HVAC System Selection Affects Building Energy Use, Cost, and Occupant Comfort for the Life of the Building
The architect rarely designs the HVAC system in detail - that is the mechanical engineer's domain - but the architect participates meaningfully in HVAC system selection during schematic design and influences system placement, space allocation, and energy performance targets throughout design development. Understanding the major HVAC system types and how they compare on energy efficiency, zone control flexibility, first cost, space requirements, and suitability for different building types is a legitimate architectural design skill and is directly tested on the ARE PPD exam. The exam asks candidates to recommend an appropriate HVAC system type for a described building program, climate zone, and owner criteria - not to size duct systems or calculate load distributions.
Variable Air Volume (VAV)
VAV systems condition a central air handling unit (AHU) with chilled water or DX cooling coils, distribute conditioned air through sheet metal ductwork, and modulate the airflow rate at each zone with a VAV box (a motorized damper that throttles airflow based on zone thermostat demand). VAV is the dominant commercial HVAC system type in the United States for medium-to-large office buildings, education buildings, and institutional facilities because it is well-understood, maintainable, and cost-effective at scale. Advantages: excellent zone-by-zone control; proven technology; maintainable by standard HVAC contractors. Disadvantages: requires significant ductwork distribution space (plenum depth or exposed slab-to-slab height); central AHU room required; minimum airflow requirements at low-load conditions can limit energy savings.
Fan Coil Units (FCU)
Fan coil units are terminal devices located in each zone (often in ceilings, floor chases, or perimeter PTAC sleeves) that circulate room air through a coil supplied with chilled water (cooling) or hot water (heating) from a central plant. FCU systems distribute water (piped) rather than air (ducted), reducing the volume of distribution space required. Advantages: individual zone control; no large ductwork chases; can be supplemented with dedicated outdoor air system (DOAS). Disadvantages: more maintenance points (a fan, coil, and drain pan in every zone); risk of condensate drain pan overflow in humid climates; requires central chilled and hot water plant.
Variable Refrigerant Flow/Volume (VRF/VRV)
VRF (or VRV - variable refrigerant volume, the Daikin trademark) systems use refrigerant piping (rather than chilled water or ductwork) distributed from outdoor condensing units to multiple indoor fan coil units. VRF can simultaneously heat and cool different zones using heat pump principles (heat recovery VRF). Advantages: energy efficient (especially heat-recovery systems); precise individual zone control; compact indoor units with minimal space requirements; no central plant (chillers and boilers). Disadvantages: higher first cost than traditional systems; refrigerant line set limitations (length, height); refrigerant leak detection requirements in occupied spaces (ASHRAE 15).
Radiant Heating and Cooling
Radiant systems circulate heated or cooled water through tubing embedded in floors, ceilings, or walls. Radiant heating (hydronic floor heating) provides extremely comfortable even heat distribution with lower water temperatures (85-110°F) than forced-air systems. Radiant cooling is feasible but requires careful dew point control to prevent condensation on the cooled surface. Advantages: silent operation; no air movement; high thermal comfort. Disadvantages: slow response time; significant first cost for in-slab systems; cooling applications require supplemental dehumidification.
Comparison Table
| System | Zone Control | Distribution | Best For |
|---|---|---|---|
| VAV | Good | Ductwork | Large commercial/institutional |
| FCU | Excellent | Piping | Hotels, residential, perimeter zones |
| VRF | Excellent | Refrigerant piping | Small-medium commercial, retrofit |
| Radiant | Slow | Hydronic piping | High-comfort residential, passive buildings |
AREprep's PPD mechanical systems content covers HVAC system types, energy performance, code requirements (ASHRAE 90.1 minimum efficiency), and the design context for HVAC system selection - giving ARE PPD candidates the mechanical systems literacy they need to answer HVAC system selection questions on the PPD exam and to participate meaningfully in mechanical system coordination during actual design practice.
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AREprep's PPD flashcards and practice exams are built around the actual NCARB exam guide - so every study session targets what the exam tests.