The difference between an air handling unit (AHU) and a fresh air handling unit (FAU)
Air handling units (AHUs, also known as air handling units or air conditioning boxes) and fresh air handling units (FAUs) are both core air handling equipment in HVAC systems. They are often used together, but their core positioning, air source handling, functional load, and applicable scenarios are completely different. In short: fresh air handling units primarily focus on "air exchange and fresh air replenishment," while air handling units primarily focus on "indoor constant temperature and humidity circulation regulation." The following is a comprehensive and detailed comparison.
I. Differences in Core Positioning and Working Principle
1. Air Handling Unit (AHU)
Core positioning: Indoor air circulation and conditioning equipment, which is the main equipment for controlling the temperature and humidity of space.
Working principle: The unit uses indoor return air (80%-90%) as its core, mixed with a small amount of outdoor fresh air. After mixing, the air undergoes a complete set of treatments including filtration, cooling, heating, humidification, and dehumidification before being sent back into the room to continuously maintain stable indoor temperature and humidity. The unit has a complete supply and return air circulation duct, which can repeatedly circulate and treat indoor air.
2. Fresh Air Unit (FAU)
Core positioning: Outdoor fresh air replacement equipment is a specialized device for ensuring indoor air quality.
Working principle: It only processes 100% fresh outdoor air , without indoor recirculation. It directly draws in outdoor air, and after filtration, temperature and humidity pretreatment, heat recovery, etc., it is sent into the room in one direction. At the same time, it works with the exhaust system to remove indoor polluted air. Its core function is to refresh indoor air, replenish oxygen, and dilute pollutants, without relying on indoor recirculation.
II. Differences in Function and Load Bearing
1. Air handling unit
It can handle all indoor heating, cooling, and humidity loads , offering comprehensive and powerful functionality. Beyond air purification, its core function is to counteract temperature differences caused by people, equipment, and the building envelope, precisely controlling indoor temperature and humidity to meet comfort or process environment requirements. Some high-end modular air handling units can be adapted to special operating conditions such as cleanliness, constant temperature and humidity, and quiet operation.
2. Fresh air handling unit
It only handles the fresh air load (the temperature and humidity brought in by outdoor air) and is not responsible for indoor temperature regulation . Its core functions are ventilation, improving air quality, maintaining positive indoor pressure, and preventing the accumulation of odors and germs. It only performs basic pretreatment of fresh air and cannot counteract changes in indoor temperature. It cannot achieve the effect of air conditioning temperature control when used alone.
III. Differences in Structure and Configuration
1. Air handling unit
Modular structure, freely combinable, with complete functional sections, including primary/medium/high-efficiency filtration sections, multi-stage surface cooling sections, heating sections, humidification sections, water-blocking sections, return air sections, and fan sections. Large air volume range; large units do not have their own cold or heat source and require an external central air conditioning unit; high air pressure and strong handling capacity, suitable for large areas and high-requirement spaces.
2. Fresh air handling unit
The structure is relatively simple and fixed, with standard configuration of fresh air filtration, heat recovery, basic heating and cooling coils, and fan section, but no return air circulation module. Due to the larger temperature and enthalpy difference in the fresh air operating conditions, there are more rows of heating and cooling coils. It focuses on fresh air pretreatment and energy-saving heat recovery, with a relatively small air volume and compact structure. Most of them are pre-designed finished products.
IV. Differences in Applicable Scenarios
1. Air Handling Unit (AHU)
Suitable for large, enclosed spaces requiring precise temperature and humidity control, it is a core component of all-air systems. Typical applications include: large shopping malls, office building lobbies, stadiums, theaters, industrial cleanrooms, operating rooms, and large computer rooms, capable of independently handling the air conditioning and regulation of these spaces.
2. Fresh Air Unit (FAU)
Suitable for spaces where ventilation and air quality improvement are the core needs, it is rarely used alone and is usually used in conjunction with fan coil units or small air conditioning terminals. Typical scenarios include: residences, offices, hotel rooms, small shops, and ordinary office cubicles, solving problems of oxygen deficiency, stuffiness, and air pollution in enclosed spaces.
