HVAC Systems Overview
Heating, ventilation, and air conditioning (HVAC) systems account for the largest share of building energy consumption and are tested throughout PPD and PDD. The ARE focuses on system selection, basic load concepts, and integration with architectural design rather than detailed engineering calculations.
HVAC System Types
| System Type | Description | Typical Application |
|---|---|---|
| Packaged rooftop unit (RTU) | Self-contained unit on roof; single-zone or multi-zone | Retail, small commercial |
| Split system | Condenser outside, air handler inside | Residential, small commercial |
| Variable air volume (VAV) | Central air handler with terminal boxes controlling airflow to zones | Large commercial, office buildings |
| Fan coil units (FCU) | Individual units in each zone with hot/chilled water from central plant | Hotels, hospitals, apartments |
| Variable refrigerant flow (VRF) | Refrigerant-based system with multiple indoor units, simultaneous heating/cooling | Mixed-use, retrofits, hotels |
| Chilled beam | Ceiling-mounted units using chilled water for sensible cooling; active or passive | Offices, labs (where latent loads are managed separately) |
| Geothermal heat pump | Uses ground as heat source/sink via ground loop | Any building type; high first cost, low operating cost |
All-Air vs. Air-Water vs. All-Water Systems
- All-air: Central air handler distributes conditioned air through ductwork (VAV, constant volume). Requires larger duct chases but centralizes maintenance.
- Air-water: Combines central ventilation air with local heating/cooling via water piping (fan coils, chilled beams). Reduces ductwork, but requires both air and piping distribution.
- All-water: Uses only piped water for heating/cooling (fan coils, radiant systems). Requires a separate ventilation air system for outdoor air requirements.
Load Calculations
HVAC loads determine equipment sizing and include:
- Sensible heat: Heat that changes temperature - from solar gain, transmission through the envelope, internal gains (people, lights, equipment), and ventilation air.
- Latent heat: Heat associated with moisture - from occupants, infiltration, and outdoor air humidity.
- Heating load: Primarily driven by transmission losses through the envelope and ventilation/infiltration in cold weather.
- Cooling load: Driven by solar gain, internal gains, and outdoor conditions in warm weather.
The Building Envelope directly determines transmission loads, and Environmental Systems (passive strategies) can significantly reduce both heating and cooling loads.
Ductwork and Distribution
Duct sizing affects ceiling plenums, floor-to-floor height, and coordination with structure and other systems. Low-velocity ductwork requires larger duct sizes but produces less noise; high-velocity systems use smaller ducts but require sound attenuation. Supply and return air distribution must provide adequate air changes without drafts or stagnant zones.
Energy Efficiency
Energy codes (ASHRAE 90.1, IECC) set minimum efficiency requirements for HVAC equipment and systems. Strategies to exceed code minimums include energy recovery ventilators (ERVs), economizer cycles (free cooling with outdoor air), high-efficiency equipment, and demand-controlled ventilation (DCV) based on CO2 sensing. These align with Sustainable Design goals and LEED Energy & Atmosphere credits.
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