Calculate mixed air temperature and humidity for HVAC systems
Mixed Temperature
28.33
°C
Mixed Humidity Ratio
9.33
g/kg
Total Flow
1500.0
kg/h (dry air)
Stream 1 Ratio
66.7%
Of total mix
Stream 2 Ratio
33.3%
Of total mix
T_mix = (m₁×T₁ + m₂×T₂) / (m₁+m₂) = (1000×25 + 500×35) / 1500.0 = 28.33°C
Mixed air is the combination of two or more air streams in HVAC (Heating, Ventilation, and Air Conditioning) systems. The most common application is mixing return air from the conditioned space with fresh outside air before it enters the cooling or heating coil. This mixing is essential for maintaining indoor air quality while optimizing energy efficiency.
The temperature and humidity of mixed air are calculated using mass-weighted averages. When two air streams at different temperatures mix, the resulting temperature depends on both the temperature of each stream and its proportion of the total flow. The formula T_mix = (m₁×T₁ + m₂×T₂) / (m₁+m₂) assumes constant specific heat, which is accurate for standard HVAC applications at typical temperature ranges.
Understanding mixed air conditions is critical for HVAC system design and operation. It determines the load on cooling and heating coils, affects energy consumption, and ensures compliance with ventilation standards that require minimum percentages of outside air. Building codes typically mandate 15-20% outside air for commercial buildings to maintain air quality, though the exact requirement depends on occupancy and building type.
ℹ️ Mass Flow Requirement
This calculator requires mass flow rates (kg/h of dry air) for both accurate temperature and humidity mixing calculations. Temperature mixing alone could theoretically work with any consistent flow unit, but specifying mass flow ensures that humidity ratio calculations (which depend on mass quantities) are physically correct. If you only have volumetric flow data, convert it to mass flow using: mass flow (kg/h) = volumetric flow (m³/h) × air density (~1.2 kg/m³ at standard conditions).
Input the mass flow rate (kg/h of dry air), temperature, and optional humidity ratio for the first air stream. Typically this is return air from the conditioned space (indoor air). Mass flow rates ensure accurate humidity mixing calculations. Both streams must use mass flow in kg/h.
Input the flow rate, temperature, and optional humidity ratio for the second air stream. This is typically outside air (fresh air) brought in for ventilation. Temperature and humidity will vary based on weather conditions.
The calculator displays the mixed temperature, mixed humidity ratio (if provided), total flow, and the percentage contribution from each stream. Use these values to size coils, select equipment, or verify system operation.
An office building air handler mixes 950 kg/h of return air at 22°C (with 8 g/kg humidity) with 250 kg/h of outside air at 32°C (with 12 g/kg humidity). Calculate the mixed air conditions entering the cooling coil.
Total Flow = 950 + 250 = 1200 kg/h
T_mix = (m₁×T₁ + m₂×T₂) / (m₁+m₂)
T_mix = (950×22 + 250×32) / 1200
T_mix = (20,900 + 8,000) / 1200
T_mix = 28,900 / 1200
T_mix = 24.08°C
w_mix = (m₁×w₁ + m₂×w₂) / (m₁+m₂)
w_mix = (950×8 + 250×12) / 1200
w_mix = (7,600 + 3,000) / 1200
w_mix = 10,600 / 1200
w_mix = 8.83 g/kg
The mixed air enters the cooling coil at 24.08°C with 8.83 g/kg humidity. Key insights:
Pro Tip: During mild weather (e.g., 18-22°C outside), use 100% outside air to eliminate mixing and reduce cooling costs. This "free cooling" strategy can cut energy use by 30-50% during shoulder seasons.
Building codes require minimum outside air for indoor air quality — typically 15-25% of total supply air. Mixing reduces energy costs compared to conditioning 100% outside air, while still providing fresh air to remove CO₂, odors, and contaminants.
Yes! For typical HVAC temperature ranges (0-40°C), air density is nearly constant, so volumetric flow gives the same result as mass flow. For more precise calculations at extreme temperatures or altitudes, use mass flow rates (kg/h).
Humidity ratio (or mixing ratio) is grams of water vapor per kilogram of dry air (g/kg). Unlike relative humidity, it's independent of temperature and follows the same weighted-average mixing formula. Typical values range from 5-15 g/kg for comfort cooling.
An economizer is a damper control system that automatically increases outside air when conditions allow "free cooling." When outside air is cooler than return air, the system uses up to 100% outside air, eliminating mechanical cooling and saving energy.
Yes! Humidity ratio mixes the same way temperature does — weighted average by flow rate. If outside air is humid and return air is dry (or vice versa), the mixed air humidity will be between the two, affecting latent cooling loads and dehumidification requirements.
In summer, hot outside air mixes with cooler return air from the air-conditioned space. The resulting mixed air is warmer than return air alone, increasing the cooling load. In winter, the opposite occurs — cold outside air lowers mixed air temperature, increasing heating load.
Use a pitot tube traverse in ductwork or read the fan schedule from design drawings. Many air handlers have factory-provided airflow tables based on fan speed. For verification, HVAC balancers use calibrated instruments following ASHRAE Standard 111.
The formula works identically in Fahrenheit — mixed temperature is still the weighted average. Just keep all temperatures in the same unit. For example: (800 ft³/min at 72°F + 200 ft³/min at 90°F) / 1000 = 75.6°F.
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