On Saturday, April 14, 2012 10:29:28 PM UTC+1, js.b1 wrote:
It is a warranty / liability limitation.

Xpelair - your fan must be cleaned monthly by a qualified electrician.
Most are AC fans with bimetallic thermal cutout. Should the fan or
discharge vent become sufficiently blocked with dust the motor will
get hot - so hot that if insulated by dust buildup and grease deposits
the combination may ignite.

Cooker hoods - switch gear, motor, bearings, impellor are not IP-rated
or temperature-rated for continuous steam exposure. The RCD may trip
out, the motor may run its bearings, the plastic impellor or airflow
guide may melt. Grease could me migrated past the filters via steam
into the motor/impellor assembly creating a fire hazard.

Another point is steam is invisible, viciously hot close to a kettle's
outlet. The visible "steam" is lower temperature hot water vapour. I
suspect plastics used for motor housing, impellor & air ducting may
not be tolerant of continuous high temperature exposure such as when
used in recirculation mode - or even extract mode at low speeds
against a barrage of steam sources. Likewise steam jets from pressure
cookers I think can be a slightly higher temperature.

Most cooker hoods are made by Elica.
Twin motor (320m3/hr) units are good enough to get rid of heat, not
just odours & steam. Single motor (170m3/hr) are good at getting rid
of moisture, they struggle on a hot summer's day. Obviously you can
get single motor 550-800m3/hr units - but check the dB(A) figures
carefully because things can get loud, likewise check hole size.

Ebay sometimes has oddball super high flow hoods at cheap prices, you
do not have to pay the ?£3.5k? someone did per cooker hood on Grand
Designs.

I'm sure thats all true.
But you'd have thought they would put something in the manuals to explain whats going on, especially when most people assume a cooker hood is going to help remove "steam" (or water vapour) from their kitchen. And it will to some extent.

Anyway since I have to choose a cooker hood soon, this is a very timely discussion.

Simon.