Whether you are speccing a new commercial building, retrofitting an industrial facility, or managing a large residential development, ventilation design sits at the centre of every successful mechanical services project. Done well, it delivers compliant airflow, comfortable occupants, manageable energy bills, and a system that performs reliably year after year.
Done poorly, it creates callbacks, enforcement notices, and unhappy clients. The 15 tips in this guide are drawn from real-world professional practice and are aimed squarely at contractors, M&E consultants, and facilities managers who want to raise the standard of their ventilation design work.
Tip 1: Start With a Thorough Ventilation Needs Assessment
Every effective ventilation design begins with a detailed understanding of what the space actually requires, not what a similar past project required or what a rule-of-thumb figure suggests.
A proper needs assessment covers the occupancy type and density, the heat and moisture loads from people and equipment, the nature of any contaminants generated in the space, the building fabric characteristics including airtightness, and the regulatory requirements that apply to the specific building type and use.
Skipping or rushing the needs assessment is the root cause of most underperforming ventilation systems. A needs assessment takes time, but it is the only reliable way to establish the design airflow rate, the appropriate system type, and the performance targets the system must meet before a single product is selected or a duct route planned.
Key Actions at the Needs Assessment Stage
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Establish the ventilation objective clearly: is it general air quality, contaminant control, thermal management, or a combination of all three?
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Carry out a COSHH assessment for any space where hazardous substances may be generated, and let the assessment findings drive the ventilation design rather than the other way around
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Confirm the applicable regulatory framework, including Building Regulations Part F, COSHH Regulations, and Workplace (Health, Safety and Welfare) Regulations 1992, before setting design targets
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Document occupancy patterns and operational schedules, as a building used 24 hours a day needs a fundamentally different ventilation strategy than one used during standard working hours
Tip 2: Use the Correct Airflow Calculation Method for the Application
Airflow calculation is not a one-size-fits-all exercise. The air changes per hour (ACH) method, occupancy-based fresh air rates, and contaminant dilution calculations each suit different design scenarios, and using the wrong method produces a system that either under-ventilates the space or wastes energy moving more air than the building actually needs.
ACH-based calculations work well for general warehousing, light manufacturing, and standard commercial occupancies where a target air change rate is a reasonable starting point. Occupancy-based fresh air calculations apply where significant numbers of people are the primary driver of indoor air quality demand.
Contaminant dilution calculations are required wherever a specific hazardous substance must be maintained below its occupational exposure limit under EH40.
Common Airflow Calculation Errors to Avoid
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Using free-air fan ratings rather than the fan's rated output at the actual system static pressure, which is always lower and sometimes dramatically so
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Applying generic ACH targets without verifying them against the specific heat and moisture loads of the space in question
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Failing to add a system resistance allowance to the design airflow figure, meaning the installed fan operates below target from day one
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Ignoring the impact of derating factors such as flexible duct connections, filter resistance, and grille free-area restrictions on effective system airflow
Tip 3: Design Ductwork for Minimum System Resistance
The ductwork layout has a bigger impact on overall ventilation system performance than any individual fan or product choice. A duct system with excessive resistance forces the fan to work harder than intended, moves the fan's operating point towards its stall region, increases noise, and raises energy consumption.
Designing for minimum resistance from the outset is far more effective than trying to compensate with a bigger fan.
The principal drivers of duct resistance are air velocity, the number and sharpness of bends and fittings, changes in duct cross-section, and the total duct length. Every avoidable bend, unnecessary reduction, or sharp fitting adds resistance that compounds with every other component in the system.
Good ductwork design keeps routes short, bends long-radius, and transitions gradual wherever the building fabric allows.
Best Practice Ductwork Design Principles
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Keep main duct air velocities within the recommended range of 5 to 8 m/s for general industrial supply and extract mains, and 3 to 5 m/s for commercial office and retail systems to balance resistance and noise
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Use 45-degree branch take-offs rather than 90-degree tee junctions wherever possible, as they reduce branch resistance significantly
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Plan duct routes in coordination with other services from the earliest design stage, as ductwork forced to navigate around structural elements and other services is almost always longer and has more bends than a planned route
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Specify long-radius elbows with a centreline radius of at least 1.5 times the duct diameter rather than short-radius bends, particularly on high-velocity main duct sections
Tip 4: Select Fans Based on the System Operating Point, Not Free-Air Performance
One of the most persistent errors in ventilation design is selecting a fan based on its maximum free-air airflow rating rather than its actual output at the system's calculated static pressure. Every fan delivers its peak flow rate at zero resistance, and that figure drops as resistance increases. The real question is how much airflow the fan delivers at the total pressure loss of the specific duct system it will be connected to.
