Study guide
12.1 HVAC System Types: All-Air, Air-Water, All-Water & VRF Systems
ARE 5.0 Project Planning & Design PPD
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1 Chapter 1: Exam Overview & Schematic Design Process
1.1 ARE 5.0 PPD Format, Scoring & Case Study Navigation 1.2 The Schematic Design Phase & Multi-Discipline Workflow 1.3 Regulatory Framework: Navigating Codes, Standards & Jurisdictions
2 Chapter 2: Site Analysis, Topography & Soil Conditions
2.1 Site Topography, Grading, Slopes & Cut/Fill Calculations 2.2 Geotechnical Reports, Soil Classification & Foundation Selection 2.3 Site Hydrology, Drainage & Stormwater Management Neighborhood Context, Historic Settings & Site Integration
3 Chapter 3: Climate, Microclimate & Solar Design
3.1 Solar Geometry, Sun Angles & Sun Path Diagrams 3.2 Microclimate Analysis, Prevailing Winds & Passive Ventilation 3.3 Passive Solar Heating & Thermal Mass Strategies
4 Chapter 4: Sustainable Principles & Energy Standards
4.1 Daylighting Metrics: Spatial Daylight Autonomy (sDA) & Annual Sun Exposure (ASE) 4.2 Building Energy Standards: ASHRAE 90.1, IECC & Energy Use Intensity (EUI) 4.3 High-Performance Frameworks: LEED v4.1, Net Zero & Embodied Carbon
5 Chapter 5: Zoning Regulations & Land Use Controls
5.1 Zoning Envelopes: Setbacks, Floor Area Ratio (FAR) & Lot Coverage 5.2 Parking Requirements, Access & Site Circulation Planning 5.3 Environmental Regulations: Wetlands, Flood Zones & Brownfields
6 Chapter 6: Building Code Fundamentals: Occupancy & Construction Types
6.1 IBC Occupancy Classifications & Group Characteristics 6.2 Mixed Occupancy Strategies: Nonseparated, Separated & Accessory 6.3 IBC Construction Types I Through V & Fire-Resistance Ratings 6.4 Allowable Building Height & Area Calculations (IBC Tables 504 & 506)
7 Chapter 7: Means of Egress & Fire Protection
7.1 Occupant Load Determination & Egress Sizing (IBC Chapter 10) 7.2 Egress Components: Common Path, Travel Distance & Exit Separation 7.3 Passive Fire Protection: Fire Walls, Barriers & Smoke Partitions 7.4 Active Fire Suppression: Sprinklers, Standpipes & Fire Alarms
8 Chapter 8: Accessibility & Universal Design
8.1 Accessible Routes, Curb Ramps & Parking Layouts (ADA / ICC A117.1) 8.2 Interior Accessibility: Doors, Maneuvering Clearances & Corridors 8.3 Accessible Plumbing Facilities: Restrooms, Grab Bars & Fixture Layouts
9 Chapter 9: Structural Systems: Wood, Steel, Concrete & Masonry
9.1 Structural Selection & Spanning Rules of Thumb 9.2 Steel Framing Systems: Beams, Girders & Open-Web Joists 9.3 Concrete Framing Systems: One-Way, Two-Way & Post-Tensioned Slabs 9.4 Wood Systems: Light-Frame, Heavy Timber & Mass Timber (CLT)
10 Chapter 10: Lateral Force Resisting Systems & Foundations
10.1 Lateral Load Paths: Wind vs. Seismic Forces 10.2 Shear Walls, Braced Frames & Moment Frames 10.3 Shallow vs. Deep Foundation Systems: Footings, Mats, Piles & Caissons
11 Chapter 11: Building Envelope: Physics, Assemblies & Moisture Control
11.1 Envelope Thermal Performance: U-Values, R-Values & Thermal Bridging 11.2 Moisture Management: Vapor Retarders, Air Barriers & Water Resistive Barriers 11.3 High-Performance Wall & Roof Assemblies: Rainscreens & Inverted Roofs 11.4 Fenestration & Glazing: U-Factor, SHGC, Condensation Resistance & VT Interior Materials, Durability & Program Performance
12 Chapter 12: Mechanical & HVAC Systems
