HVAC System Selection Is an Architect-Engineer Coordination Issue
The mechanical engineer selects and designs the HVAC system, but the architect makes decisions that determine which systems are even possible - the ceiling height, shaft locations, floor-to-floor heights, and structural bay sizes all constrain what HVAC options are available. The ARE PPD tests HVAC system knowledge because architects must understand system types well enough to coordinate effectively with mechanical engineers, make informed space planning decisions, and evaluate trade-offs between different system approaches during schematic design.
All-Air Systems
All-air systems distribute conditioned air from a central air-handling unit (AHU) to spaces through ductwork. The AHU contains the cooling coil (chilled water or DX), heating coil, fans, filters, and mixing box. Terminal devices (diffusers, registers) distribute air into occupied spaces. All-air systems can provide excellent ventilation, humidity control, and air filtration - qualities that make them common in healthcare and laboratory applications.
VAV (Variable Air Volume)
The most common all-air system in commercial buildings. A central AHU provides conditioned air at a fixed temperature (typically 55°F supply air) at variable volume. Each zone has a VAV box that modulates the volume of supply air delivered based on the thermostat call. As load decreases, the VAV box closes down, reducing airflow. This saves energy compared to constant-volume systems because the supply fan only moves the air that is actually needed. VAV systems require a minimum airflow setting to maintain acceptable ventilation rates even at low load.
Air-Water Systems
Air-water systems use both conditioned air (for ventilation and humidity control) and chilled or hot water (for sensible heating and cooling at the zone level). The central air system provides only ventilation air (minimum code-required fresh air), while fan coil units or chilled beams at each zone handle most of the sensible load using water coils. This dramatically reduces ductwork size (only ventilation air moves through ducts) and can provide excellent occupant control.
Fan Coil Units (FCUs)
Fan coil units are small terminal units (cabinet or concealed above ceiling) that contain a fan, a heating/cooling coil, and a filter. Chilled or hot water is piped to each unit; the fan draws room air across the coil to heat or cool it. FCUs provide excellent zone-by-zone control and are common in hotels, residential, and retrofit applications. They require piping to each unit (a coordination challenge) but use minimal ductwork.
VRF/VRV Systems (Variable Refrigerant Flow)
VRF systems distribute refrigerant directly to indoor units at each zone (rather than chilled water or air). A single outdoor condensing unit connects via refrigerant piping to multiple indoor units (ceiling cassettes, duct-connected units, wall-mount units). Advanced VRF systems can simultaneously heat some zones while cooling others by recovering heat from the cooling zones. VRF offers excellent efficiency, individual zone control, and compact equipment rooms, but requires specialized refrigerant piping installation and commissioning.
All-Water Systems
All-water systems use only water as the heating and cooling medium at each zone, typically through radiant systems (floor, ceiling, or wall panels) or finned-tube radiators. All-water systems provide excellent thermal comfort (radiant heat is perceived as very comfortable) but cannot effectively control humidity or provide ventilation - a separate ventilation system is required to meet code minimums.
Key System Comparison
| System | Ventilation Control | Humidity Control | Zone Control | Space Needed |
|---|---|---|---|---|
| VAV (all-air) | Excellent | Excellent | Good | Large ducts, shafts |
| Fan coil (air-water) | Good | Moderate | Excellent | Small ducts + piping |
| VRF | Poor (needs DOAS) | Poor | Excellent | Minimal |
| Radiant (all-water) | None (needs DOAS) | None | Excellent | Minimal ducts |
Key Exam Points
- VAV: all-air, variable volume, most common commercial system, good humidity control.
- Fan coil: air-water, individual zone units with water coils, small ductwork.
- VRF: refrigerant to each zone, heat recovery possible, no ductwork, needs DOAS for ventilation.
- Radiant: excellent comfort, no humidity control, always needs supplemental ventilation (DOAS).
- DOAS = Dedicated Outdoor Air System, required when the primary system cannot provide ventilation.
Study PPD on AREprep
AREprep’s PPD flashcards cover every concept on this exam with spaced repetition, and the practice exams mirror the real question formats so the actual test feels familiar.