I have a cooker hood mounted on an island in my kitchen . Problem is that to get to the outside the ducting has to go through a series of bends. Ducting needs to be at least 4 meters long. I am worried that the extraction rate will suffer rendering the hood useless. Any ideas on how I can attach an extractor fan to the end of the ducting (outside) and get the extractor fan to switch on as soon as the extractor fan is switched on? Is there anything like a wind activated sensor maybe? or do i have to hack the cooker hood switch - I would not like to do that since I'd loose the warrenty.
There are air operated switches but I would consider rather than two devices sucking out have one extracting and one feeding air into the kitchen.
It does not matter if it's a coal fire, a tumble drier or a cooker hood if you take air out of the house it has to be replaced. If it is replaced by sucking air where ever it can then it will cause drafts and send air already heated by central heating out. However if you blow air into the house you can control exactly where it feeds so stop it causing drafts elsewhere and ensure the air you have heated with the central heating is not sent outside.
Honeywell do a sail air flow switch S6040A1003 but normally the switches are part of a kit. A cheap V3 micro switch is normally the electric bit which will be mounted inside the duct so guarding the terminals. The other method is a reed switch and a magnet on an flap. However to build or buy will cost and to use positive pressure plus negative being able to switch independent would give more control.
I have found hob extractors are required with gas but with electric the moisture is much reduced and often no or little extraction is required. Gas burns air and produces water even when there is nothing on the hob and also puts far more heat into the kitchen. My father-in-law uses gas and we use an induction electric hob in identical houses next door but one to each other. In summer our kitchen is far cooler and in winter far dryer then his. So I would tend to try it first specially if using electric to cook with.