Non-Domestic Ventilation Requirements Under the FBS

Ventilation in non-domestic buildings is governed by Approved Document F, Volume 2 (AD F2), which sets minimum outdoor air supply rates, extract rates, and indoor air quality criteria for buildings other than dwellings. AD F Vol 2 is the 2021 edition (effective 15 June 2022) and was not updated for the FBS - the Building Circular 01/2026 only published a new AD F Volume 1 (domestic). Because ventilation heat loss feeds directly into the SBEM calculation, the interaction between Part F and Part L Volume 2 is critical to achieving compliance.

Part F Volume 2 and the FBS

Part F requires that adequate means of ventilation be provided for people in the building (Requirement F1). For non-domestic buildings, AD F2 takes a performance-based approach: designers may use any ventilation solution provided they can demonstrate to the building control body that it achieves the indoor air quality criteria set out in the document (AD F2, para 1.3). The prescriptive ventilation rates provide a deemed-to-satisfy route but are not the only means of compliance.

Although AD F Vol 2 was not revised for the FBS, the ventilation parameters that feed into the SBEM calculation - heat recovery efficiency, specific fan power, and outdoor air supply rates - are set in the NCM Modelling Guide and AD L2 2026. Designers must satisfy AD F2 ventilation rates while meeting the tighter energy targets in AD L2 2026.

Ventilation rates by building and room type

AD F2 sets minimum outdoor air supply rates for non-domestic buildings. These are minimum rates and apply whether ventilation is natural, mechanical, or mixed-mode (AD F2, paras 1.32–1.41 and Table 1.1).

Offices

AD F2 gives detailed prescriptive rates for offices (paras 1.24–1.37). Whole building outdoor air supply must be the higher of (AD F2, para 1.32):

• 10 l/s per person
• 1 l/s per m² floor area

Extract rates for specific spaces within offices include (AD F2, paras 1.24–1.28):

• Sanitary accommodation: 15 l/s per shower/bath and 6 l/s per WC pan or urinal (intermittent extract)
• Printer/photocopier rooms: 20 l/s per machine during use
• Food preparation (microwave/drinks only): 15 l/s
• Food preparation (hob/cooker, adjacent extract): 30 l/s

Educational buildings

AD F2 refers to Building Bulletin 101 (BB101) for schools and other educational buildings (AD F2, Table 1.1). BB101 sets ventilation rates to achieve CO₂ below 1,000 ppm averaged over the occupied period.

Other building types

For buildings other than offices and car parks, AD F2 states that adequate ventilation may be demonstrated by meeting the relevant standards in CIBSE Guide A. Table 1.1 lists specific guidance sources for each building type, including hotels, retail, healthcare, sports facilities, and industrial buildings (AD F2, para 1.41).

Specialist and high-pollutant spaces

• Commercial kitchens: specific guidance in BESA DW 172 and HSE Catering Information Sheet No. 10 (AD F2, Table 1.1)
• Sanitary accommodation: extract rate of 6 l/s per WC pan or urinal, 15 l/s per shower/bath - no recirculation permitted (AD F2, para 1.26)
• Enclosed car parks: CO-controlled to maintain≤ 30 ppm CO (8-hour average) and ≤ 90 ppm CO (15-minute peak at ramps/exits) (AD F2, para 1.38)

Natural, mechanical, and mixed-mode ventilation

AD F2 permits three ventilation strategies for non-domestic buildings: natural, mechanical, or mixed-mode (hybrid). The choice depends on building depth, external noise, air quality, and security. Naturally ventilated buildings should follow CIBSE AM10; mixed-mode buildings should follow CIBSE AM13 (AD F2, paras 1.9–1.10).

Natural ventilation

Natural ventilation may be the sole means of ventilation where conditions allow. CIBSE AM10 provides design guidance on single-sided and cross-ventilation strategies for non-domestic buildings.

Purge ventilation must be available in all occupied spaces to rapidly dilute pollutants. For naturally ventilated offices, extract ventilators should be located as high as practicable, no more than 400 mm below the ceiling (AD F2, para 1.30).

