Comparison of the applicable scenarios, advantages and disadvantages of all-air systems and fan coil unit + fresh air systems
I am an HVAC expert. This issue is one of the core decision points in HVAC system schematic design, directly affecting building energy consumption, indoor comfort, initial investment, and O&M complexity. Based on my engineering experience, in different scenarios such as large public buildings (e.g., shopping malls, theaters) and hotel guest rooms, system selection often determines the final outcome. Below, with reference to GB50736-2012 "Design Code for Heating Ventilation and Air Conditioning of Civil Buildings" and practical project experience, I will analyze in detail the applicable scenarios, advantages and disadvantages, and cases of both.
All-Air System
Applicable Scenarios:
- Large spaces with high occupant density: such as shopping malls, gymnasiums, theaters, exhibition halls, large conference rooms, etc. These areas have concentrated loads and open spaces, facilitating centralized air supply.
- Buildings with high air quality requirements: such as hospital operating rooms, laboratories, and clean factories. All-air systems can achieve centralized treatment through high-efficiency filtration, humidification, dehumidification, etc., meeting strict hygiene standards.
- Buildings with greater floor-to-floor height: such as industrial plants and airport terminals, where space can be used for duct layout.
1. Strong air handling capability: Can centrally achieve multiple functions such as cooling, heating, humidification, dehumidification, and filtration, especially suitable for occasions requiring precise temperature and humidity control (e.g., constant-temperature and constant-humidity equipment rooms). Article 7.3.1 of GB50736 explicitly states that all-air systems should be capable of centralized air treatment.
2. Centralized maintenance: All equipment (cooling/heating sources, fans, filters, etc.) is concentrated in the equipment room, facilitating inspection and management and reducing decentralized terminal maintenance costs.
3. Good noise control: The main noise sources are located in the equipment room. Through sound attenuation treatment, indoor noise can meet the requirements of GB50118 "Code for Sound Insulation Design of Civil Buildings" (e.g., hotel guest rooms ≤35dB(A)).
4. No condensate problems: Condensate is uniformly handled in the equipment room, avoiding terminal dripping.
Disadvantages:
1. Large duct space occupation: Requires extensive ductwork and high floor-to-floor height (typically clear height ≥3.5m), not suitable for buildings with low floor height or compact spaces.
2. High energy consumption: The air system has high resistance, and fan energy consumption accounts for a large proportion; moreover, it is difficult to achieve independent room-by-room control, resulting in low operating efficiency under partial load (Article 7.3.2 of GB50736 recommends: all-air systems should adopt variable air volume for energy savings).
3. High initial investment and operating costs: Large-area ducts, insulation, and equipment room civil works are costly, and operating energy consumption is high.
Case: A large shopping mall (building area 50,000 m²) adopted an all-air constant air volume system. Summer cooling load was approximately 3000kW, with ducts laid along the main corridors, and indoor temperature controlled at 24-26℃. Advantages: uniform air distribution and high comfort; disadvantages: high energy consumption (annual operating electricity cost approximately 1.2 million yuan), and large temperature deviation in local areas (e.g., corners). After later retrofit to a variable air volume (VAV) system, energy was saved by 20%.
Fan Coil Unit + Outdoor Air System
Applicable Scenarios:
- Buildings with multiple rooms and zones requiring independent control: such as hotel guest rooms, office buildings, hospital wards, residential apartments, etc. Each room can independently adjust temperature.
- Buildings with limited floor height: Fan coil terminal equipment occupies little space (typically ceiling height ≤300mm), suitable for locations with clear height of 2.8-3.2m.
- Retrofit projects: Fan coil systems are flexible and easy to install in existing buildings.
1. Flexible independent control: The fan coil unit in each room can be started/stopped independently and adjust air volume and temperature, meeting personalized needs. Article 7.3.3 of GB50736 recommends: multi-room buildings should adopt fan coil unit plus outdoor air systems.
2. Small duct space occupation: The outdoor air system requires only a small amount of ductwork (typically only supplying outdoor air to rooms), with small main duct dimensions, saving ceiling space.
3. Lower initial investment: Compared with all-air systems, duct, insulation, and equipment room civil works costs are significantly reduced, especially suitable for small and medium-sized projects.
4. Energy savings under partial load operation: Rooms not in use can be shut down, avoiding full-system operation and reducing energy consumption.
Disadvantages:
1. Condensate risk: Fan coil units generate condensate in summer. If drainage slope is insufficient or insulation is poor, it can easily lead to ceiling leakage and mold growth. Drainage slope must be strictly set in accordance with Article 5.3.4 of GB50243 "Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Works" (≥1%).
2. Limited air handling capability: Fan coil units can only adjust temperature and cannot independently handle humidity, filtration, or outdoor air, relying on the outdoor air system for supplementation. If outdoor air volume is insufficient, indoor CO₂ concentration can easily exceed standards (Article 3.0.6 of GB50736 requires: outdoor air volume ≥30m³/h·person).
3. Decentralized maintenance: The fan coil unit in each room requires regular cleaning of filters and inspection of condensate pans, making management difficult, especially when there are many hotel guest rooms.
4. Noise issues: Fan coil units have built-in fans. If improperly selected or poorly installed, noise may exceed 40dB(A), affecting guest room comfort.
Case: A five-star hotel (300 guest rooms) adopted fan coil units + independent outdoor air systems. Outdoor air handling units were set on the roof, supplying air to guest rooms after cooling and dehumidification, with fan coil units adjusting room temperature. Advantages: guest rooms can independently control temperature and humidity, meeting different guest needs; disadvantages: in summer, blocked condensate pipes caused water leakage in multiple guest rooms. This was later resolved by installing drainage risers on each floor and increasing the slope. Operating energy consumption was approximately 30% lower than that of an all-air system.
Summary Conclusion
All-air systems are suitable for locations with large spaces, high loads, and centralized treatment requirements (e.g., shopping malls, theaters). Their advantages lie in strong air handling capability and low noise, but they have high energy consumption and large space occupation. Fan coil unit + outdoor air systems are suitable for buildings with multiple rooms requiring independent control (e.g., hotels, office buildings). Their advantages lie in flexibility and low initial investment, but they have condensate risks and decentralized maintenance issues. Selection should be based on a comprehensive trade-off of building function, floor height, load characteristics, and budget.
The above are reference opinions. Actual project conditions still need to be further evaluated and implemented in combination with specific operating conditions, site conditions, and other factors.