ΔT °C × 9/5 |
Practical Tips
For cooling applications, typical supply air temperatures range from 12–18°C (54–65°F). Supply air below 12°C risks condensation on ducts and diffusers. Supply air above 18°C may not provide adequate dehumidification.
For heating applications, supply air temperatures above 50°C (122°F) can cause thermal stratification in tall spaces. Consider destratification fans for spaces with ceiling heights above 4 m (13 ft).
Air density matters at altitude. At 1,500 m (5,000 ft) elevation, air density drops to approximately 1.06 kg/m³, which increases the required ΔT by about 13% compared to sea level. Always adjust air density for high-altitude installations.
Important: This calculator provides steady-state estimates using standard air properties. Actual supply air temperature may vary with humidity, duct losses, fan heat gain, and system part-load conditions. For critical applications, verify with detailed psychrometric analysis and equipment manufacturer data.
Key Facts
- Supply air temperature is the single most important parameter for occupant thermal comfort in mechanically ventilated buildings.
- Typical commercial cooling supply air temperature is 12–16°C (54–61°F) — cold enough to dehumidify but warm enough to avoid condensation.
- Every 1°C increase in cooling supply air temperature can reduce chiller energy consumption by 2–4%.
- Fan heat gain typically adds 1–3°C to supply air temperature between the AHU and the diffuser.
- ASHRAE Standard 55 recommends that supply air velocity at the occupied zone not exceed 0.25 m/s to avoid draft complaints.
Applications
- AHU coil selection and chilled water temperature design.
- VAV system zone control and minimum airflow setpoints.
- Rooftop unit (RTU) supply air temperature scheduling.
- Fan coil unit (FCU) sizing for hotel guest rooms and offices.
- Data center cooling design and hot aisle/cold aisle temperature planning.
- Heat pump supply air temperature verification for residential systems.
Example Calculation
Example — Cooling Mode
Given:
- Room temperature = 24°C
- Sensible cooling load = 5,000 W
- Supply airflow = 2,500 m³/h
- Air density = 1.2 kg/m³
Calculation:
V̇ = 2,500 / 3,600 = 0.694 m³/s
ṁ = 1.2 × 0.694 = 0.833 kg/s
ΔT = 5,000 / (0.833 × 1,005) = 5.97°C
T_supply = 24 − 5.97 = 18.03°C
Result: T_supply ≈ 18.0°C (64.5°F)
Example — Heating Mode
Given:
- Room temperature = 21°C (69.8°F)
- Sensible heating load = 8,000 W
- Supply airflow = 3,000 m³/h
- Air density = 1.2 kg/m³
Calculation:
V̇ = 3,000 / 3,600 = 0.833 m³/s
ṁ = 1.2 × 0.833 = 1.0 kg/s
ΔT = 8,000 / (1.0 × 1,005) = 7.96°C
T_supply = 21 + 7.96 = 28.96°C
Result: T_supply ≈ 29.0°C (84.1°F)
Standards & References
- ASHRAE Standard 55 — Thermal Environmental Conditions for Human Occupancy
- ASHRAE Fundamentals Chapter 18 — Nonresidential Cooling and Heating Load Calculations
- ASHRAE Standard 62.1 — Ventilation for acceptable indoor air quality
- ASHRAE 90.1 — Energy standard for buildings (supply air temperature reset requirements)
- SMACNA HVAC Systems Duct Design — Duct insulation and condensation prevention
Limitations
- This calculator provides steady-state sensible heat estimates only.
- Latent heat load (moisture removal) is not included — use a psychrometric calculator for total cooling coil load.
- Fan heat gain (1–3°C rise) is not included — actual supply temperature at the diffuser will be higher than the coil leaving temperature.
- Duct heat gain or loss is not modeled — long duct runs or uninsulated ducts will change the effective supply temperature.
- Air density is assumed constant — for precise results at non-standard conditions, use the Air Density Calculator.
Common Mistakes to Avoid
- Using volume airflow (CFM or m³/h) directly in the sensible heat equation without converting to mass flow rate.
- Forgetting to account for fan heat gain, which raises the effective supply air temperature by 1–3°C.
- Ignoring altitude effects on air density — at 1,500 m elevation, air density is ~12% lower than at sea level.
- Confusing sensible load with total load — latent load does not affect supply air temperature directly.
- Setting supply air temperature too low, causing condensation on uninsulated ducts and diffusers.
Frequently Asked Questions
What is a typical supply air temperature for cooling?
How does airflow affect supply air temperature?
What happens if supply air temperature is too low?
What is supply air temperature reset?
Does altitude affect supply air temperature calculation?
How do I account for fan heat gain?
Frequently Used Together
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Calculate
Target or current indoor dry-bulb temperature
Total sensible cooling or heating load for the space
Volume airflow rate of supply air
Standard air density is 1.202 kg/m³. Adjust for altitude or temperature if needed.