Short answer: Every filter, treatment casing, bend and attenuator resists airflow. The fan delivers the volume available at the point where its pressure capability meets the total system resistance. Adding filtration without recalculating that operating point can reduce extract volume and allow heat, fumes and combustion products to escape the canopy.
A fan is not a fixed-airflow machine unless a properly commissioned control system measures and maintains the duty within its capability. A percentage speed setting does not prove airflow. The design needs a pressure-loss schedule for the clean system and a defined loaded condition, followed by balancing measurements after installation.
Important: This guide explains design questions; it is not a product selection, planning approval or performance guarantee. A competent designer should assess the actual kitchen, air system, discharge and maintenance conditions.
Clean, loaded and failed conditions
Mechanical filters gain resistance as they load. Carbon beds and panels also have product-specific pressure characteristics. ESP cells can accumulate grease or become obstructed even if their initial resistance is relatively low. The designer should decide the service trigger and fan reserve rather than waiting until capture visibly fails.
- Record initial clean pressure losses.
- Define an acceptable loaded-filter condition.
- Provide gauges or alarms where useful.
- Link maintenance frequency to observed loading and cooking hours.
The fan operating point
The fan curve shows the relationship between volume and pressure at a given speed. The system curve represents the resistance of the installed route. Their intersection is the working point. If resistance increases, the point moves and delivered volume normally falls unless controls and available fan capacity compensate.
- Select against total calculated resistance.
- Check motor and drive capability at the required speed.
- Include attenuators, terminals and replacement air interactions.
- Avoid selecting only from free-air volume.
Why the kitchen may notice first
Reduced extract airflow can appear as heat, steam or smoke escaping the canopy, doors becoming difficult due to pressure imbalance, or gas interlock problems where fan proving is involved. These symptoms should prompt measurement and safe investigation; simply turning the fan faster can create noise, draughts and ESP velocity problems.
- Observe containment during peak cooking.
- Measure rather than infer from sound.
- Check make-up air and building pressure.
- Recommission after changing filter grade or adding stages.
What the observation may be telling you
| Observation | What it may mean | Correct next step |
|---|---|---|
| New filters, correct airflow | Design operating point is being achieved | Record baseline readings |
| Airflow gradually falls | Filters loading or duct/fan deterioration | Inspect and compare pressure readings |
| Airflow low immediately after retrofit | Fan/system not selected for added resistance | Recalculate and recommission |
| Fan speed increased, noise excessive | Attempt to overcome resistance without full review | Check fan, duct velocity and acoustic design |
| Uneven capture across canopy | Branch balance or make-up air problem | Measure distribution, not only total flow |
A practical design and handover workflow
Treat the filtration unit as one component of an air system. A dependable design starts with the cooking process and ends with a maintainable discharge, rather than starting with a catalogue airflow. The following workflow gives the client, designer and installer a shared audit trail.
- Define the source. List every appliance, fuel, cooking method, duty, diversity assumption and operating period. Chargrilling, frying and light reheating do not create the same grease, smoke or odour load. Record the future menu where the tenant is not yet trading.
- Establish the air duty. Calculate the extract volume and capture arrangement, then record the expected operating range. Check make-up air, duct velocity, leakage class, fan curve and the resistance of clean and loaded treatment stages.
- Select the treatment train. State what each stage is intended to remove, the evidence used for selection and the conditions attached to any performance claim. Include bypass sealing, controls, interlocks, alarms and the effect of failure.
- Prove and maintain it. Commission airflow and functions at the installed condition. Give the operator baseline readings, access instructions, cleaning intervals, consumables and a clear trigger for inspection or replacement.
Keep a useful record: Keep the approved drawing, design airflow, fan duty, clean pressure readings, control cause-and-effect, commissioning results and maintenance log together. When performance changes, comparison with this baseline is more useful than replacing parts by guesswork.
Project or service checks
- Request a component-by-component pressure schedule.
- Show fan duty in volume and external static pressure.
- Define clean and loaded commissioning targets.
- Include replacement air in the balance.
- Provide accessible pressure tappings where appropriate.
- Record inverter settings but do not treat them as airflow readings.
- Recommission after any material filtration change.
Turn the agreed checks into named deliverables. Each item should have an owner, due date and acceptance method. Where a measurement is required, record the value, unit, instrument or source, operating condition and result. This makes the information usable at handover and during a later investigation.
Common mistakes to avoid
- Selecting a fan from maximum airflow at zero pressure.
- Adding carbon after commissioning without fan review.
- Ignoring the loaded state of disposable filters.
- Increasing speed without checking ESP operating range.
- Failing to document the baseline pressure and airflow readings.
Avoid closing the issue on appearance alone. A new filter, reset controller or revised drawing may change the symptom without resolving the underlying duty, utility, access or maintenance problem. Confirm the completed work under a representative operating condition and keep the evidence with the asset or ventilation record.
Frequently asked questions
Does an ESP have no pressure loss?
No real duct component has zero resistance. ESP products may have relatively low clean resistance, but the manufacturer data and complete casing arrangement must be used.
Can a variable-speed drive solve filter loading?
It can be part of a control strategy if the fan has capacity and airflow is measured, but it does not remove the need for maintenance.
Why does a new higher-grade filter reduce airflow?
Denser or deeper media can have a different pressure characteristic. The fan/system must be checked before substituting filter types.
Related Kitchen Install Guide reading
- Commercial Kitchen Make-Up Air Explained
- What Is DW/172?
- What Should a Kitchen Extraction Quote Include?
Capture the installation and commissioning detail
Use the KIG 318 Commercial Kitchen Installation Checklist to record interfaces, checks, outstanding actions and the evidence needed at handover.
Sources and further guidance
- BESA: DW/172 Specification for Kitchen Ventilation Systems
- HSE: Ventilation of kitchens and catering establishments
- Purified Air: Commercial kitchen multi-filter units
- Extechnology: Airflow rates and ESP performance
Last reviewed: 14 August 2026. This article provides general commercial-kitchen design information. It does not replace a site-specific ventilation design, manufacturer instructions, risk assessment, planning conditions or advice from a competent specialist.