It should really go somewhere outside. If the hob is on an outside wall, then the hood can discharge very easily via a pipe through the wall, otherwise it would need a longer pipe with a more circuitous route. The discharge (mainly steam) can be via a duct up a few inches and then back through the outside wall, or up through the ceiling and then to an outside wall somewhere, or around the kitchen just below the ceiling to an outside wall. It all depends on the circumstances.
There are two types.
Recirculating: these don't extract to the outside. They just let the filters soak it up.
Extraction: They need the outlet that Atheist is talking about. (By far the best and more efficient.)
I've fitted extraction hoods that aren't on an outside wall. You either have to duct acroaa the ceiling through a hole in the wall...
... or, if you're lucky, put the duct up inside the ceiling IF the floor joists are running the right way.
Barsel, the recirculating ones clean the stuff that wafts up from the hob and then discharge it back into the room. The other type sends it outdoors. It seems to me that it's better to send the steam and oil fumes etc outdoors. Builder will be able to give you more relevant and practical info.
Atheist, yes I thought that too, but I'm just wondering if it would make a big difference to the price of having it fitted.
Perhaps the builder might come back to me.
Yes that's what I wanted to clear up.
So, would it cost a great deal more to have it fitted so the steam etc goes out of the kitchen?
The type I have at the moment just recirculates but the extractor fan on the outside wall seem to take most of the steam out.
Builder; not quite understanding your post.
"Any of them can go anywhere you like, but the extraction ones need to be connected by duct to the outside air."
Isn't there a contradiction there?
Too technical to answer here without seeing the room.
Your cooker is not on an outside wall, so you would have to have quotes for installing ducting.
There is of course the matter of a maybe unsightly duct across the kitchen ceiling if you can't put it between the joists.
I'd love to give you a rough figure, but it all depends............ ;o(
I understand.
Think I will just go with one that can be recirculating and put the outside extractor fan on as I have been doing for the pat 16 years.
Thanks for your answers boys.