This requires reading and understanding fan performance curves rather than simply comparing headline flow rate figures. The operating point of the fan, where the fan's performance curve intersects the system resistance curve, must fall within the stable operating region of the fan's curve. A fan operating near its stall point is noisy, inefficient, and subject to accelerated motor wear.
Fan Selection Checklist for Professionals
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Calculate total system resistance for the index circuit before selecting any fan, accounting for all ductwork, bends, fittings, filters, terminals, and grilles
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Plot the calculated system resistance curve on the fan's performance graph and confirm the intersection falls in the efficient, stable operating zone
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Select a fan with a rated motor power that includes an adequate safety margin above the maximum likely shaft power at the operating point, typically 10 to 15 percent
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For variable-flow systems, check that the fan operates stably across the full intended speed range, not just at design conditions
eFans stocks axial, centrifugal, mixed-flow, and inline fans from brands including Systemair, S&P, Elta, Vent-Axia, and Hydor, with full performance data available to support operating point verification. Browse our full range of extractor fans.
Tip 5: Specify EC Motor Fans to Meet Energy Performance Requirements
AC motor fans served the ventilation industry well for decades, but for any new installation or major refurbishment where the ventilation system will run for significant daily hours, EC motor technology is now the professional standard.
EC (electronically commutated) motors are brushless DC motors with integrated electronics that adjust motor speed in response to load, delivering efficiencies of 80 to 90 percent compared to 55 to 70 percent for comparable AC units.
The energy saving from EC motor fans is most pronounced at partial-load conditions, which is where most ventilation systems spend the majority of their operating time.
On a continuously running commercial or industrial system, the cumulative annual energy saving from EC over AC can justify the modest premium in capital cost within one to two years of operation. Building Regulations Part L and ErP Directive requirements are also progressively mandating higher fan efficiency standards that AC motor fans increasingly struggle to meet.
When EC Motor Specification Is Essential
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New-build commercial and industrial buildings where Part L compliance requires minimum Specific Fan Power (SFP) targets to be met
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MVHR systems and whole-building mechanical ventilation installations that run continuously throughout occupied hours
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Large multi-fan industrial systems where the aggregate energy consumption of the fan plant is a meaningful contributor to the building's operational carbon footprint
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Retrofit projects replacing older fixed-speed AC fan installations where energy cost reduction is a primary client objective
Tip 6: Apply Variable Speed Control Wherever Demand Varies
A ventilation system that runs at full speed and full airflow at all times is almost always wasting energy. Occupancy in industrial and commercial buildings varies by hour of day, day of week, and season.
Contaminant generation in process environments varies with production schedules. A fixed-speed system designed for peak demand over-ventilates the space during every other operating condition, consuming fan energy needlessly around the clock.
Variable speed drives (VSDs) solve this by allowing the fan motor speed, and therefore airflow, to be continuously adjusted in response to a demand signal. Because fan power consumption reduces with the cube of speed reduction, even a modest reduction in fan speed delivers a disproportionately large energy saving.
A fan running at 80 percent speed consumes only about 51 percent of the energy of the same fan at 100 percent speed.
Demand Control Strategies Worth Specifying
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CO2-based demand control uses occupancy-linked CO2 sensors to vary fresh air supply in proportion to actual occupancy, a well-established approach for commercial offices, schools, and meeting rooms
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Temperature-controlled extract for industrial heat management adjusts fan speed in response to space temperature, increasing airflow during peak heat-load periods and reducing it during cooler or quieter operational periods
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Occupancy sensor integration links ventilation directly to PIR-detected occupancy, ensuring washrooms, changing areas, and intermittently used production spaces are ventilated on demand rather than on a fixed schedule
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Production schedule-based control links ventilation to process control systems in manufacturing environments, ramping extract ventilation up when hazard-generating processes are active and reducing it during idle periods
Tip 7: Account for Duct Leakage in Your Design Calculations
Duct leakage is one of the most underestimated performance issues in ventilation design, and it is almost universal in real-world ductwork installations to some degree. Air leaking out of supply ductwork before it reaches the intended terminal reduces the effective airflow delivered to the space.
