An exhaust fan that is too small does not half-work. It fails quietly: the mirror still fogs, the smell lingers, and within a year or two you have mould in the corners of the ceiling. Getting the size right at the outset costs nothing extra.
The rule of thumb
A bathroom should have its air replaced roughly 8 to 10 times an hour. A kitchen, where you are dealing with heat, steam and cooking oil, needs 15 or more.
Work out the room volume - length x width x height in metres - then multiply by the air changes you need, and divide by 60 for the airflow per minute.
A typical Sri Lankan bathroom of 2 m x 2 m x 3 m is 12 cubic metres. At 10 air changes an hour that is 120 cubic metres per hour, or 2 m3/min. Most wall-mount bathroom fans clear that comfortably, which is why the 10EGKB and 15EGKB suit the great majority of bathrooms.
Choosing the type, not just the size
- Bathroom series (10EGKB, 15EGKB) - compact, quiet, straightforward wall mounting. The default choice for a domestic bathroom.
- Wall mount with automatic shutter (20AUA, 25AUA, 30AUA) - the shutter closes when the fan stops, which keeps insects, dust and monsoon rain out. Worth having on any external wall.
- Reversible (20RGA, 25RGA, 30RGA) - switch between extracting and drawing air in. Useful in a kitchen that gets stuffy as well as smoky.
- Ceiling mount, ducted, super quiet (17CUH, 24CUH, 24CDH, 24CHH) - the fan sits above the ceiling and only a slim grille shows. The right answer where noise matters or there is no suitable external wall.
- Industrial high pressure (35GSC to 60GSC, 40KAQA) - factories, commercial kitchens, workshops. These move serious volumes of air against duct resistance.
Where people go wrong
Fitting the fan on the wrong wall. Air has to come in as well as go out. A fan on the same wall as the only door and window short-circuits: it pulls fresh air straight back out without ever crossing the room. Put the fan high and opposite the air inlet.
Ducting a fan that was not meant to be ducted. A wall-mount fan pushes air a short distance. Add several metres of duct and a couple of bends and its actual output collapses. If the air has to travel, use a ducted sirocco model designed for the static pressure.
Undersizing a kitchen fan. Kitchen extraction has to deal with grease as well as steam. Size generously, and clean the blades.
Noise is a specification, not an accident
KDK's Super Quiet ceiling series exists because a bathroom fan you can hear through the wall at 6 am gets switched off and never used again. If the bathroom adjoins a bedroom, spend the extra on the quiet model.
Tell us the room
Send us the dimensions and where the external wall is, and we will tell you which model and where to put it. Call +94 11 250 0823 or visit us at 35/3 Nawala Road, Narahenpita. Elite Radio is the authorised KDK distributor in Sri Lanka.