Rangehoods are not only a functional appliance but in many kitchens are a focal point. The functional aspect of the range hood involves removing cooking fumes to the outside but the size and prominence of some range hoods means that they must satisfy style considerations too.
Extractors can be hidden in cabinets for kitchen designs that require the air extraction device to be hidden. Whereas this type used to be small units with low extraction rates, they have evolved to be just as powerful as full size canopies.
Rangehoods can be used two ways: to vent to the outside or to re-circulate air within the kitchen. Re-circulating rangehoods usually have a grease filter, which must be replaced regularly if it is the paper type or be cleaned regularly if it is a screen. Re-circulating rangehoods tend not to be very satisfactory in use. Venting range hoods are more effective at removing grease and cooking fumes, they usually have screens that can be cleaned in the dishwasher. The better ones also have grease traps.
The most efficient way to vent a range hood is straight up through the roof. A straight pipe works best; bends in the vent pipe add back pressure that requires a stronger motor to overcome. Venting capacity is usually given in cubic meters per hour; that is the number of cubic meters of air that can be moved under ideal conditions. Conditions are rarely ideal of course, which means that you should usually go for the highest capacity you can, especially if you have a long run or bends or both.
Generally speaking 450 cubic meters per hour is the minimum you should look for. Anything less is unlikely to be satisfactory in use except in the smallest kitchens.
Noise is the next consideration. The more powerful the motor or the greater the number of motors, the greater the potential noise. Noise levels are expressed in decibels and you will tend to want a range hood with a lower decibel rating.
If low noise is important to you, you should consider one of the new silent rangehood systems. Since most of the rangehood noise is generated by the fan, it can be greatly reduced by moving the motor to the loft space or even outside the building. The difference is dramatic, that between a hiss and a roar. With a remote motor, the only noise you are left with is that of the air moving through the rangehood.
Remember too that flexible hosing, which is corrugated, cause greater air resistance and therefore the need for a more powerful motor. Use smooth ducting wherever possible.