# HVAC Fundamentals Heating, Ventilation, and Air Conditioning (HVAC) systems maintain acceptable indoor environmental conditions — thermal comfort, air quality, and humidity — for building occupants. HVAC typically accounts for 40-60% of a commercial building's total energy consumption and dominates the building services coordination effort. For the practicing architect, HVAC decisions profoundly affect floor-to-floor heights, ceiling void depths, riser sizes, plant room locations, facade design, and the building's operational carbon footprint. Early integration of HVAC strategy into the architectural design is essential — retrofitting ductwork into an undersized ceiling void is expensive, disruptive, and architecturally damaging. --- ## Table of Contents - [Thermal Comfort Criteria](#thermal-comfort-criteria) - [Heating Systems](#heating-systems) - [Cooling Systems](#cooling-systems) - [Ventilation Systems](#ventilation-systems) - [Air Distribution Systems](#air-distribution-systems) - [HVAC System Types](#hvac-system-types) - [Plant Room Requirements](#plant-room-requirements) - [Ductwork and Pipework Sizing](#ductwork-and-pipework-sizing) - [Controls and Building Management Systems](#controls-and-building-management-systems) - [Energy Efficiency Strategies](#energy-efficiency-strategies) - [See Also](#see-also) --- ## Thermal Comfort Criteria Thermal comfort is governed by six factors: air temperature, radiant temperature, air velocity, humidity, metabolic rate (activity), and clothing insulation. **Design internal conditions** (CIBSE Guide A / ASHRAE 55): | Space Type | Winter Heating (°C) | Summer Cooling (°C) | Relative Humidity (%) | |-----------|---------------------|---------------------|----------------------| | Office — general | 20-22 | 23-25 | 40-60 | | Office — executive | 21-23 | 22-24 | 40-60 | | Retail — sales floor | 19-21 | 21-24 | 40-60 | | Hotel — bedroom | 19-21 | 23-25 | 40-60 | | Hospital — ward | 22-24 | 23-25 | 40-60 | | Hospital — operating theatre | 18-22 | 18-22 | 40-60 | | School — classroom | 19-21 | 23-25 | 40-70 | | Residential — living room | 21-23 | 25-28 (overheating limit) | No control typical | | Data centre | 18-27 (ASHRAE A1) | 18-27 | 20-80 | | Swimming pool hall | 27-29 | 27-29 | 50-70 | **Overheating criteria** (CIBSE TM59 / TM52): - Occupied spaces should not exceed 26°C for more than 3% of occupied hours (Category II) - The absolute upper limit is 28°C for bedrooms during sleeping hours - Adaptive comfort model applies in naturally ventilated buildings — occupants tolerate higher temperatures when they have control --- ## Heating Systems ### Heat Sources | System | Efficiency/COP | Fuel | Application | |--------|---------------|------|------------| | **Gas condensing boiler** | 90-95% (gross) | Natural gas | Commercial; residential; retrofit | | **Air-source heat pump (ASHP)** | COP 2.5-4.0 | Electricity | Low-carbon heating; new-build; retrofit | | **Ground-source heat pump (GSHP)** | COP 3.5-5.0 | Electricity | Larger buildings; available ground area/boreholes | | **Water-source heat pump** | COP 3.5-5.0 | Electricity | Adjacent to river, lake, or aquifer | | **District heating** | Varies (network dependent) | Various | Urban; masterplan; campus | | **Biomass boiler** | 80-92% | Wood pellet/chip | Rural; off-gas sites; carbon reduction | | **Electric boiler** | 99% (point-of-use) | Electricity | Small loads; peak backup | | **Combined heat and power (CHP)** | 80-90% (total) | Gas (typically) | Large constant heat demands; hospitals; hotels | **UK policy direction**: The Future Homes Standard (2025) and Future Buildings Standard effectively mandate heat pumps for new buildings by prohibiting fossil fuel heating. Architects should design for low-temperature heating systems (45°C flow, ideally 35°C) to maximise heat pump efficiency. ### Heat Emitters | Emitter | Flow Temperature | Response Time | Spatial Impact | |---------|-----------------|---------------|---------------| | **Radiators** (steel panel) | 55-75°C (gas boiler); 35-55°C (heat pump compatible) | Fast | Wall-mounted; affects furniture layout | | **Underfloor heating (UFH)** | 35-45°C (screed); 45-55°C (timber overlay) | Slow (thermal mass) | Zero wall impact; floor build-up +50-100mm | | **Fan coil units (FCU)** | 45-70°C heating; 6-12°C cooling | Fast | Ceiling void; access panels; condensate drainage | | **Chilled beams** | 14-18°C cooling; 35-45°C heating | Moderate | Ceiling-mounted; no condensate risk (active); silent | | **Trench heating** | 55-75°C | Moderate | Perimeter; below glazing; floor trench required | | **Radiant ceiling panels** | 35-45°C | Moderate | Concealed above ceiling; low-temp compatible | --- ## Cooling Systems ### Cooling Sources | System | COP/EER | Capacity Range | Application | |--------|---------|---------------|------------| | **Air-cooled