Air leaking into extract ductwork from unconditioned spaces dilutes the extract stream and reduces the effective extraction rate at the capture points.
HVCA DW/144 classifies ductwork into four leakage classes (A to D) with progressively tighter leakage tolerances, and specifying the appropriate leakage class for the system type is a fundamental design decision. Class A (lowest standard) is acceptable for low-pressure general ventilation.
Class C or D is required for systems serving cleanrooms, pressurised zones, or COSHH-controlled environments where leakage has safety or process implications.
Reducing Duct Leakage in Practice
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Specify the appropriate DW/144 leakage class for the system in the project specification documents rather than leaving it to contractor discretion
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Use proprietary flanged duct joint systems with factory-applied sealant or site-applied mastic rather than relying on standard slip-socket joints sealed with tape alone
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Include leakage testing in the commissioning programme for higher-specification systems, particularly LEV, cleanroom, and pharmaceutical applications
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Check all penetration seals where ductwork passes through walls, floors, and ceilings, as these are consistently the highest-leakage locations in a completed ductwork installation
Tip 8: Design for Maintenance Access From Day One
A ventilation system that cannot be properly maintained will not perform to design specification for long. Fan impellers accumulate dust and become unbalanced. Filters fill and restrict airflow. Heat exchanger surfaces foul and lose efficiency. Dampers seize in incorrect positions.
Every one of these failure modes is preventable with the right maintenance regime, but only if the system was designed with adequate access to all serviceable components from the outset.
Maintenance access is one of the design considerations most frequently compromised in the pressure to minimise ceiling void depth, maximise plant room space for other uses, or reduce ductwork installation costs. The long-term consequence is a system that is either inadequately maintained because access is too difficult, or one that requires expensive building work to access for every service visit.
Designing for Maintainability
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Position fan plant in accessible locations with enough clear working space for impeller removal, motor replacement, and bearing inspection without removing adjacent ductwork or services
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Include inspection hatches in ductwork at every fan, filter housing, fire damper, and change-of-direction point, sized to allow physical access for inspection and cleaning
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Specify filter housings with tool-free filter access on MVHR and air handling units, as filters that are difficult to change are changed less frequently, directly degrading system performance
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Coordinate maintenance access space requirements with the structural and architectural design team at the earliest stage, before ceiling voids and plant rooms are dimensionally fixed
Tip 9: Integrate Fire Dampers and Smoke Control Requirements at Design Stage
Fire dampers and smoke control ventilation requirements have a direct and significant impact on ductwork design, fan selection, and system controls, and they cannot be treated as an afterthought once the general ventilation design is complete.
Every penetration of a fire-rated compartment wall or floor by ductwork requires a fire damper or fire-resisting duct enclosure, and each fire damper adds resistance to the duct system that must be accounted for in the pressure calculations.
For buildings with dedicated smoke control ventilation systems, the mechanical ventilation strategy must be designed in coordination with the smoke control strategy from the outset.
In some building types, the general mechanical ventilation system is required to switch modes and operate as smoke exhaust in a fire scenario, which has specific implications for fan sizing, duct construction standard, and control system design.
Fire Safety Coordination Checklist
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Identify all fire compartment boundaries that the ductwork crosses at the earliest design stage and confirm the fire damper or duct protection requirement for each with the fire engineer
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Account for the pressure loss contribution of fire dampers in the system resistance calculation, as a fully-open fire damper blade still adds meaningful resistance to the duct
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Specify fire dampers with remote position indication and test capability where the damper is inaccessible for manual inspection after installation
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Confirm with the project fire engineer whether any ventilation fans are required to operate in fire mode, as this affects motor specification, control panel design, and cable specification requirements
Tip 10: Coordinate Ventilation Design With the Building Thermal Model
Ventilation and thermal performance are inseparable in modern building design, and treating them as independent disciplines produces buildings that either over-ventilate and waste heating energy, or under-ventilate and overheat.