12.1 HVAC System Types: All-Air, Air-Water, All-Water & VRF Systems 12.2 Central Plant Equipment: Chillers, Boilers, Cooling Towers & Geothermal 12.3 Ductwork Distribution, Air Handling & Plenum Space Requirements 12.4 Indoor Air Quality, Ventilation Rates & ASHRAE 62.1 Baselines
13 Chapter 13: Plumbing, Electrical & Life Safety Systems
13.1 Domestic Water Supply & Pressure Boosting Systems 13.2 Sanitary Drainage, Venting & Fixture Count Calculations 13.3 Electrical Service, Transformers, Switchgear & Backup Generators 13.4 Lighting Design, Controls & Lighting Power Density (LPD) Limits
14 Chapter 14: Specialized Building Systems & Vertical Circulation
14.1 Architectural Acoustics: STC, NRC, RT60 & Noise Criteria (NC) 14.2 Elevator Systems: Traction vs. Hydraulic, Core Layout & Sizing 14.3 Building Telecommunications, Security & Building Automation Systems (BAS)
15 Chapter 15: Systems Integration & Coordination
15.1 Floor-to-Floor Height & Plenum Coordination (Structure + MEP + Architecture) 15.2 Structural Grid Planning & Spatial Adjacencies 15.3 Building Massing, Orientation & Envelope Integration 15.4 Case Study Strategy: Synthesizing Drawings, Code Sheets & Schedules
16 Chapter 16: Project Costs, Budgeting & Life-Cycle Analysis
16.1 Schematic Estimating Techniques: Area, Volume & Parametric Costs 16.2 Project Budgets: Contingencies, Overhead, Profit & Escalation 16.3 Life-Cycle Cost Analysis (LCCA) & Value Engineering
12.1 HVAC System Types: All-Air, Air-Water, All-Water & VRF Systems
Key Takeaways Explain This Topic AI
HVAC selection compares loads, ventilation, humidity, zoning, distribution space, maintenance and cost.
VAV needs air distribution; VRF still needs a designed outdoor-air solution and service access.
Heat recovery can transfer energy between zones, but annual performance depends on climate and loads.
Refrigerant charge, eligible room volume and permitted mitigation require the applicable safety analysis.
Last updated: October 2026
12.1 HVAC System Types: All-Air, Air-Water, All-Water & VRF Systems
Heating, ventilation, and air conditioning (HVAC) systems represent one of the most critical integration challenges in commercial architectural design. The selection of an HVAC system directly determines vertical floor-to-floor heights, ceiling plenum clearances, structural framing depths, rooftop equipment loading, exterior façade louvers, and vertical shaft square footage. During schematic design, architects must evaluate how thermal conditioning media—air, water, refrigerant, or hybrid combinations—align with building programmatic zoning, acoustical criteria, energy conservation goals, and capital cost constraints.
Primary Classification of HVAC Systems
HVAC systems are fundamentally classified by the fluid medium used to deliver heating and cooling to occupied thermal zones:
All-Air Systems : Air handles both the sensible and latent thermal loads as well as mandatory ventilation requirements. Conditioned air is treated in central Air Handling Units (AHUs) and ducted throughout the building.
Air-Water Systems : Air supplies mandatory ventilation and latent dehumidification, while recirculated hydronic water (chilled or hot) circulates through local terminal devices to handle the majority of sensible cooling and heating.
All-Water Systems : Hydronic piping circulates hot or chilled water directly to terminal heating/cooling units within the space. Because water carries no outdoor air, a separate ventilation system must be provided.
