Calculate the temperature of mixed air streams for HVAC applications
Typically return air from conditioned space
Typically outside air for ventilation
Mixed Air Temperature
26.20
°C (or °F)
Total Flow
1500
Combined flow rate
Stream 1
80.0%
Of total mixture
Stream 2
20.0%
Of total mixture
T_mix = (m₁×T₁ + m₂×T₂) / (m₁+m₂) = (1200×24 + 300×35) / 1500 = 26.20°
Mixed air temperature is the temperature that results when two or more air streams at different temperatures are combined. This is a fundamental calculation in HVAC (Heating, Ventilation, and Air Conditioning) system design, particularly when mixing return air from a building with fresh outside air before it enters heating or cooling equipment.
The calculation follows the principle of conservation of energy. The resulting temperature is a weighted average based on the mass (or volume) flow rates of each stream. The formula T_mix = (m₁×T₁ + m₂×T₂) / (m₁+m₂) assumes negligible heat loss during mixing and constant specific heat, which is accurate for standard air-conditioning applications.
Knowing the mixed air temperature is essential for sizing HVAC equipment. It determines the entering air conditions for cooling or heating coils, affects fan power requirements, and influences energy consumption. Building codes require minimum percentages of outside air for indoor air quality (typically 15-25%), so understanding mixed air conditions allows engineers to balance fresh air requirements with energy efficiency.
Input the flow rate and temperature for the first air stream. Use consistent units — either all metric (m³/h, °C) or all imperial (CFM, °F). The first stream is typically return air from the conditioned space.
Input the flow rate and temperature for the second air stream using the same units. This is typically outside air brought in for ventilation. Make sure flow units match between both streams (both volumetric or both mass-based).
The calculator displays the resulting mixed air temperature, total flow, and the percentage contribution from each stream. This mixed temperature is what enters your heating or cooling equipment.
A classroom HVAC unit mixes 850 m³/h of return air at 21°C with 150 m³/h of outside air at 30°C to meet ventilation requirements. Calculate the mixed air temperature.
Outside Air % = 150 / (850 + 150) = 150 / 1000 = 15%
✓ Meets 15% minimum requirement
T_mix = (m₁×T₁ + m₂×T₂) / (m₁ + m₂)
T_mix = (850×21 + 150×30) / (850 + 150)
T_mix = (17,850 + 4,500) / 1,000
T_mix = 22,350 / 1,000
T_mix = 22.35°C
The mixed air enters the cooling coil at 22.35°C, which is 1.35°C warmer than the return air alone. Key points:
Design Note: During peak summer (35°C+ outside), consider pre-cooling outside air with energy recovery ventilators (ERV) or heat wheels to reduce the mixed air temperature and save cooling energy.
Yes! The principle extends to any number of streams. For three streams: T_mix = (m₁T₁ + m₂T₂ + m₃T₃) / (m₁ + m₂ + m₃). Calculate pairwise — mix two streams, then mix that result with the third stream.
Convert to the same unit first. For example: 1000 CFM = 1699 m³/h, or 1 kg/s = 3600 kg/h. Online unit converters or HVAC handbooks provide conversion factors. The calculator assumes you've already matched units.
At high altitude, air density decreases but the temperature calculation remains the same. However, volumetric flow rates represent less mass, so use mass flow (kg/h) for precision at elevations above 1500m. Temperature mixing is unaffected by altitude.
In summer, hot outdoor air (30-40°C) mixes with cool indoor return air (22-24°C), raising the mixed temperature above the return air. This increases cooling loads. In winter, cold outdoor air lowers mixed temperature, increasing heating loads.
ASHRAE Standard 62.1 specifies minimum outdoor air for commercial buildings — typically 15-25 CFM per person or 15-20% of total supply air, whichever is greater. Residential buildings follow ASHRAE 62.2 with lower requirements (usually 50-100 CFM total).
Motorized dampers control the ratio of outside to return air. Fully open outside air damper = 100% outdoor air (no mixing). Partially open = proportional mixing. Dampers are adjusted based on economizer logic, occupancy sensors, or CO₂ levels.
Yes! Humidity ratio (g/kg) mixes using the same weighted-average formula. However, relative humidity does NOT mix linearly — it must be converted to humidity ratio first, mixed, then converted back to RH if needed.
This calculator uses dry-bulb temperature (standard thermometer reading). Wet-bulb temperature accounts for evaporative cooling and is always lower than dry-bulb. For complete HVAC calculations, use psychrometric charts or calculators that include both.
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