Is that Duck or Duct tape???

Kitchen extractor hoods come in two flavours; ducted and recirculation - only of these (ducted) achieves all the aims: Elimination of exce...

Kitchen extractor hoods come in two flavours; ducted and recirculation - only of these (ducted) achieves all the aims:

• Elimination of excessive condensation/steam
• Elimination of cooking odours
• Elimination of "dirty" air to allow fresh air to flow in

The problem is the duct; this is typically anything from 100mm to 150mm circular or rectangular, made of plastic or spiral wound galvanised steel and it needs to get from the extractor hood to an outside wall. This makes them difficult to use with low ceiling heights, non-existent ceiling voids (no you can't notch floorboards by 100mm let alone 150mm) or wall cupboards to hide them behind/in.

We therefore face a challenge, well several actually. Firstly we definitely want to show ducted extraction and secondly we want to press the "Strange/Unusual/Custom" button on the extractor hood selection.

At this point we recommend you get a coffee refresh, pull up a chair and settle in for a big one...

Lets break this monster into some main parts and discuss them - starting with the easiest bit first...
Hole in the wall

The ducting needs to get out to the exterior of the house somehow. There's the mechanical aspect of this - how do you get 100mm of duct out through a wall that's composed of a "club sandwich" consisting; plaster, render, single brick, rubble infill and knapped flint. Then there's the aesthetic aspect - once you've made that hole how do you make it "neat" inside?

Making the hole in this kind of wall is not as simple as using a core-drill - the core drill won't touch the flint, no honestly it won't. So the first part is to remove the flint(s) and rubble infill to get back to the outside of single brick skin. Then the core drill can be wielded from the inside to cut the hole.

At this point we feel obliged to point out the following handy core drilling hints:

• Check for pipework and electrocity cables
• Make sure you've picked the right wall
• Drill the pilot hole WITH hammer and the core WITHOUT hammer
• Take the pilot drill out once the core drill has established a reasonable depth of cut
• Cut from the inside out
• Use a core drill, a proper one, they have a safety clutch - something you'll appreciate when the drill jams in the cut

Making the hole was the easiest bit, making the inside appear neat was trickier and for this we took a different approach to the most common one. Usually, assuming the duct is not hidden in a cupboard, people will fill round the edges of the hole. It's very hard to do this neatly even with experience and then after a few years if you're lucky, a few months more likely it will crack due to expansion/contraction of the duct.

For this work we chose to machine a proper escutcheon plate from a 180mm round bar of aluminium. The inside was bored out to be a comfortable fit for the duct (allowing about 10mm all round). The outside was given a very large chamfer and then drilled with 5 holes at 72 degrees before the splitting the whole thing in two on the lathe.

Escutcheon - you can see the 3 additional holes for screwing the back plate to the wall 

This gave us two rings, the ring with the chamfer was taken and the drilled holes opened out and counterbored slightly. In these holes would sit 5 flanged head titanium bolts.

Assembled escutcheon - the bolt hole were given a 1mm counterbore prior to installation

The other ring was tapped to accept the titanium bolts and then was drilled again this time to allow us to screw the plate to the wall.

Once completed we had two rings; one was screwed to the wall covering the hole, the other was then bolted to the that ring to create a neat and suitably industrial looking escutcheon.

Escutcheon in-situ

Duck; Hood(s)

Now things are starting to get complicated; this project would be using exposed duct work. The decision process was driven both by the constraints of the specific installation and the fact that the industrial aesthetic would "handle" exposed ducting. We chose to use spiral wound, galvanised ducting with special unions that use a multi-lip rubber seal to avoid the need for duct tape (yes that's how it's spelt!).

In a true industrial environment duct work is typically suspended using simple bent brakets, stainless steel wire or a combination there-of. We wanted to add a certain refinement whilst still retaining a mechanical look and this drove multiple iterations of a design for duct brackets.

The selected design used two plates for each bracket. The plates are separated by blocks placed at 120 degrees spacing. Each block is threaded internally to carry an M8 cap head, stainless bolt. The plates also have two dowell pins between them onto which small rod-ends are threaded (a rod end is special kind of bearing which affords articulation in multiple directions). Each bracket is suspended from the ceiling by a special mounting that uses tied rods between the rod ends on the plates and those on the mounting. Using left and right hand threads on the tie rod gives a range of adjustment allowing the duct to be leveled in-situ.

CAD drawing for the jig plate, 3d model and CAD drawing for ceiling mounts

Now it has to be said this was not something that was completed in a weekend; we produced CAD drawings to show both the bracket design (including a 3d model) and the ceiling mounting. We also used CAD to generate a design for a jig-plate which gave us the repeatability across the 3 brackets we needed to make.

CAD on the left, reality on the right - this approach ensures absolute consistency across the parts 

When you view the photo album you'll see the sheer amount of work that went into this seemingly simple part of the design. Each bracket comprises a significant number of parts.

A little light assembly required 

One small detail not mentioned above is that the M8 bolts in the block allow the duct to be centered in the bracket (or moved slightly) - in order to do this we milled small concave recesses into the end of each bolt and then used a 10mm ball-bearing between the bolt and the duct.
Installation

We knew this would be "fun" - the ceiling is nearly 200 years old and is made of lath and plaster that has then been boarded over. Like any old house some settlement has occured and therefore flat is not an adjective one can use when describing the ceiling. Indeed this lack of flatness was a key driver in the complexity of the overall bracket design; we knew we'd need to accomodate potentially significant variations in the ceiling.

A completed bracket clearly showing the tie rods that allow the adjustment of the duct height 

We did get some "luck" in so far as we were able to attach two of the ceiling mounts to joists, the other bracket used a spreader plate installed in the ceiling by taking up the floor above. You see if some of the pictures that we ultimately used a multiple level set-up to get the duct work installed and levelled. You can also clearly see just how much the ceiling varies - in fact it varies enough that we had to remake the tie rods on once bracket because we ran out of adjustment.

Take one spirit level to site? Not when you're doing this you don't! 

Conclusion(s)...

This project was a really good example of how to do the industrial aesthetic but in a refined way. It was also a good example of how installation constraints can, will, should drive design.

Bracket, duct, escutcheon!

Lots of pictures to accompany this post;
This album has all the pictures showing the making of the brackets
Then this one has all the pictures showing the making of the escutcheon
And if you're not full by now, here's an album showing the install