Mechanical ventilation

Mechanical ventilation is the standard approach for most new non-domestic buildings under the FBS. Common system types include:

• Balanced supply and extract with heat recovery (MVHR): centralised AHU providing supply and extract with a heat exchanger – the standard for new buildings under the FBS
• Mechanical extract ventilation (MEV): centralised or decentralised extract fans with replacement air entering via purpose-provided openings
• Variable air volume (VAV): supply and extract adjusted based on zone demand using CO₂, temperature, or occupancy sensors
• Fan coil units with fresh air supply: local recirculating units for heating and cooling with a separate outdoor air supply system

Mixed-mode (hybrid)

Mixed-mode buildings combine natural and mechanical ventilation, typically with automatic changeover based on conditions. This is a common approach for schools and offices with openable windows plus mechanical backup. Part F ventilation rates must be satisfied at all times, regardless of which mode is active (AD F2, para 1.9; CIBSE AM13).

MVHR and heat recovery requirements

Heat recovery efficiency is not specified in AD F2 - it is set by the NCM Modelling Guide and enforced through the SBEM calculation. The current Part L 2021 notional building assumes a sensible seasonal heat recovery efficiency of 76% for balanced mechanical ventilation systems (NCM 2021, para 49). The FBS impact assessment raises this to 80% for the preferred option specification (FBS Impact Assessment, Tables 7–8, p48). The notional building heat recovery assumption feeds directly into the BTER and BTPER targets.

Systems serving spaces where extract air is contaminated (commercial kitchens, fume cupboards, car parks) are typically exempt from heat recovery requirements due to cross-contamination risk. However, run-around coils or other indirect heat recovery methods should be considered where practicable.

In practice, many designers specify heat recovery efficiencies of 80–90% and lower SFPs to create headroom against the compliance targets. Higher heat recovery directly reduces ventilation heat loss, which is one of the largest energy flows in a well-insulated non-domestic building.

Specific fan power (SFP) limits

Specific fan power is the total electrical power consumed by all fans in the system divided by the total ventilation air flow rate, expressed as W/(l/s). Lower SFP means less electrical energy consumed for ventilation. SFP limits are set in AD L2 2026 (not AD F2) and enforced through the SBEM/NCM calculation. The backstop (maximum allowable) values are (AD L2 2026, Table 5.9):

• Centralised balanced mechanical ventilation: 1.6 W/(l/s) maximum (supply and extract combined)
• Centralised mechanical extract only: 0.8 W/(l/s) maximum
• Local mechanical extract (e.g. toilet extract): 0.5 W/(l/s) maximum

AD F2 notes that energy efficiency should be considered when specifying ventilation systems, and refers designers to Part L (AD F2, para 0.19). Ventilation system design must satisfy both Part F (adequate ventilation) and Part L (energy efficiency) simultaneously.

Achieving low SFP in practice requires careful ductwork design (short, straight runs with appropriate sizing), high-efficiency fans (backward-curved centrifugal or EC motor technology), variable speed drives, and filter selection that balances air quality against pressure drop.

Indoor air quality and CO₂ targets

CO₂ concentration is the primary proxy indicator of ventilation adequacy in occupied spaces. AD F2 Appendix C provides guidance on CO₂ monitoring (AD F2, Appendix C):

• Outdoor baseline: approximately 400 ppm (AD F2, Appendix C)
• Well-ventilated space: consistent indoor CO₂ below 800 ppm indicates good ventilation
• Poor ventilation indicator: an average of 1,500 ppm over the occupied period indicates poor ventilation and action should be taken

For spaces with continuous talking, singing, dancing, sport or exercise, AD F2 recommends keeping CO₂ levels below 800 ppm (AD F2, Appendix C). CO₂ monitors should be NDIR type, mains powered, and placed at breathing height away from windows, doors, and ventilation openings (AD F2, paras 1.21–1.23).

AD F2 Appendix B also sets indoor air pollutant guidance values including formaldehyde (100 μg/m³, 30-minute average), TVOC (300 μg/m³, 8-hour average), and nitrogen dioxide limits (AD F2, Appendix B, Table B1). Where buildings are near busy roads or industrial sources, Section 2 provides guidance on minimising the ingress of external pollutants (AD F2, paras 2.1–2.9).

Demand-controlled ventilation (DCV)

Demand-controlled ventilation adjusts ventilation rates based on measured CO₂ or occupancy rather than running at a fixed design maximum, reducing energy consumption while maintaining air quality. AD F2 permits lower ventilation rates where automatic controls can demonstrate that performance criteria in Appendix B are met, though expert advice should be obtained (AD F2, para 1.13).

