Ventilation Design: A Practical Guide for UK Engineers

What ventilation design covers

Ventilation in building services has four main purposes: providing fresh outdoor air to occupants, removing airborne contaminants (CO₂, VOCs, moisture, odours), controlling condensation risk, and contributing to thermal comfort in summer through free cooling or purge ventilation.

In UK practice, ventilation design is governed by multiple overlapping standards — from the statutory Building Regulations to CIBSE industry guidance to sector-specific requirements like BB101 for schools. This guide walks through the practical design process: which standards apply, how to determine flow rates, which system types to consider, and how to ensure compliance with energy and commissioning requirements.

The key standards

The table below summarises the main standards that affect ventilation design in UK building services. Most projects will need to comply with several of these simultaneously.

Source: Various — see individual standards for current editions.

Fresh air requirements

Ventilation rates in UK practice are specified in two main ways:

• Per-person rates (l/s per person) — used where the primary driver is occupant-generated pollutants (CO₂, body odour). Common in offices, classrooms, and meeting rooms.
• Per-area rates (l/s per m²) or air changes per hour (ACH) — used where the primary driver is process contaminants, moisture, or heat removal. Common in kitchens, plant rooms, and car parks.

The table below provides common ventilation rates for typical UK building types. Always check the specific standard for your project type.

Source: CIBSE Guide A, Approved Document Part F. [VERIFY: exact table references in current editions.]

Ventilation system types

The choice of ventilation system depends on building type, use, energy targets, and available space. The main system types used in UK building services are:

Natural ventilation

Relies on opening windows, trickle ventilators, stack effect, and cross-ventilation. Suitable for low-density occupancies near the building perimeter. Zero fan energy but limited control, dependent on weather, and requires careful facade design. Common in UK schools (where BB101 allows it, subject to overheating assessment) and low-rise residential.

Mechanical extract ventilation (MEV)

Continuous low-rate extract fans with passive supply air through trickle vents or purpose-built inlets. Common in UK residential (flats and houses). Simple, low energy, but no heat recovery from extract air.

Mechanical ventilation with heat recovery (MVHR)

Balanced supply and extract with a heat exchanger recovering warmth from extract air. Increasingly common for Part L compliance because the recovered heat reduces space heating demand. Typical heat recovery efficiency: 70–90% [VERIFY]. Requires dedicated ductwork for both supply and extract.

Mixed-mode ventilation

Combines natural and mechanical ventilation, with controls to switch between them based on conditions. Common in UK education and office buildings. Offers the energy benefits of natural ventilation when conditions allow, with mechanical backup for extreme weather or high-occupancy periods.

Full mechanical (central AHU)

Central air handling unit with supply and extract ductwork serving the entire building or floor. Standard for deep-plan offices, healthcare, retail, and any building where natural ventilation alone cannot meet requirements. Offers the best control over indoor air quality, temperature, humidity, and filtration, but has the highest capital and running costs.

The design process

A practical step-by-step process for ventilation design in UK building services:

1. Determine ventilation requirements — for each space, identify the applicable standard and look up the required supply and/or extract rate. Use per-person rates for occupied spaces and ACH or per-area rates for process spaces.
2. Calculate total flow rates — sum the supply and extract requirements across all spaces. Check that the building-level supply and extract are balanced (or intentionally pressurised/depressurised as required).
3. Select the system type — based on building use, energy targets (Part L), acoustic requirements, available space for ductwork, and project budget.
4. Size ductwork — use the velocity or equal friction method. See the Complete Guide to Ductwork Sizing for the full process.
5. Select air handling units and fans — match the AHU to the required flow rate and total system pressure drop. Include filtration, heating/cooling coils, and heat recovery as needed.
6. Check SFP against Part L limits — calculate the specific fan power and verify it meets the applicable limit (see the SFP section below).
7. Design the controls strategy — consider CO₂ demand control ventilation, occupancy-based scheduling, time-clock operation, and temperature-based free cooling modes.
8. Specify diffusers and grilles — select terminal devices that meet the noise, throw, and pressure drop requirements for each space. Record design flow rates for commissioning.
9. Prepare commissioning data — document the design flow rate for every terminal device, duct section, and AHU. This is the basis for testing and balancing.

Air changes per hour vs flow rate

Air changes per hour (ACH) and volume flow rate (l/s or m³/s) are different ways of expressing the same thing. The relationship between them:

Where: Q = volume flow rate (m³/s), V = room volume (m³).

Or, rearranging to find flow rate from ACH:

Where: V = room volume (m³), Q = flow rate (l/s).

When to use which

• ACH — appropriate for car parks, plant rooms, kitchens, stores, and other spaces where ventilation relates to the volume of the space rather than occupancy.
• Per-person rates (l/s per person) — appropriate for offices, classrooms, meeting rooms, and other spaces where fresh air demand is driven by occupant density.

Specific fan power (SFP)

Specific fan power is the total electrical power consumed by all fans in a ventilation system divided by the total design air flow rate through the system. It is the primary energy metric for ventilation systems under Part L of the Building Regulations.

Units: W/(l/s).

Part L sets maximum SFP limits for different system types:

Source: Approved Document L2, Table 6.3 [VERIFY: exact table reference in current edition].

These limits apply to new buildings and major refurbishments. Keeping duct velocities reasonable, minimising pressure drop through fittings, and selecting efficient fans are the most effective ways to stay within SFP limits.

School ventilation (BB101)

Building Bulletin 101 (BB101) sets ventilation and thermal comfort requirements specific to school buildings in England. Its requirements are in addition to the Building Regulations — you must comply with both Part F minimums and BB101 limits.

Key BB101 requirements:

• CO₂ limit: Daily average must not exceed 1000 ppm during occupied hours.
• Maximum CO₂: Must not exceed 1500 ppm for more than 20 consecutive minutes during occupied hours.
• Minimum purge ventilation: 8 l/s per person capability (for rapid air exchange).
• Overheating: Adaptive thermal comfort thresholds assessed using the TM52 methodology (CIBSE TM52). All three TM52 criteria must be evaluated.

In practice, BB101’s CO₂ limits are tighter than Part F’s general ventilation rates. This often drives higher flow rates for teaching spaces than Part F alone would require. The overheating assessment can also force fundamental changes to the ventilation strategy — from purely natural to mixed-mode or full mechanical — so it should be done early in the design process.

Commissioning and testing

Commissioning is the process of verifying that the installed ventilation system delivers the design flow rates at every terminal device. Without proper commissioning, even a well-designed system may perform poorly.

• Design flow rates must be documented for every terminal device (diffuser, grille, valve) on commissioning schedules issued with the design.
• Commissioning procedure: Follow BSRIA BG 30/2007 procedures for air-side systems. This includes proportional balancing of branches before fine-tuning individual terminals.
• Allowable tolerance: Typically +/- 10% of the design flow rate for individual terminals, and +/- 5% for main branches [VERIFY: check BSRIA BG 30 for exact tolerances].
• Record keeping: Measured values for every terminal must be recorded in commissioning certificates and included in the O&M manual.
• Post-occupancy: For demand-controlled systems (CO₂ or occupancy-based), post-occupancy checks are recommended to verify the control strategy responds correctly under real conditions.

Quick reference tables

A consolidated summary of the key reference data from this guide.

Common ventilation rates

SFP limits (Part L)

Key formulas