Exhaust Fans and Ventilation: Getting Air Changes Right

Ventilation removes moisture, odour and heat. Whether it succeeds depends on the airflow actually achieved, not the airflow printed on the box.

Free air versus installed performance

Fan ratings are often quoted at free delivery — no ducting, no resistance. Add a length of duct, some bends, a grille and a backdraught damper, and the delivered airflow drops, sometimes dramatically. Selection should use the fan curve at the calculated system resistance, in the same way a pump is selected at its duty point.

Long duct runs need the right fan type

An axial fan moves air efficiently against low resistance. Where the duct run is long or has several bends, a centrifugal or inline duct fan sustains flow far better. Fitting an axial fan to a long duct is a common cause of a bathroom that never clears.

Make-up air is half the system

Air extracted must be replaced. If a room is sealed, the fan works against a partial vacuum and moves very little air regardless of its rating. A door undercut, transfer grille or dedicated supply is part of the design, not an afterthought.

Where ventilation matters most

Toilets, kitchens, plant rooms, battery rooms, car parks and store rooms each have their own requirement, often set by regulation. Kitchen extract additionally requires grease filtration and an accessible cleaning regime.

Commissioning and verification

Measure the actual airflow at the grille rather than assuming the design value. Where a system underperforms, check for crushed flexible duct, a closed damper, a blocked external louvre or reversed rotation before questioning the fan.

Noise

Fan noise drives whether occupants leave a system running. An oversized fan throttled by resistance is usually noisier and less effective than a correctly selected one.