The ventilation design must be integrated with the building's thermal model from the early design stages, particularly for new-build commercial and industrial projects where overheating risk assessment and Part L energy calculations both depend on accurate ventilation inputs.
In buildings with significant internal heat gains from people, equipment, or industrial processes, mechanical ventilation contributes directly to thermal management.
The interaction between ventilation rate, heat recovery efficiency, and the building's heating and cooling systems determines the overall energy performance of the building, and that interaction must be modelled and optimised rather than assumed.
Thermal and Ventilation Integration Points
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Confirm the ventilation heat loss and heat gain contributions with the energy assessor before the thermal model is submitted for Part L compliance, as ventilation is typically the largest single energy loss mechanism in well-insulated modern buildings
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For MVHR systems, provide the energy assessor with the manufacturer's certified heat recovery efficiency figures and specific fan power data for accurate SAP or SBEM input
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In industrial buildings with significant process heat gains, evaluate whether heat recovery from extract air is feasible and cost-effective before defaulting to a simple mechanical extract strategy
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Coordinate free-cooling strategies, where increased night-time ventilation reduces daytime cooling loads, with both the ventilation control design and the building thermal model
Tip 11: Consider Acoustic Performance as a Core Design Requirement
Noise from ventilation systems is one of the most common sources of occupant complaints in completed commercial and industrial buildings, and it is almost always easier and cheaper to address at the design stage than after handover.
Ventilation noise reaches occupied spaces through two pathways: airborne sound propagating through the ductwork and being radiated by grilles and diffusers, and structure-borne noise transmitted from the fan and motor through its supports and connections into the building fabric.
A comprehensive acoustic design approach addresses both pathways. It starts with selecting a fan that operates in its quiet, efficient operating zone rather than near stall. It continues with anti-vibration mounting for the fan, flexible duct connections at the fan inlet and outlet, and acoustic attenuators sized to reduce airborne sound to the target levels in occupied spaces.
Acoustic Design Best Practices
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Obtain octave-band sound power level data for the selected fan at its actual operating point, not just a single overall dB(A) figure, to allow accurate attenuator sizing
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Position fan plant as far as practical from noise-sensitive spaces such as offices, meeting rooms, and healthcare consultation areas within or adjacent to industrial buildings
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Specify flexible duct connections at both the fan inlet and outlet as a standard detail, not only on systems where noise is a formal specification requirement
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Line the first two to three metres of ductwork downstream of the fan with acoustic insulation to attenuate fan-generated noise before it propagates further into the distribution network
Tip 12: Plan the Commissioning Programme Before Construction Starts
Commissioning is not something that happens at the end of a project. It is a planned activity that must be resourced, scheduled, and designed into the project from the outset.
The information required to commission a ventilation system correctly, including design airflow targets for every terminal, system resistance calculations, and fan operating point data, must be prepared during the design stage and handed over to the commissioning engineer before site work starts.
BSRIA Guides BG 16 and BG 52 provide the standard methodology for commissioning mechanical ventilation systems in the UK, and compliance with these guides is expected on most commercial and industrial projects.
A commissioning specification that references these guides and sets out clear acceptance criteria for airflow measurement and system balancing gives the commissioning team a clear brief and the client a clear standard against which handover performance is measured.
Commissioning Preparation Actions for Design Engineers
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Prepare a ventilation schedule listing the design airflow rate for every supply and extract terminal in the system before construction starts, not at practical completion
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Include balancing dampers in the ductwork design at every branch take-off to enable the commissioning team to balance the system without modifying the ductwork
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Specify measurement points, such as pitot traverse sections or fixed flow measurement stations, in the ductwork design for systems where ongoing performance monitoring is required after commissioning
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Confirm with the project programme that adequate time is allowed for commissioning before practical completion, as ventilation commissioning on complex systems takes days rather than hours
Tip 13: Specify the Right External Terminals for Each Application
External terminals, including roof cowls, wall grilles, louvres, and weather-protected outlets, are the interface between the ventilation system and the external environment. They are also one of the most frequently underspecified components in a ventilation design.
