Do rangehoods have to vent outside?

The two types do genuinely different jobs. A ducted hood captures air and expels it outside the building. A recirculating hood passes air through a filter, usually charcoal, and returns it to the kitchen. The filter removes some odour. It does not remove water vapour, and it does not remove combustion products.

Why it matters more with gas

Burning gas indoors produces water vapour, carbon dioxide, nitrogen dioxide and, with incomplete combustion, carbon monoxide. A recirculating hood returns all of it to the room. That is the argument for ducting a hood over a gas cooktop specifically, and it is a stronger argument in a well-sealed modern home than it was in a draughty older one.

Getting a ducted hood right

• Duct as short and straight as possible, with smooth rather than flexible ducting where you can.
• Duct diameter matched to the hood, since reducing it strangles the airflow and creates noise.
• Discharge outside, not into the roof space or a wall cavity, for the same reason a bathroom fan must not.
• A backdraft flap at the outlet.
• A hood wide enough to cover the cooktop, mounted at the manufacturer's height.

The interaction with open flued appliances

A powerful extraction fan in a sealed house can create enough negative pressure to pull combustion products back down the flue of an open flued gas heater or hot water unit. This is a recognised hazard and it is one of the checks a gasfitter performs when servicing an open flued appliance. If you have added a strong rangehood, mention it at the next service.