Mixed Air in HVAC
In HVAC (Heating, Ventilation, and Air Conditioning) systems, mixed air refers to the combination of return air from the building and fresh outdoor air before it enters the air handling unit. The mixed air temperature determines the heating or cooling load required to condition the air to the desired supply temperature. Proper mixing calculations are essential for energy-efficient HVAC design and indoor air quality compliance.
Building codes require minimum outdoor air ventilation rates (typically 15-20 CFM per person) to maintain indoor air quality. The economizer mode uses increased outdoor air when conditions are favorable, reducing mechanical cooling costs. Mixed air calculations ensure the system can handle all operating conditions from extreme cold to extreme heat.
Mixing Formula
This is a weighted average by volume flow rate. For enthalpy-based calculations (including humidity effects), replace temperatures with enthalpies.
Applications
| Application | Stream 1 | Stream 2 | Goal |
|---|---|---|---|
| Standard mixing box | Return air (75°F) | Outside air (varies) | Pre-condition air |
| Economizer | Return air | Cool outside air | Free cooling |
| Dual duct | Hot duct (110°F) | Cold duct (55°F) | Zone control |
Frequently Asked Questions
What is the typical mixed air temperature?
In summer, with 75% return air at 75 F and 25% outdoor air at 95 F, the mixed air temperature is about 80 F. In winter, with the same ratio and outdoor air at 20 F, mixed air is about 61 F. The mixed air temperature determines whether the coil must heat or cool the air to reach the desired supply temperature (typically 55 F for cooling).
What is an economizer?
An economizer increases outdoor air percentage when outdoor conditions are favorable for free cooling (typically when outdoor air is cooler and drier than return air). Instead of mechanically cooling return air, the system uses more outdoor air directly. This can reduce cooling energy by 20-40% in mild climates. Enthalpy-based economizers consider both temperature and humidity.
Does this formula account for humidity?
This simplified formula calculates dry-bulb temperature mixing only. For accurate HVAC design, humidity must also be considered using enthalpy mixing on a psychrometric chart. The mixed enthalpy follows the same weighted average formula, and the resulting mixed air state can be found on the psychrometric chart.