# Central Air Conditioning Systems Central air conditioning systems provide cooling, heating, ventilation, and humidity control from a centralised plant to multiple zones within a building. For the architect, understanding central system types is essential because they determine plant room sizes, riser requirements, ceiling void depths, floor-to-floor heights, and facade integration. System selection directly impacts spatial planning, energy performance, and construction cost. --- ## Table of Contents - [System Classification](#system-classification) - [All-Air Systems](#all-air-systems) - [Air-Water Systems](#air-water-systems) - [All-Water Systems](#all-water-systems) - [Ductwork Design](#ductwork-design) - [Zoning Principles](#zoning-principles) - [Plant Room Requirements](#plant-room-requirements) - [Energy Efficiency](#energy-efficiency) - [See Also](#see-also) --- ## System Classification | Classification | Medium | Examples | |---------------|--------|---------| | **All-Air** | Conditioned air only | CAV, VAV, dual-duct, multizone | | **Air-Water** | Air (ventilation) + water (thermal) | Fan coil units + DOAS, chilled beams + DOAS | | **All-Water** | Water only (no ducted air) | Fan coil units (with natural ventilation) | | **Refrigerant-Based** | Refrigerant direct to terminal | VRF/VRV, split systems | --- ## All-Air Systems ### Constant Air Volume (CAV) | Feature | Description | |---------|-------------| | Principle | Fixed air volume; temperature varied to meet load | | Application | Single-zone spaces (auditoria, theatres, clean rooms) | | Advantages | Simple, reliable, good air quality | | Limitations | Poor multi-zone control, high fan energy at part load | | Duct size | Large — full design air volume at all times | ### Variable Air Volume (VAV) | Feature | Description | |---------|-------------| | Principle | Air volume modulated per zone via VAV boxes; supply temperature constant | | Application | Open-plan offices, large multi-zone buildings | | Advantages | Energy efficient at part load; good zone control | | Limitations | Reduced ventilation at low load; reheat may be needed | | Terminal units | VAV boxes with damper ± reheat coil | | Duct size | Medium — reduces at part load | ### Dedicated Outdoor Air System (DOAS) | Feature | Description | |---------|-------------| | Principle | Separate system handles 100% outdoor air; local terminals handle thermal load | | Application | Paired with fan coils, chilled beams, or radiant systems | | Advantages | Decoupled ventilation and thermal conditioning; excellent IAQ | | Limitations | Two systems to coordinate | --- ## Air-Water Systems ### Fan Coil Units (FCU) + DOAS | Feature | Description | |---------|-------------| | Principle | Central DOAS provides ventilation air; local FCUs provide heating/cooling via chilled/hot water | | Pipe system | 2-pipe (heating or cooling), 4-pipe (simultaneous heating and cooling) | | Terminal | Ceiling cassette, wall-mounted, or ducted FCU | | Advantages | Individual zone control; reduced ductwork; smaller ceiling voids | | Limitations | Maintenance access to FCUs; condensate drainage; noise | | Typical void | 350–500mm (FCU + DOAS duct) | ### Chilled Beams + DOAS | Type | Description | |------|-------------| | **Passive chilled beam** | No fan; cooling by natural convection over chilled water coil | | **Active chilled beam** | Primary air induces room air over chilled water coil | | Advantages | Silent operation; low energy; minimal maintenance; low ceiling voids | | Limitations | Limited cooling capacity (~80–100 W/m²); condensation risk; requires low-humidity supply air | | Application | Offices, laboratories, hospitals | --- ## All-Water Systems | System | Description | |--------|-------------| | **Fan coils only** | No central air supply; FCUs with fresh air from openable windows or local intake | | **Radiators/convectors** | Heating only via hot water; cooling by other means | | **Radiant panels** | Ceiling or floor panels with embedded pipes | | **Underfloor heating** | Embedded pipes in floor screed | All-water systems require separate ventilation provision (natural or mechanical) to meet [[Indoor Air Quality Standards]]. --- ## Ductwork Design | Parameter | Guideline | |-----------|-----------| | **Maximum velocity (supply)** | 5–8 m/s in occupied areas; 10–15 m/s in risers | | **Maximum velocity (return)** | 4–6 m/s in occupied areas | | **Pressure drop** | 0.8–1.2 Pa/m (low pressure); 1.5–2.5 Pa/m (medium pressure) | | **Minimum void depth** | 200mm (small duct) to 600mm+ (main duct) | | **Insulation** | All cold supply ducts insulated to prevent condensation | | **Fire dampers** | At all fire compartment boundaries | | **Access panels** | At dampers, coils, filters, junctions | ### Duct Shapes | Shape | Use | Notes | |-------|-----|-------| | Rectangular | Main runs, risers | Flat profile for limited voids; higher friction | | Circular/spiral | Branch runs, high-velocity | Lower friction; quieter; stiffer | | Flat oval | Limited void height | Compromise between rectangular and circular | --- ## Zoning Principles | Zone Factor | Response | |-------------|----------| | **Orientation** | Separate zones for N, S, E, W facades (solar load varies) | | **Internal vs perimeter** | Core zones have constant load; perimeter zones vary with weather | | **Occupancy pattern** | Separate zones for areas with different schedules | | **Activity type** | Server rooms, kitchens, meeting rooms = separate zones | | **Floor-by-floor** | Each floor independently controlled | --- ## Plant Room Requirements | Equipment | Typical Space Allowance | |-----------|------------------------| | **AHU** | 0.5–1.5 m² per 1000 m² served (plus clearance for filter access) | | **Chiller** | 30–60 m² per 1000 kW cooling capacity | | **Boiler** | 15–30 m² per 1000 kW heating capacity | | **Cooling tower** | Roof-mounted; 50–100 m² per 1000 kW | | **Pump room** | Adjacent to chillers/boilers | | **BMS/controls** | 10–20 m² | Plant rooms should be located to minimise duct/pipe runs, with adequate structural loading, vibration isolation, and acoustic separation from occupied spaces. Roof plant requires screening and planning consideration. --- ## Energy Efficiency | Strategy | Saving | |----------|--------| | Variable speed drives on fans/pumps | 30–50% fan energy | | Heat recovery (plate, rotary, run-around) | 50–80% exhaust heat recovered | | Free cooling (economiser) | Bypass chiller when outdoor temp < required supply | | Demand-controlled ventilation | Reduce outdoor air at low occupancy | | High-efficiency chillers | COP 5–7 (modern magnetic bearing) | | Chilled beams over VAV | 30–40% less fan energy | --- ## See Also - [[HVAC Fundamentals]] - [[Variable Refrigerant Flow Systems]] - [[Chilled Water Systems]] - [[Air Distribution Design]] - [[Heat Pump Systems]] - [[Indoor Air Quality Standards]] - [[Energy Modeling for Buildings]] --- #hvac #air-conditioning #vav #fan-coil #chilled-beam #ductwork #building-services