What is Fresh Air Fraction in an HVAC system?

Understanding the Fresh Air Fraction in an HVAC System

In the realm of heating, ventilation, and air conditioning (HVAC) systems, ensuring the provision of high-quality indoor air is paramount for the health and comfort of occupants. One essential parameter used to achieve this is the fresh air fraction, which plays a crucial role in maintaining indoor air quality. In this article, we will explore what fresh air fraction is, why it is important, and how it is implemented in HVAC systems.

Defining Fresh Air Fraction:

Fresh air fraction refers to the proportion of outside air introduced into an HVAC system compared to the total supply of air circulated within a building. It represents the ratio of the volume of fresh outdoor air to the total supply air volume, including recirculated air. Expressed as a percentage or a decimal value between 0 and 1, the fresh air fraction determines the amount of outdoor air brought into the system for ventilation purposes.

Importance of Fresh Air Fraction:

The fresh air fraction is vital in maintaining a healthy indoor environment. By introducing a sufficient amount of outdoor air, pollutants, odors, and contaminants that accumulate indoors can be diluted and expelled, thereby improving indoor air quality. Fresh air is essential for removing airborne particles, such as dust, allergens, and volatile organic compounds (VOCs), which can have adverse effects on human health and comfort.

Optimal fresh air fraction levels can vary depending on factors such as building occupancy, outdoor air quality, and climate conditions. Striking the right balance is crucial to prevent stagnant air, excessive humidity, or unnecessary energy consumption.

Implementing Fresh Air Fraction in HVAC Systems:

HVAC systems utilize various methods to control the fresh air fraction, ensuring a consistent flow of outdoor air throughout the building. The following are common approaches employed:

  1. Ventilation Modes: HVAC systems offer different ventilation modes, including constant volume, variable air volume (VAV), and demand-controlled ventilation (DCV). These modes enable the adjustment of fresh air intake based on occupancy levels and other parameters.
  2. Air Dampers: Dampers control the flow of air in HVAC systems. By modulating the position of dampers, the fresh air fraction can be adjusted to maintain optimal indoor air quality. Dampers may be manually or automatically controlled, with advanced systems integrating sensors and actuators for precise regulation.
  3. Energy Recovery Systems: To minimize energy loss while introducing fresh air, HVAC systems often incorporate energy recovery systems. These systems recover heat or coolness from the exhaust air and transfer it to the incoming outdoor air, reducing the energy requirements for conditioning the fresh air.
  4. Air Quality Sensors: Modern HVAC systems utilize air quality sensors to monitor pollutant levels, CO2 concentrations, and humidity. These sensors provide real-time feedback to adjust the fresh air fraction as needed, ensuring adequate ventilation based on current conditions.


The fresh air fraction is a critical aspect of HVAC systems that directly impacts indoor air quality and the well-being of occupants. By effectively managing the introduction of outdoor air, HVAC systems can mitigate indoor pollutants, maintain optimal humidity levels, and promote a healthier indoor environment. The proper implementation of fresh air fraction involves a combination of ventilation modes, air dampers, energy recovery systems, and air quality sensors.

As awareness of indoor air quality increases, building owners, HVAC professionals, and occupants need to understand the significance of the fresh air fraction and its role in creating comfortable and healthy indoor spaces. By prioritizing fresh air intake and implementing appropriate HVAC strategies, we can foster environments that promote well-being and productivity for all.