Refrigerant-Based (Direct-Expansion / VRF) Systems : Refrigerant is piped directly from an outdoor condensing unit or heat recovery compressor to multiple indoor terminal fan coil evaporators.
All-Air Systems
All-air systems condition spaces solely through supply air. Because air has a relatively low specific heat capacity ( c p ≈ 0.24 BTU/lb ⋅ ∘ F c_p \approx 0.24\text{ BTU/lb}\cdot^\circ\text{F} c p ≈ 0.24 BTU/lb ⋅ ∘ F ) compared to water ( c p = 1.0 BTU/lb ⋅ ∘ F c_p = 1.0\text{ BTU/lb}\cdot^\circ\text{F} c p = 1.0 BTU/lb ⋅ ∘ F ), massive volumes of air must be circulated, necessitating large supply and return ductwork, expansive ceiling plenums, and tall vertical mechanical chases.
Variable Air Volume (VAV) Systems
Variable Air Volume (VAV) systems are the prevailing standard for multi-tenant commercial office buildings, institutional facilities, and schools. A central AHU delivers conditioned air at a constant supply temperature (typically 55 ∘ F 55^\circ\text{F} 5 5 ∘ F ) through a main supply duct network.
Modulation Mechanism : Each thermal zone contains a terminal VAV box equipped with a motorized damper. As the space thermostat approaches its setpoint, the damper throttles airflow down to a minimum threshold (typically 20 % to 30 % 20\%\text{ to }30\% 20% to 30% of design maximum to preserve minimum ventilation rates).
Terminal Reheat : When cooling demand is minimal or the perimeter envelope loses heat during winter, further reduction of cold air would starve the space of fresh air. A terminal reheat coil—circulating hydronic hot water or utilizing electric resistance elements—heats the reduced airflow before discharge.
Architectural Advantages : Excellent central humidity control, seamless integration of air-side economizers (free cooling), and flexible re-zoning by repositioning downstream terminal boxes during tenant fit-outs.
Spatial Liabilities : Large supply duct cross-sections ( 14 to 24 inches 14\text{ to }24\text{ inches} 14 to 24 inches deep) dictate deep ceiling plenums ( 24 to 36 inches 24\text{ to }36\text{ inches} 24 to 36 inches ). Vertical mechanical shafts typically consume 2 % to 4 % 2\%\text{ to }4\% 2% to 4% of the gross floor area.
Constant Air Volume (CAV) Systems
Constant Air Volume (CAV) systems deliver a constant volume of supply air (cubic feet per minute, CFM) while varying the supply air temperature to satisfy zone thermal loads.
Single-Zone CAV : Best suited for large, undivided spaces with uniform thermal loads and continuous occupancy, such as gymnasiums, auditoriums, convention halls, arenas, and retail superstores. Because the entire space experiences uniform heating or cooling demand, a single thermostat controls the supply air temperature without complex zoning dampers.
Multizone CAV (Historic) : Historically used hot-deck and cold-deck air streams blended at central mixing dampers. Due to catastrophic energy waste caused by simultaneous heating and cooling of mixed air, traditional multizone CAV is prohibited by modern energy codes (ASHRAE 90.1 and IECC).
Packaged Rooftop Units (RTU)
Packaged Rooftop Units (RTUs) are self-contained, factory-assembled HVAC packages that incorporate fans, direct-expansion (DX) refrigeration compressors, air-cooled condensers, air filters, and heating modules (natural gas burners or electric coils) within a single weatherized exterior cabinet.
Application : Ubiquitous in low-rise commercial developments, strip malls, big-box retail stores, and single-story suburban schools.
Architectural Trade-offs :
Capital Cost : Lowest initial cost and fastest installation schedule, eliminating interior central boiler/chiller mechanical rooms.
Rooftop Footprint & Loading : RTUs impose significant dead weight ( 80 to 150 psf 80\text{ to }150\text{ psf} 80 to 150 psf localized) and structural vibration onto roof framing. Units must be screened architecturally per municipal zoning ordinances.
