I searched high an low on the internet and found very little info on fitting a cooker hood setup for external extraction so I thought I write a post on what I did in case it helps anyone.
We have just re-fitted the kitchen and went for a 110 Range Cooker, its a 1930's brick built house with cavity walls and in the winter when cooking the walls are dripping wet so we replaced the old broken extractor fan and decided that the cooker hood would have to be setup to vent the cooking fumes externally.
This guide is based on my house so I apologies if its not for you.
White Plastic Pipe - http://www.screwfix.com/prods/54391/Heating-Cooling/Ducting/Round-Pipe-125-x-350mm
Cooker Hood Kit - http://www.screwfix.com/prods/17489/Heating-Cooling/Ducting/Manrose-Cooker-Hood-Kit-125mm
Large Jubilee Clip for ducting (only one in kit above)
Masonry Drill BIts
The plastic pipe and cooker hood from screwfix are excellent and a lot cheaper than many other kits or nits and pieces I found in B&Q and Wickes, I.E the 125mm hood kit from Screwfix is only £9.06 but a similiar kit from B&Q was £26.00. Screwfix also do a 100mm version for those with smaller hoods.
1. First step is to mark the position of the hood on the wall, this has to be done in order to locate the correct position for the extractor hole.
2. Once the hood is marked out temporarily fix it to the wall, once in place work out where the hole will be ensuring that it is as close to the hood as possible but far enough away to allow the ducting to be installed. When locating the hole take into account any electrical cables and water pipes both inside and outside, also check the external wall from the outside to ensure the vent will come out in a clear area.
3. Once everything is marked out its time to drill the hole, the hole will need to be large enough to take the ducting. Check your hood manual and ensure that you stick to whatever size is recommended. My hood recommended a 130mm hole but most ducting comes in either 100mm or 125mm sizes so I opted for 125mm ducting.
4. My hole was bored out by a plumber using a 5inch Diamond Core Bit and it only took him 1/2 hour and it was a very clean not to messy job.
If you choose to do it yourself get someone to help with a hoover on hand to extract any dust.
5. Its best to sleeve the hole to prevent damage to the ducting from debris falling down the cavity, I sleeved mine with a 127mm plastic piping from Screwfix which was just the right size for the 5inch hole but first you may need wire brush out the hole and remove any imperfections and any loose mortar.
6. The plastic sleeve may need a little gentle persuasion but be careful not to get it stuck half way in. Also ensure that it is flush with the outside wall where the vent will be installed and leave enough on the internal wall on which to connect the ducting.
7. Seal around the edge of the pipe inside and outside the house with sealant, colour of your choice, for the brickwork outside I used brown.
8. Fit the vent to the pipe outside, and if required secure it to the wall with screws (Holes drilled with masonry drill and holes fitted with rawl plugs).
9. Mount your hood in accordance with the manufacturers instructions fit the ducting between the cooker hood and the plastic vent pipe going through the wall, using a piece of aluminum ducting cut to size.
10. Switch on fan and check that the gravity flaps on the vent cover open (dam, just remembered I didn't do this, made a mental note in case any sealant had spilled out onto the flaps).
11. Finish off as required, covering the ducting with whatever you have, my ducting was hidden behind a false chimney which came with to hood but you may have to box yours in with hood cabinet it all depends on your setup.
Hope this guide helped someone, its more than I had when I started this job!!
I have pictures but will add links when I get them on tinypic or whatever and edit the post.