A poorly chosen terminal introduces unnecessary resistance into the system, allows wind-driven rain or pests into the ductwork, generates noise in windy conditions, or fails to prevent air recirculation between exhaust and fresh air intakes.
For bathroom and toilet extract in residential and light commercial applications, a standard wall terminal with back-draught shutter is usually adequate. For industrial extract carrying moisture, grease, or chemical contamination, a purpose-designed outlet cowl with the appropriate material specification is essential.
For fresh air intakes on commercial and industrial buildings, louvres with an adequate free area and weather protection standard must be selected to ensure the intake resistance does not become a significant portion of total system resistance.
External Terminal Selection Criteria
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Check that the free area of the selected terminal, expressed as a percentage of the duct cross-sectional area, does not contribute excessive resistance at the design airflow velocity
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Ensure the separation distance between exhaust outlets and fresh air intakes meets the minimum recommended by CIBSE and the equipment manufacturer to prevent contaminated recirculation
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For roof-mounted terminals on industrial buildings, select cowls rated for the wind exposure category of the site to prevent wind-induced backpressure affecting extract fan performance
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Confirm that terminal material specification is appropriate for the airstream content, particularly for kitchen, chemical, or moisture-laden extract where standard powder-coated steel terminals corrode prematurely
Tip 14: Document the Design Fully and Hand Over a Complete O&M Manual
A ventilation system is only as useful to the client as the information available to operate and maintain it correctly. An incomplete or absent operation and maintenance manual means building managers and maintenance teams do not know what the system is designed to do, what airflow rates it should be delivering, what maintenance intervals are required, or what to check when performance deteriorates.
A complete ventilation O&M manual covers the system description and design intent, the design airflow rates and pressure parameters, the commissioning results including measured terminal airflows, the maintenance schedule and intervals for every serviceable component, spare parts information, and the contact details for the design engineer, contractor, and equipment suppliers.
For LEV systems, the O&M pack must also include the first thorough examination record and the 14-month re-examination schedule.
O&M Manual Minimum Contents for Ventilation Systems
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As-built ductwork drawings showing all duct sizes, routes, fittings, damper locations, and terminal positions as actually installed rather than as originally designed
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Commissioning results schedule showing measured airflow at every terminal alongside the design target, confirming the system was balanced to specification at handover
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Equipment data sheets and installation manuals for every fan, unit, and control device in the system
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Planned preventative maintenance schedule with specific tasks, frequencies, and competency requirements for each activity, referenced to the relevant BSRIA or manufacturer guidance
Tip 15: Stay Current With Regulatory and Standards Updates
UK ventilation regulations and technical standards evolve regularly, and professional practice requires staying ahead of those changes rather than designing to requirements that have already been superseded. Approved Document F was significantly updated in 2021 with revised ventilation rates, new requirements for airtightness and overheating risk assessment, and updated guidance on mechanical ventilation system types.
Part L has been progressively tightened in successive revisions, and further updates in support of the Future Homes Standard will continue to raise minimum energy performance requirements for ventilation equipment and systems.
Keeping up with CIBSE guidance updates, HSE guidance revisions including EH40 and HSG258, and changes to British Standards relevant to ventilation, including BS EN ISO 16000 for indoor air quality and BS EN 13779 for non-residential building ventilation, is a professional obligation rather than optional continuing development.
Ventilation designs that reference outdated standards expose the design engineer to technical and legal risk.
Resources for Staying Current
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Subscribe to CIBSE technical updates and the BSRIA technical guidance library, both of which publish amendments and new guidance relevant to ventilation design practice
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Monitor HSE enforcement publications and case summaries for LEV and workplace ventilation, which provide practical insight into where compliance failures most commonly occur
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Review NHBC Technical Standards and LABC warranty provider guidance annually, as these bodies implement regulatory changes on the ground for new-build residential and commercial projects
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Engage with eFans' free expert advice service for product-level specification support, including guidance on which fan and ventilation products currently meet Part L SFP requirements and ErP Directive standards at
Frequently Asked Questions
What is Specific Fan Power and why does it matter in ventilation design?
