There are three distinctive types of ventilating systems: Updraft, downdraft, and re-circulators.
Updraft ventilation systems – as the name suggests – funnel the air upwards, normally to an outside duct. Popular in commercial kitchens, these systems are the most effective in removing heat and food odours, thanks to the natural process of rising air. These systems can be expensive, however, and will require alterations to the existing wall space.
Downdraft systems use either a single, powerful fan or multiple fans to effectively suck the air downwards. Whilst also effective and slightly lower in price, they can be very noisy. They also consume a lot of energy, and pose the risk of those aromas rising back up from the vents if not left on for long enough after cooking.
Re-circulatory systems became popular around the turn of the millenium, as they take up less space than conventional types of ventilation systems, are cheaper, and offer a vast array of unique designs. However, these systems require heavy and frequent maintenance. You will need to replace the carbon filter on a frequent basis, as the filter effectively acts as a permanent sponge. Failure to do so can also lead to serious health hazards.