How to Commission Ventilation Systems Properly

A ventilation system can look complete, sound as though it is running and still fail to protect a property from excess humidity, mould and stale air. That is why understanding how to commission ventilation systems matters. Commissioning proves that the installed system delivers the airflow it was designed to provide, in the rooms where people need it.

For a family home, rental property, office or multi-unit development, this is the point where good design becomes measured performance. It is also where issues such as crushed ductwork, poorly adjusted valves, blocked grilles and unsuitable fan settings are identified before they become costly call-backs or persistent condensation problems.

What ventilation commissioning actually involves

Commissioning is the process of testing, adjusting and recording the performance of a mechanical ventilation system after installation. It applies to intermittent extract fans, continuous mechanical extract ventilation, decentralised single-room heat recovery units and centralised mechanical ventilation with heat recovery systems.

The aim is not simply to confirm that air is moving. The system must achieve the required extract and supply rates while operating safely, quietly and efficiently. For systems serving new homes and many significant renovation projects, the evidence also needs to support compliance with the applicable Irish Building Regulations and Technical Guidance Document F, 2022.

A proper commissioning visit checks the whole installation: fan operation, duct routes, terminals, filters, controls, airflow rates and the final settings left for the occupier. The result should be a clear commissioning record, not a verbal assurance that everything is working.

Prepare the system before testing begins

Reliable readings start with a system that is ready to test. Measuring airflow too early can produce misleading results and send installers looking for problems that are simply part of an unfinished build.

All ductwork should be connected, sealed and supported, with no sharp kinks or unnecessary restrictions. External grilles and roof terminals must be fitted and free from protective packaging, debris or wet trades residue. Internally, supply and extract valves should be installed, clean and accessible.

For heat recovery systems, check that the correct filters are fitted, the condensate drain is in place where required and both intake and exhaust terminals are clear. A blocked filter, a missing duct cap or an unsealed branch can substantially affect performance.

Controls also need attention before airflow testing. Set boost switches, humidistats, timers and trickle settings in accordance with the design intent. If a bathroom fan is expected to boost with the light switch, for example, it must be tested in that operating mode. A unit measured only at its background setting may appear underpowered when it has not been set up correctly.

How to commission ventilation systems step by step

Confirm the design airflow targets

The commissioning engineer begins with the design information. This identifies the required total ventilation rate, individual room extract or supply targets, and the intended operating modes. The correct target depends on the type of property, room use, occupancy assumptions and ventilation strategy.

A kitchen, utility room, bathroom and WC may each have different extract requirements. In a centralised heat recovery system, bedrooms and living areas also require planned supply volumes. The goal is balanced, controlled movement of air from habitable rooms towards wet rooms, rather than random draughts through the building.

Where no design information exists, particularly in an older home, a competent survey is advisable before adjustments are made. Increasing one fan setting without considering make-up air, transfer paths and other extracts can create new problems elsewhere.

Measure airflows at each terminal

Airflow is measured using suitable calibrated equipment, commonly an airflow capture hood or other approved measuring device appropriate to the terminal type. Readings are taken at each supply and extract valve, grille or fan outlet, then compared with the required figures.

Measurements should be repeated in relevant modes. A continuous system may need readings at normal and boost speeds. An intermittent extractor should be tested while switched on. For demand-controlled ventilation, the engineer should confirm that sensors and controls trigger the expected response.

This stage often reveals why a room still feels damp despite having a new fan. The fan may be capable of the right airflow, but resistance from a long duct run, multiple bends, an undersized external grille or poor installation can reduce what actually reaches the outside.

Balance the system, not just the fan

Balancing means adjusting valves, dampers and fan speeds so each room receives or extracts its intended airflow. It is particularly important for centralised MVHR systems, where an over-open valve in one room can deprive another room of the air it needs.

Adjustments should be methodical. Restricting a valve too far can increase noise, while running the whole system at unnecessarily high speed can increase energy use and disturb occupants. The best result meets the design requirement with sensible fan settings and low noise levels.

There are trade-offs. A compact retrofit may have more duct resistance than a new-build installation, and a larger fan is not always the answer. Improving duct routes, reducing restrictions or selecting an appropriate ventilation approach may provide a better long-term outcome than forcing a system to work harder.

Check controls, noise and user operation

Commissioning is also a practical handover check. Test boost controls, run-on timers, humidity sensors and any smart controls. Confirm that kitchen and bathroom users can operate the system easily and that the controls are labelled clearly.

Listen for excessive vibration, rattling ducts or whistling valves. Some background sound is normal in mechanical ventilation, especially during boost operation, but intrusive noise can lead residents to switch systems off. A system that is turned off cannot manage moisture or protect indoor air quality.

In homes with heat recovery ventilation, explain the difference between normal operation and boost. The unit should generally run continuously at its commissioned background rate, with boost used during showers, cooking and other moisture-producing activities. Opening windows remains a personal choice, but it should not be relied upon as a substitute for a correctly commissioned mechanical system.

Record the final results and issue documentation

The final airflow readings, system settings, equipment details and test date should be recorded on a commissioning sheet or report. This documentation provides evidence of performance at the time of handover and gives future installers a useful baseline for servicing or fault finding.

For projects requiring formal compliance evidence, records should be completed accurately and retained with the building documentation. Landlords, developers and property managers benefit from this audit trail, particularly where several properties have the same ventilation design.

A good handover also includes practical maintenance advice. Filters need regular inspection and replacement, extract valves should not be painted over or blocked, and external grilles need to remain clear. Commissioning is not a one-time cure for neglect, but it establishes the correct starting point.

Common commissioning mistakes that affect performance

The most common mistake is treating ventilation commissioning as a paperwork exercise. A completed form cannot correct a system that has not been measured properly. Another is changing fan speed at the unit without testing every terminal afterwards, which can leave some rooms over-ventilated and others below target.

Poor duct installation is another frequent cause of failed readings. Flexible ducting that is compressed, excessively long or sharply bent adds resistance. Leaks at joints can waste airflow into roof spaces or service voids instead of delivering it to the occupied rooms.

It is equally risky to close valves because a room feels draughty without investigating the cause. The issue may be an incorrect balance, a poorly positioned supply valve, excessive fan speed or a missing transfer route beneath an internal door. The answer should protect both comfort and airflow performance.

When professional commissioning is the right choice

A simple replacement bathroom fan may be straightforward, but whole-house systems, new builds, extensions, blocks of flats and commercial premises require a more detailed approach. Professional commissioning is particularly valuable where certification is required, occupants are experiencing mould or condensation, or the property has been altered since the original ventilation design.

PureAir Solution provides installation, airflow testing, balancing, reporting and certification for residential and commercial projects across Ireland. Bringing supply, installation and commissioning together helps avoid the gaps that occur when each stage is handled separately.

Healthy indoor air is not achieved by fitting equipment alone. It comes from a system that has been designed thoughtfully, installed carefully, measured properly and explained clearly to the people who live or work with it every day.