AD F2 requires CO₂ monitors in new non-domestic buildings for occupiable rooms in offices, rooms with singing or aerobic activity, and rooms where members of the public gather. Monitors must be NDIR type, mains powered, and positioned at breathing height at least 500 mm from occupants (AD F2, paras 1.21–1.23). DCV is particularly effective for meeting rooms, lecture theatres, open-plan offices with variable occupation, and retail premises with seasonal footfall variation.

In the SBEM calculation, DCV receives an energy credit because it reduces average ventilation rates (and associated heating energy) in variable-occupancy spaces.

Relationship between Part F and Part L

The interaction between Part F and Part L is one of the most important design considerations for non-domestic buildings under the FBS. AD F2 explicitly notes that energy efficiency should be considered when specifying ventilation systems (AD F2, para 0.19). Ventilation heat loss is a major component of the energy balance, and the BPER and BER calculated by SBEM include ventilation energy - fan power, heat loss through ventilation air, and heat recovery credits.

Part F sets the minimum ventilation rates – these cannot be reduced to save energy. Part L sets the maximum energy consumption – ventilation systems must deliver the Part F rates with minimum energy use. Designers must balance both requirements simultaneously.

The key ventilation parameters that affect the SBEM calculation are:

• Outdoor air supply rate: higher rates increase ventilation heat loss but must meet Part F minimums
• Heat recovery efficiency: higher efficiency reduces ventilation heat loss - the current notional building assumes 76% (NCM 2021, para 49); the FBS preferred option targets 80% (FBS Impact Assessment, Table 8, p48)
• Specific fan power: lower SFP reduces electrical energy for fan operation
• Ductwork leakage: leaky ductwork wastes conditioned air – pressure testing demonstrates integrity
• DCV: reduces average ventilation rates and associated energy in variable-occupancy spaces
• Free cooling (economiser): using outdoor air for cooling when external temperature is below internal setpoint reduces mechanical cooling energy
• Night purge ventilation: reduces peak cooling demand and mechanical cooling energy

Controls and commissioning

All mechanical ventilation systems must be commissioned in accordance with Regulation 44 and CIBSE Commissioning Code M. Commissioning should also follow BESA guidance: DW 144 for metal ductwork and DW 143 for ductwork air leakage testing (AD F2, paras 4.1–4.3).

AD F2 requires that controls allow ventilation in each room to be adjusted (AD F2, para 1.36). Key control requirements include:

• Time control: systems must operate during occupied hours and reduce or switch off during unoccupied hours
• Zone control: multi-zone buildings should have independently controllable ventilation zones matching occupancy patterns
• Local override: occupants should have local control where practicable (openable windows, local fan speed, boost buttons)
• BMS integration: in larger buildings, ventilation systems should be integrated with the building management system for monitoring, scheduling, and fault detection

Sufficient information about the ventilation system and its maintenance requirements must be given to the building owner, including design flow rates, in a building log book following CIBSE TM31 (AD F2, paras 4.4–4.9). The BRUKL report documents the as-built ventilation specifications alongside other building services.

FBS implications for ventilation

Although AD F Vol 2 was not updated for the FBS, the tighter energy targets in AD L2 2026 have significant implications for ventilation system design:

• Heat recovery efficiency: the Part L 2021 notional building assumes 76% sensible seasonal efficiency (NCM 2021, para 49); the FBS impact assessment raises this to 80% for the preferred option (FBS Impact Assessment, Tables 7–8, p48)
• Tighter SFP backstops: AD L2 2026 sets maximum SFP of 1.6 W/(l/s) for centralised balanced systems (AD L2 2026, Table 5.9)
• CO₂ monitoring: AD F2 requires NDIR CO₂ monitors in new non-domestic buildings for offices and public-facing spaces (AD F2, paras 1.21–1.23)
• DCV energy credit: demand-controlled ventilation reduces average ventilation rates in variable-occupancy spaces, helping to meet the tighter BPER targets
• Part O interaction: AD F2 acknowledges that overheating risk assessment may require higher purge ventilation rates for buildings within Part O scope (AD F2, para 0.21)