chiller** | 2.5-3.5 | 5-2,000 kW | Roof-mounted; no water infrastructure needed | | **Water-cooled chiller** | 4.0-6.0+ | 50-10,000 kW | Basement plant; cooling tower on roof | | **VRF/VRV system** | 3.0-5.0 | 10-150 kW (per system) | Multi-zone; refurbishment; mixed-use | | **Split/multi-split** | 3.0-4.5 | 2-30 kW | Small offices; retail; supplementary | | **Absorption chiller** | COP 0.7-1.4 | 100-5,000 kW | Waste heat driven; CHP integration | | **Adiabatic cooling** | N/A | Supplementary | Pre-cooling of condenser air; reduced chiller load | | **Free cooling** | Very high | System-dependent | Waterside or airside economiser; cool-climate benefit | ### Refrigerants | Refrigerant | GWP | Status | |------------|-----|--------| | R-410A | 2,088 | Current dominant; phase-down under F-gas regulation | | R-32 | 675 | Lower GWP alternative for splits and VRF | | R-134a | 1,430 | Centrifugal chillers; being replaced | | R-1234ze | 7 | Ultra-low GWP; new centrifugal chillers | | R-290 (propane) | 3 | Natural refrigerant; small systems; flammable | | R-744 (CO₂) | 1 | Natural; high-pressure; specialist applications | **F-gas regulation** (EU/UK): Phasing down HFC refrigerants to 21% of 2015 baseline by 2030. Architects should specify systems compatible with low-GWP refrigerants. --- ## Ventilation Systems ### Ventilation Rates | Space | Minimum Outdoor Air (l/s per person) | Standard | |-------|-------------------------------------|----------| | Office | 10 | CIBSE Guide A; EN 16798 Category II | | Classroom | 8-10 | BB101 (UK); ASHRAE 62.1 | | Retail — general | 10 | CIBSE | | Hotel bedroom | 10 (per room, minimum) | CIBSE | | Restaurant/dining | 10 | CIBSE | | Kitchen (commercial) | Based on hood extract rate | DS 101 | | WC/bathroom | 15 l/s per WC pan (extract) | Building Regulations Part F | | Car park | 3 ACH (normal); 6 ACH (peak) | Approved Document B/F | ### Ventilation Strategies | Strategy | Description | Typical Application | |----------|------------|-------------------| | **Natural ventilation** | Wind and stack-driven airflow through openings | Shallow-plan buildings (<15m); schools; residential | | **Mechanical ventilation with heat recovery (MVHR)** | Supply and extract fans with plate or rotary heat exchanger | Airtight buildings; residential; Passivhaus | | **Mechanical extract ventilation (MEV)** | Extract fans in wet rooms; trickle vents for supply | Residential (standard) | | **Mixed-mode** | Natural ventilation with mechanical backup/supplement | Offices; education; best of both | | **Displacement ventilation** | Low-velocity supply at floor level; extract at ceiling | Offices; auditoria; low energy | | **Demand-controlled ventilation (DCV)** | CO₂ or occupancy sensors modulate fresh air rate | Variable occupancy spaces; energy saving | See [[Natural Ventilation Strategies]], [[Passive House Standard]], and [[Building Envelope Fundamentals]] for integration with envelope design. --- ## Air Distribution Systems ### Ductwork Types | Duct Type | Material | Typical Velocity (m/s) | Application | |-----------|---------|----------------------|------------| | **Rectangular galvanised steel** | Galvanised mild steel | 4-8 (supply); 6-10 (main) | Standard commercial distribution | | **Circular galvanised steel** | Spiral-wound | 5-10 | Preferred acoustically; less pressure drop | | **Flexible duct** | Aluminium/polyester | 3-5 (short runs only) | Final connections to diffusers; max 1.5m | | **Fabric duct** | Polyester fabric | 2-4 | Exposed distribution; food factories; sports halls | | **Phenolic/PIR duct board** | Insulated rigid board | 4-8 | Low-rise; refurbishment; reduced ceiling void | ### Ceiling Void Requirements | HVAC System | Minimum Void Depth | Notes | |-------------|-------------------|-------| | Fan coil units only | 250-350mm | Plus 50mm for sprinklers, 100mm for lighting | | VAV ductwork (medium velocity) | 400-600mm | Main ducts 400-500mm deep | | High-velocity ductwork | 300-400mm | Smaller ducts; more fan energy; noise risk | | Chilled beams + ventilation ducts | 350-500mm | Beams 200-300mm; primary air ducts alongside | | Displacement (floor supply) | 300-500mm (raised floor) | Void below floor, not ceiling | **Architectural implication**: Ceiling void depth directly determines floor-to-floor height. A typical office requires 400-600mm void for services (all trades combined), yielding 3.6-4.2m floor-to-floor with 2.7m finished ceiling. See [[Office Building Design]] for dimensional coordination. --- ## HVAC System Types | System Type | Heating/Cooling Medium | Zone Control | Typical Application | |------------|----------------------|-------------|-------------------| | **All-air (VAV)** | Conditioned air via ductwork | VAV boxes per zone | Large open-plan offices; retail | | **All-air (CAV)** | Constant volume air supply | Limited (thermostat per