Acoustics & Durability : Located directly over occupied spaces, compressor and fan vibration can transmit low-frequency structure-borne noise. Rooftop equipment faces harsh weather exposure, resulting in shorter operational lifespans ( 15 years 15\text{ years} 15 years versus 25 to 30 years 25\text{ to }30\text{ years} 25 to 30 years for indoor central plants).
Air-Water Systems
Air-water systems leverage water's superior thermal density to dramatically reduce duct dimensions. Water holds approximately 3,500 times more heat per unit volume than air, meaning a small 1 -inch 1\text{-inch} 1 -inch hydronic pipe carries the equivalent thermal energy of an 18 -inch × 18 -inch 18\text{-inch} \times 18\text{-inch} 18 -inch × 18 -inch air duct.
Fan Coil Units (FCU) with Dedicated Outdoor Air Systems (DOAS)
In modern high-performance architecture, Fan Coil Units (FCUs) are paired with a Dedicated Outdoor Air System (DOAS) :
Decoupled Operation : The DOAS handles 100 % 100\% 100% of the outdoor ventilation air, filtering, dehumidifying, and tempering it before delivering it to occupied spaces via small, constant-volume ducts. Meanwhile, localized FCUs recirculate room air across hydronic coils to satisfy sensible heating and cooling demands.
Two-Pipe vs. Four-Pipe Hydronic Distribution :
Two-Pipe Systems : Consist of a single supply and single return pipe. The entire building must operate in either heating mode or cooling mode, requiring seasonal changeover. If a cold autumn morning transitions to a sunny afternoon, south-facing zones overheat because the system cannot provide cooling while in heating mode.
Four-Pipe Systems : Feature dedicated chilled water supply/return lines and dedicated hot water supply/return lines connected to dual coils within each FCU. Any zone can heat or cool independently at any time, eliminating seasonal changeover limitations.
Applications : Hotels, motels, multi-family apartments, college dormitories, and hospital patient rooms, where individual room acoustic and thermal control is required.
Induction Systems
Induction units utilize high-pressure primary ventilation air ducted from a central AHU. As this primary air discharges through internal nozzles at high velocity, it creates a localized low-pressure venturi effect that induces room air across a secondary hydronic coil, providing sensible conditioning without an internal fan motor.
All-Water Systems
All-water systems circulate hydronic fluid exclusively, utilizing no ductwork for space temperature control. A separate mechanical ventilation or natural ventilation system is mandatory to meet building code indoor air quality baselines.
Hydronic Baseboard Convectors
Finned-tube baseboard convectors circulate hot water along the perimeter base of exterior walls. Room air enters the bottom of the enclosure, warms via natural convection as it passes across heated aluminum fins, and discharges into the space to counteract cold downdrafts from exterior glazing. Baseboard systems provide no cooling or dehumidification.
Radiant Floor Heating & Cooling Slabs
Radiant systems circulate hot or chilled water through continuous cross-linked polyethylene (PEX) tubing embedded directly within concrete floor slabs or topping screeds.
Thermal Comfort Dynamics : Radiant systems exchange heat primarily via radiation ( > 50 % >50\% > 50% ) directly with occupants and surrounding room surfaces, rather than heating or cooling air volumes. This controls Mean Radiant Temperature (MRT) , allowing occupants to feel comfortable at lower winter air temperatures ( 65 ∘ F 65^\circ\text{F} 6 5 ∘ F instead of 70 ∘ F 70^\circ\text{F} 7 0 ∘ F ) and higher summer air temperatures ( 78 ∘ F 78^\circ\text{F} 7 8 ∘ F instead of 74 ∘ F 74^\circ\text{F} 7 4 ∘ F ), yielding significant energy savings.