Specific Fan Power (SFP) is a measure of ventilation system energy efficiency expressed in Watts per litre per second (W/l/s). It represents the total electrical power consumed by all fans in a ventilation system divided by the total design airflow rate. Building Regulations Approved Document F sets maximum SFP limits for different ventilation system types in new-build and notifiable refurbishment projects.
A lower SFP indicates a more energy-efficient system. Achieving compliant SFP requires careful selection of fan type and motor efficiency, minimising ductwork resistance, and avoiding undersized duct sections, excessive bends, and high-resistance terminals. For most commercial mechanical ventilation systems, SFP targets of 0.5 to 1.5 W/l/s are typical, though the specific limit depends on system type and building category as set out in Part F and the relevant CIBSE guidance.
How do you prevent condensation in ventilation ductwork?
Condensation in ventilation ductwork occurs when warm, moisture-laden air contacts a duct surface that is below the dew point temperature of the airstream. It is most common in extract ductwork passing through cold loft spaces, unheated plant rooms, or external walls in UK climates.
Prevention requires insulating any section of extract ductwork that passes through an unheated space, using insulation with a vapour-barrier outer facing to prevent moisture from the surrounding cold air migrating through the insulation and reaching the duct surface.
Ductwork should also be installed with a slight fall towards the external terminal or a condensate drain point so that any condensation that does form can drain away rather than pooling inside the duct and causing corrosion or returning into the occupied space.
For MVHR systems, the heat exchanger core and surrounding ductwork must be insulated to the manufacturer's specification to prevent condensation at the point where warm extract air meets cold supply air within the unit.
What is the difference between supply ventilation and balanced ventilation?
Supply-only ventilation introduces fresh air into a building using fans and ductwork without providing a corresponding powered extract route, relying on adventitious infiltration through the building fabric for air to escape. This creates positive pressure in the building that can cause moisture-laden air to be pushed into the building fabric under certain conditions, potentially causing interstitial condensation.
Balanced ventilation provides both powered supply and powered extract at matched rates, maintaining a neutral or designed pressure condition in the building. Mechanical ventilation with heat recovery (MVHR) is the most widely specified form of balanced ventilation in UK new-build projects, as it combines the benefits of balanced airflow with heat recovery efficiency.
Building Regulations Part F increasingly favours balanced mechanical ventilation in tightly constructed modern buildings, and supply-only systems require careful analysis of moisture risk before specification.
How often should ventilation systems in commercial buildings be serviced?
Commercial ventilation systems should be serviced at least annually as a minimum, with more frequent maintenance required for components in demanding operating environments or those with significant safety implications. Fan impellers should be inspected and cleaned annually, with bearings checked and lubricated according to the manufacturer's schedule.
Filters in supply air handling units and MVHR systems require inspection every three to six months and replacement when pressure drop across the filter bank exceeds the specified maximum. Heat exchanger surfaces on MVHR units should be cleaned annually to maintain heat recovery efficiency. Fire dampers in ductwork require inspection and operational testing every year in accordance with BS 9999 and the fire strategy for the building.
LEV systems require thorough examination and testing every 14 months under COSHH Regulation 9, with records retained for five years. Planned preventative maintenance contracts for ventilation systems should be structured around the O&M manual requirements and the specific operating conditions of each installation.
What is the role of pressure testing in ventilation system quality assurance?
Pressure testing of ductwork verifies that the completed installation meets the specified leakage class under DW/144 before the system is commissioned and the building is occupied. The test involves sealing all terminals and openings in the duct system, pressurising the ductwork to a specified test pressure, and measuring the leakage rate against the pass/fail criteria for the specified leakage class.
Ductwork that fails the leakage test must be inspected, defective joints identified and re-sealed, and the test repeated until the specified leakage class is achieved. Pressure testing is a mandatory requirement for ductwork serving cleanrooms, pharmaceutical manufacturing, and other environments where DW/144 Class C or D is specified.
For general commercial and industrial ventilation, pressure testing is increasingly specified by clients and project managers as a quality assurance measure even where it is not a regulatory requirement, because it provides objective evidence that the ductwork installation has been completed to the specified standard before practical completion.