Mixed Air Temperature Calculator

The calculation relies on the principle of conservation of energy applied to air mixing. Depending on your available data, you can approach this using either fractional percentages or mass/volume flow rates. When using percentages, the first percentage represents the proportion of the initial airstream, while the second percentage automatically accounts for the remainder: second_percentage = 1 - first_percentage. The combined temperature is then found via mixed_temperature = (first_temperature × first_percentage) + (second_temperature × second_percentage). Alternatively, when dealing with specific volumetric flow rates, the calculation uses the mass-weighted average formula: mixed_temperature2 = (flow_rate_outside × temperature_outside + flow_rate_return × temperature_return) / (flow_rate_outside + flow_rate_return).

Imagine you are designing an HVAC ventilation cycle for an office building. You have 2,000 CFM (cubic feet per minute) of outdoor air entering at 20°F during a cold winter day, mixing with 8,000 CFM of indoor return air circulating at 75°F. First, calculate the total flow rate: 2,000 + 8,000 = 10,000 CFM. Next, apply the flow rate formula. Multiply the outdoor airflow by its temperature (2,000 × 20 = 40,000) and the return airflow by its temperature (8,000 × 75 = 600,000). Add these values together to get 640,000. Finally, divide by the total flow rate of 10,000 CFM. The resulting mixed air temperature is 64°F, ensuring the mixture entering the heating coil is well above freezing.

Always ensure your temperature units match consistently throughout the equation, whether utilizing Fahrenheit or Celsius. When working with volumetric flow rates, verify that the units of measurement for both the outdoor and return streams are identical, such as CFM or cubic meters per hour. Keep in mind that this mathematical model assumes complete, uniform mixing and does not account for thermal losses through ductwork walls.