Radiant cooling : Control slab temperature above the actual indoor dew point with an appropriate margin, sensors and coordinated dehumidification/ventilation. A DOAS is one common strategy, but not the only permitted way to manage latent load. A universal 58°F dew-point threshold cannot describe every room and operating condition.
Variable Refrigerant Flow (VRF / VRV) Systems
Variable Refrigerant Flow (VRF) systems circulate direct-expansion refrigerant directly between an exterior compressor unit and multiple interior fan-coil evaporators. Using variable-speed, inverter-driven scroll compressors, the system dynamically modulates refrigerant flow to match instantaneous part-load requirements.
Heat Pump vs. Heat Recovery VRF
Heat Pump VRF (Two-Pipe) : The entire network of indoor units operates simultaneously in either cooling mode or heating mode. Suitable for buildings with uniform solar orientation or open floor layouts.
Heat Recovery VRF (Three-Pipe or Two-Pipe with Branch Controllers) : Incorporates a high-pressure vapor line, liquid line, and suction line (or branch circuit controllers) allowing simultaneous heating and cooling across different zones. Heat extracted from interior zones demanding cooling is transferred directly through the refrigerant loop to satisfy heating demand in shaded perimeter zones, achieving seasonal part-load Coefficients of Performance (COP) exceeding 4.5 4.5 4.5 .
Architectural Integration & Space Savings
VRF eliminates bulky supply and return ducts, replacing them with pairs of insulated copper refrigerant lines ( 1 / 2 inch to 1 - 1 / 8 inch 1/2\text{ inch to }1\text{-}1/8\text{ inch} 1/2 inch to 1 - 1/8 inch diameter). This reduces ceiling plenum requirements from 30 inches 30\text{ inches} 30 inches down to 12 to 14 inches 12\text{ to }14\text{ inches} 12 to 14 inches , enabling lower floor-to-floor heights or taller finished ceilings in height-restricted urban zoning envelopes.
Code Safety: ASHRAE Standard 15 & 34 (Refrigerant Concentration Limit)
Because refrigerants displace breathable oxygen and pose toxic/asphyxiation hazards in high concentrations, ASHRAE Standard 15 and Standard 34 mandate strict limits on total system refrigerant charge relative to occupied room volume.
Refrigerant Concentration = W total V room ≤ RCL \text{Refrigerant Concentration} = \frac{W_{\text{total}}}{V_{\text{room}}} \le \text{RCL} Refrigerant Concentration = V room W total ≤ RCL
Where:
W total W_{\text{total}} W total = Total refrigerant charge in the connected VRF system (lbs)
V room V_{\text{room}} V room = Volume of the smallest occupied, enclosed room served by the piping (cu ft)
RCL \text{RCL} RCL = Refrigerant Concentration Limit (e.g., 26 lbs / 1 , 000 cu ft 26\text{ lbs}/1,000\text{ cu ft} 26 lbs /1 , 000 cu ft for hypothetical refrigerant)
If a single VRF system serving an entire floor leaks into a small private office or enclosed phone booth, the concentration must not exceed the RCL. If it exceeds the RCL, the architect and mechanical engineer must split the system into smaller, separate VRF circuits or install continuous mechanical relief exhaust with refrigerant leak sensors.