AHU) | Cinemas; auditoria; simple spaces | | **Fan coil unit (FCU)** | Piped water + local fan + outdoor air | Per FCU (individual) | Multi-tenanted offices; hotels | | **Chilled beam (passive)** | Piped water; natural convection | Per beam zone | Open offices; low cooling loads | | **Chilled beam (active)** | Piped water + primary air induction | Per beam zone | Offices; labs; quality environments | | **VRF/VRV** | Refrigerant piped to indoor units | Per indoor unit | Multi-zone; commercial; refurbishment | | **Split/multi-split** | Refrigerant | Per indoor unit | Small commercial; retail; supplementary | | **Radiant ceiling/floor** | Piped water; radiant exchange | Zone circuits | Silent; uniform comfort; low energy | | **DOAS + radiant** | Dedicated outdoor air + radiant panels | Separate zones | High-performance; Passivhaus-adjacent | **Selection factors**: cooling load density, zone control requirement, ceiling void availability, noise sensitivity, capital cost, running cost, maintenance access, and architectural preference for exposed/concealed services. --- ## Plant Room Requirements | Plant | Typical Area Allowance | Location | |-------|----------------------|----------| | **Boiler plant** | 0.5-1.0% GFA | Basement or roof | | **Chiller plant** | 0.5-1.5% GFA | Basement (water-cooled) or roof (air-cooled) | | **AHU plant** | 1.5-4.0% GFA | Dedicated floor or roof; penthouse | | **Main riser** | 0.5-1.0% per floor | Central core; one per 1,000-1,500 m² | | **Cooling towers** | Roof area 0.5-1.0% GFA | Roof; setback from occupied areas | | **Electrical switchroom** | 0.3-0.5% GFA | Ground/basement; DPC level | | **Generator** | 15-40 m² per 500 kVA | Ground/basement; exhaust route; fuel storage | | **Transformer** | 8-15 m² per 1 MVA | Ground; external access for replacement | **Access requirements**: All plant rooms require clear maintenance access, equipment removal routes, and structural capacity for heavy equipment. Chillers and boilers typically require double-door openings and crane access or removal panels. --- ## Ductwork and Pipework Sizing **Ductwork sizing rules of thumb**: - Maximum duct velocity: 6 m/s (occupied areas for noise); 10 m/s (risers, plant rooms) - Maximum pressure drop: 1.0 Pa/m (low velocity); 2.0 Pa/m (high velocity) - Duct cross-sectional area = volume flow rate (m³/s) ÷ velocity (m/s) **Pipework sizing rules of thumb**: - Maximum water velocity: 1.0-1.5 m/s (small pipes); 2.0-3.0 m/s (mains) - Pipe sizing based on pressure drop ≤400 Pa/m and velocity limits - LTHW (low-temperature hot water): Flow 45-80°C; return 35-60°C - CHW (chilled water): Flow 6°C; return 12°C --- ## Controls and Building Management Systems A Building Management System (BMS) monitors and controls HVAC, lighting, and other building services centrally: **Key control functions**: - **Temperature control**: Zone setpoint control via thermostats, actuators, and valves - **Time scheduling**: Optimised start/stop; occupancy scheduling - **Free cooling**: Automatic switch to outside air when conditions allow - **Demand control**: CO₂-based ventilation modulation; occupancy sensing - **Fault detection**: Alarm generation; trend logging; energy monitoring - **Integration**: Interface with fire alarm (damper closure), access control, lighting **Commissioning**: HVAC systems must be commissioned and balanced before handover — air flow rates measured and adjusted at every terminal device; water flow rates balanced through all circuits. Post-occupancy seasonal commissioning is essential. See [[RIBA Plan of Work]] Stage 6-7 for handover requirements. --- ## Energy Efficiency Strategies | Strategy | Energy Saving Potential | Complexity | |----------|----------------------|-----------| | **Heat recovery (MVHR, run-around coils)** | 50-90% of ventilation heat loss | Standard | | **Free cooling (economiser)** | 20-40% of annual cooling energy | Low | | **Variable speed drives on fans/pumps** | 30-60% of fan/pump energy | Standard | | **Demand-controlled ventilation** | 20-40% of ventilation energy | Moderate | | **Heat pump (replacing gas boiler)** | 50-70% reduction in heating energy | Moderate-high | | **Thermal storage (ice/PCM)** | Peak load shifting; smaller plant | High | | **Desiccant dehumidification** | Latent load management; lower reheat | Specialist | | **Night purge ventilation** | Reduced mechanical cooling | Low (if design allows) | See [[Passive House Standard]], [[BREEAM Assessment Framework]], and [[LEED Certification System]] for performance benchmarks. --- ## See Also - [[Building Envelope Fundamentals]] - [[Office Building Design]] - [[Passive House Standard]] - [[Natural Ventilation Strategies]] - [[Thermal Comfort Fundamentals]] - [[Fire Safety in Building Design]] - [[BIM Fundamentals and LOD]] --- #services #hvac #heating #cooling #ventilation #mechanical #energy