The ventilation meets the calculations. Why does the room still feel stuffy?

The unit is on, the controller shows no faults, and the design specifies sufficient airflow. Yet after a meeting you want to open a window, odours linger in the far office, and there is a draught near the grille. Before increasing fan speed, establish whether there really is too little air or whether it is simply not reaching where it is needed.

The design describes conditions but cannot guarantee they will remain unchanged

Ventilation is designed for specific rooms, occupancy levels and patterns of use. Once the building is in use, a store room may become a meeting room, an open-plan space may be divided into offices, and workstations may end up behind a high partition. The unit itself remains unchanged.

When investigating complaints, the design must therefore be compared not only with the installed equipment but also with how the building is used today. ‘Installed in accordance with the design’ answers only half the question: what matters is whether the design itself matches the current load.

Total airflow does not show what happens in each room

A hypothetical office receives 3000 m³/h. Two large zones each receive 1200 m³/h, while the remaining rooms share 600 m³/h. If a small room is converted into a densely occupied meeting room, the unit’s total airflow may remain correct while the air distribution no longer is.

An extra 500 m³/h in the main duct does not guarantee that it will reach the meeting room. First, measure branch airflows and check the extract air path. Sometimes balancing is enough; sometimes the ductwork needs altering; and sometimes a larger unit really is required.

Air distribution within the room is checked separately. The air jet must serve the occupied zone, rather than create a draught beneath the grille or leave the far end of the room without mixing.

One sensor cannot represent the entire floor

A CO₂ sensor in the common extract duct measures a mixture of air streams and may miss a problem in a small overcrowded room. If 90% of the extract air is at 600 ppm and 10% comes from a meeting room at 1600 ppm, perfect mixing would give a reading of only 700 ppm at the common sensor.

C = 0,9 × 600 + 0,1 × 1600 = 700 ppm

This illustrates averaging; it is not a regulatory limit. A reassuring overall reading can conceal a very different situation in an individual room. CO₂ also does not replace an assessment of temperature, humidity, odours and pollutants from materials or processes.

How to find the cause rather than mask the complaint

Start by mapping complaints: where, when and at what occupancy the problem occurs. Then compare this with the layout, airflows, damper positions and unit operating mode. Next, check actual conditions in the affected zone, and only then choose an intervention.

A command is not an outcome. ‘Fan running’ does not mean the room is receiving its design airflow. An open damper on the screen does not confirm the physical position of its blades, and a normal temperature does not prove that the outdoor air supply is sufficient.

Effective ventilation is not about having enough cubic metres in the design. It means delivering the right air to the right room at its actual occupancy.