Multi-Criteria HVAC Selection Matrix
HVAC System Typology Relative First Cost Operating Efficiency Ceiling Plenum Required Vertical Shaft Impact Zoning Flexibility Acoustical Characteristics
VAV with Terminal Reheat Moderate High (with economizer) Heavy ( 24 – 36 in. 24\text{–}36\text{ in.} 24 – 36 in. ) High ( 2 % – 4 % 2\%\text{–}4\% 2% – 4% of GSF) High (individual terminal boxes) Excellent (equipment in central room; duct silencers)
Packaged Rooftop (RTU) Low Moderate Moderate ( 18 – 24 in. 18\text{–}24\text{ in.} 18 – 24 in. ) Minimal (direct roof drops) Moderate (single-zone or VAV) Fair to Poor (compressor vibration & roof breakout noise)
4-Pipe FCU + DOAS Moderate to High High Compact ( 10 – 14 in. 10\text{–}14\text{ in.} 10 – 14 in. ) Low (small DOAS risers) Superior (individual room control) Good (quiet ECM fan coils; locate over entry vestibules)
Radiant Slabs + DOAS High Superior Minimal ( 8 – 12 in. 8\text{–}12\text{ in.} 8 – 12 in. DOAS) Low Moderate (thermal mass slows zone response) Exceptional (completely silent hydronic slab)
Heat Recovery VRF High Superior (part-load) Minimal ( 10 – 12 in. 10\text{–}12\text{ in.} 10 – 12 in. ) Negligible (small pipe chases) Superior (simultaneous heat/cool) Good (quiet indoor cassettes; no large fan noise)
- Worked Scenario: System Selection & Refrigerant Safety Analysis
- Scenario
A 4-story suburban commercial office building features 15 , 000 GSF 15,000\text{ GSF} 15 , 000 GSF per floor ( 60 , 000 GSF 60,000\text{ GSF} 60 , 000 GSF total). The municipal zoning envelope caps total building height at 52 ft 52\text{ ft} 52 ft . The ground floor contains an open corporate conference center, while upper floors contain executive private offices, including an enclosed lactation room measuring 10 ft × 10 ft 10\text{ ft} \times 10\text{ ft} 10 ft × 10 ft with an 8.5 ft 8.5\text{ ft} 8.5 ft ceiling height ( V = 850 cu ft V = 850\text{ cu ft} V = 850 cu ft ). The mechanical engineer proposes a Heat Recovery VRF system with a total system charge of 32 lbs 32\text{ lbs} 32 lbs of hypothetical refrigerant per floor.
Step-by-Step Resolution
Zoning & Plenum Coordination :
Available floor-to-floor height = 52 ft / 4 stories = 13 ft 0 in. 52\text{ ft} / 4\text{ stories} = 13\text{ ft } 0\text{ in.} 52 ft /4 stories = 13 ft 0 in.
With a target finished ceiling height of 9 ft 6 in. 9\text{ ft } 6\text{ in.} 9 ft 6 in. and a 20 in. 20\text{ in.} 20 in. structural steel framing depth, the remaining clear ceiling plenum space is: 156 in. − ( 114 in. + 20 in. ) = 22 in. 156\text{ in.} - (114\text{ in.} + 20\text{ in.}) = 22\text{ in.} 156 in. − ( 114 in. + 20 in. ) = 22 in.
A conventional all-air VAV system with main supply ducts ( 18 in. 18\text{ in.} 18 in. duct + 2 in. + 2\text{ in.} + 2 in. external insulation + 4 in. + 4\text{ in.} + 4 in. clearance = 24 in. = 24\text{ in.} = 24 in. ) cannot fit beneath or through structural framing without costly web penetrations. The VRF system's small refrigerant piping ( 2 to 3 in. 2\text{ to }3\text{ in.} 2 to 3 in. bundle with insulation) fits easily within the 22 in. 22\text{ in.} 22 in. plenum.
Resolution : Under the example's supplied concentration limit, a 16-pound circuit in 850 cubic feet gives 18.8 lb/1,000 ft³, below 26. This is one arithmetic screen, not a complete safety approval. Confirm the connected charge, eligible volume, refrigerant classification, approved mitigation and applicable standard with the mechanical engineer. An arbitrary opening to a general corridor is not automatically an allowed remedy.
Mechanical planning values here are preliminary examples. Equipment selection, refrigerant safety, outdoor-air rates, filtration, economizers, fire/smoke protection and service clearances require the applicable adopted standards, current manufacturer data and project loads. A single capacity, CO2 reading or equipment efficiency does not establish whole-system compliance or annual performance.