Calculate mixed air temperature from 2-3 airstreams in HVAC systems. Find weighted average temperature, flow fractions, and mass flow rates.
The **Mixed Air Temperature Calculator** determines the resulting temperature when two or three airstreams of different temperatures are combined — the fundamental calculation in HVAC system design. Every building air handling unit (AHU) mixes return air with outside air, and getting this temperature right is critical for occupant comfort, energy efficiency, and system sizing.
The mixed air temperature is a simple flow-weighted average: T_mix = (T1×F1 + T2×F2) / (F1 + F2). But the implications are significant — in winter, too much cold outside air wastes heating energy; in summer, an economizer can use cool outside air to reduce cooling loads. This balance point is central to energy-efficient building operation.
This calculator handles 2 or 3 airstreams with selectable temperature and airflow units, shows the mixing ratio visually, provides a lookup table for different ratios, and calculates mass flow and heat transfer rates. Check the example with realistic values before reporting.
Mixed air temperature is the starting point for all HVAC coil sizing, energy modeling, and economizer control. This calculator makes it easy to explore different mixing ratios and optimize your system. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
T_mix = (T₁ × F₁ + T₂ × F₂) / (F₁ + F₂) Where: T_mix = mixed air temperature, T₁, T₂ = stream temperatures, F₁, F₂ = airflow rates For 3 streams: T_mix = (T₁F₁ + T₂F₂ + T₃F₃) / (F₁ + F₂ + F₃)
Result: 27.0°C
T_mix = (35 × 2000 + (-5) × 500) / (2000 + 500) = (70000 - 2500) / 2500 = 27.0°C. With 80% return air at 35°C and 20% outside air at -5°C, the mixed air is 27°C — requiring minimal additional heating to reach a supply temperature of 35°C.
Every air handling unit has a mixing section where building return air combines with outdoor air. The outdoor air fraction is the single most impactful variable in HVAC energy consumption — it determines how much heating or cooling the coils must provide.
At 100% recirculated air, the coil only needs to overcome internal heat gains. At 100% outside air, the coil must condition the entire supply from outdoor conditions — a vastly larger load in extreme weather. Finding the minimum outside air that satisfies ventilation codes while minimizing energy use is a core HVAC engineering challenge.
An economizer is an HVAC control strategy that increases outside air intake when outdoor conditions can provide free cooling. In a typical sequence: when outside air temperature drops below the return air temperature, dampers modulate to admit more outside air. When outside air is cold enough (below ~55°F), the system may use 100% outside air, completely eliminating mechanical cooling.
Advanced economizers also monitor enthalpy (temperature + humidity) to prevent admitting air that is cool but very humid, which would increase the latent cooling load. Economizer faults — stuck dampers, failed sensors — are among the most common HVAC maintenance issues and can waste 10-30% of cooling energy.
The sensible cooling/heating load is directly proportional to the temperature difference between mixed air and supply air. Every degree that mixed air differs from the supply setpoint translates to increased coil load. In large commercial buildings, optimizing mixed air temperature through economizer control, demand-controlled ventilation, and energy recovery ventilators can reduce HVAC energy by 20-40%.
ASHRAE Standard 62.1 requires minimum outside air for ventilation — typically 15-25% of total supply air for offices. Economizer mode can increase this to 100% when outdoor conditions favor free cooling.
When outside air is cooler than return air (but warm enough to condition), the economizer increases the outside air fraction, reducing mechanical cooling. Below 55°F (13°C) outside air temp, many systems can meet cooling loads with 100% outside air.
For temperature alone, this flow-weighted average is accurate. For full psychrometric analysis (enthalpy, humidity ratio), you need to mix both temperature and moisture content independently — see the psychrometric calculator.
Most chilled-water coils are designed for entering air of 55-75°F (13-24°C). Mixing too-cold outside air with return air can cause coil freeze issues unless proper controls are in place.
1 CFM = 0.4719 L/s. A typical office ventilation rate is about 15-20 CFM (7-9.5 L/s) per person.
Yes — if warm humid return air mixes with cold outside air, the mixture can reach its dew point, causing condensation. This is a common problem in cold climates and requires proper drainage and insulation.