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Welcome to your complimentary tour through The Upside-Down Extractor Hood

Why have a special range hood project?

Short answer - we found nothing on the market that would capture 100% of the cooking vapours, run whisper quiet, minimise heat losses, could be installed on a window wall, and that you couldn't bang your head on.

This is a walk through of what we ended up designing, having gone to a number of hood makers, fan makers and filter makers. Now it's built (and we know that it works) this is a record of what it looked like before we prettied it up with paint - along with an explanation of how it works, and maybe more importantly, why it works.

Enjoy the tour!

fan outside

We start with the exterior, where the fan and motor live inside this neat casing. In a few minutes we'll see into the heart of the fan from inside the house, as we dismantle the hood to show its workings. Hopefully it will all become much clearer then.

This is the inside unit of the upside-down hood. Yes, it is supposed to be like that. There's a gap at the top, looking like a shadow, that provides the magic. The fumes get out through that gap.

And, unlike the conventional hood, which you'd think was designed as a megaphone for fan noises, this one more or less contains the noises, and any that do try and get downwards are immediately reflected back up towards the ceiling - where they do no harm, because in our house that's not where we go to have quiet conversations.

Note the gas hob - pre-existing, like the 'splashback window'. Gas hobs are another source of humidity. Why use them? For your "plan B" cooking because the electricity supply hereabouts is not 100% reliable. Where does one get their gas from? In our district, remote from piped reticulation, we use bottled gas. When we bought here, people put a 9kg BBQ bottle under the hob, and refilled it at BP once or twice a year. But that's no longer legal. As we were doing a kitchen make-over we were required to install underground piping and the big bottles you see through the window. At great expense.

At least this will prevent the accidental asphyxiation of any hob-goblin (get it?) if you happened to have one living in what little space would be left for him (and his family no doubt) once you had jammed the BBQ gas bottle into said cavity. Each of these monsters is 45kg, I think. They'll need refilling in 10x9 months - that's over seven years away! But I think that when we empty the first - almost four years from now - we'll switch to the full one and get the empty refilled immediately. Like, before we forget! The fat filter might need cleaning about then, too. More about that below.

Oh yes - we are most decidedly putting a decorative camouflage around those cylinders.

This is the magic gap. It's where the upside-down hood almost meets the ceiling. For this prototype there are adjusting screws at each corner so that the gap could be varied during the initial setting up.

It's the width and location of the gap that lets it do its job, which is really to exploit the phenomenon of convection.

Convection is the real magician. Free of charge, with neither noise nor pumping costs, it delivers the fumes right up to the ceiling where they form a thin layer from which they can be 'skimmed' with a very slight vacuum and just a gentle airflow, through the magic gap between ceiling and vent hood, thence through the filter, the fan, and finally outside.

This is the fat filter. A good hood deserves a good filter. It's needed so you don't get grime down the outside of the building, and to prevent the motor and fan getting gunged up - which rather destroys their efficiency. On the principle that spare capacity is a "good thing" we got ourselves an industrial filter, which has less resistance to airflow and so can be used with a lower fan setting. It will need less frequent cleaning (like only when we change a gas bottle - as noted above!), and it also further attenuates the fan noise. For the techies, at high flow its back pressure is from 20 Pa clean to 70 Pa dirty. and even less at lower fan speeds. So, for our high-flow/high-pressure fan, this is a good match, even when the system is barely whispering. This is the appearance when properly installed.

This shows the element properly installed, in place behind the retaining bar, and with the bar catch closed. For cleaning - first rotate and open the bar catch. The filter will still be held in by the retaining bar.

To remove the filter, using the handle, pull it firmly down from behind the retaining bar. The element is a tight fit in its holder, to prevent leakage and rattles. Insertion is the reverse - and the filter must be inserted the same way round. The end that must be inserted first has a notch cut into the frame of the element. That's the end that's furthest away in this photo.

This is the working end of the filter. Handle it by its outer frame. The aluminium 'labyrinth' elements are relatively fragile. They would be bent by a high-pressure jet, so wash gently in hot water, with a non-caustic degreaser or detergent, as the aluminium may be discoloured or corroded by alkali. Do not clean in your dishwasher unless you use a special low-pH detergent.

Replacing the clean filter is the reverse of removal. With the handles downwards, insert it 'notch-first' to fit the retaining strap on the back wall, guide it into place between the side guides, then firmly press it by the corners in and up behind its retaining bar so that you can close the catch as pictured further above. You may press the filter's aluminium frame very firmly, but not the aluminium 'labyrinth' elements for they are relatively fragile. If the filter does not press into place, check its orientation, as it can only be inserted one way round.

At the back of the filter casing is the fan duct, connecting the hood to the centrifugal fan, mounted outside.

At the far end of the fan duct, the fan motor and blades are clearly visible in their location on the outside of the building. The inside ends of the centrifugal fan blades are curved and angled to 'scoop' the air through the system. From the shape of the fan blades you may get a sense of the way their rotation draws the air from the room, through the filter, to the outside.

This one has a huge fan capacity (for the technocrats among you it's capacitor run so can you can use a cheap phase-chop electronic controller on the main winding to limit the power down to a minimum useful level of about 10w. Uncontrolled it draws 65w at full chat, and its response curve then goes from 90 L/sec @ 100 Pa down to 20 L/sec @ 280 Pa, or any intermediate combination. Controlled down to about 10w it will move 20 L/sec @ 20 Pa which is all that's presented by a clean filter even at maximum flow) - and so for us, with all the other noise mitigation features, it's whisper quiet.

fan outside

The fan and motor are outside, covered by this unobtrusive, weatherproof housing, which has a vermin-proof grill out of the weather, underneath, through which the filtered air is discharged.

Putting the motor outside, to reduce the inside noise, unfortunately means that you increase the outside noise. But then, when the fan is running, we are usually inside, which deals with that one. And in the great outdoors the noise isn't nearly as obtrusive as it is in the enclosed confines of a kitchen. Besides, the nearest neighbour's living quarters are effectively out of the sound range.

This brings us back to where you joined our tour. Thank you for being with us today, and if you enjoyed yourself, please do tell your friends.

Now the detail - why have a special range hood project?
Building changes

This house was built only seven years before we installed the hood. It was the first that we had lived in that was - as nearly as possible - hermetically sealed. "In our day" it was customary to have a cast plaster grille, variously decorated, in every room, high on the outside wall, to let the old stale air leak to outside through the wall cavity, with 'fresh' (read 'cold') make-up air leaking in around the doors and windows, and through the floorboards.

Humidity problems

But excess cold isn't healthy, which the modern house deals with - it retains the heat very well. It also retains the humidity, and smells, and chemical vapours. The chemical vapours can diffuse from the treated timber of the frame, through the plasterboard, into the unventilated home, and damage your health. So we change to untreated timber framing. We then find that humidity can diffuse into the walls, and rot the untreated frame. The excess humidity also weeps down the window frames, and eventually cooking smells permeate the house, the windows fog up when you boil a kettle in winter. Altogether, it's not a good scene.

Range hood ideals

It's not uncommon these days, to find homes with all or any of dehumidifiers, exhaust fans, range hoods, and the like. But many of them suck heat out, along with the fumes and vapours - sort of defeats the object, eh? The ideal is to suck out of the house the very minimum of air, have the hood inconspicuous, safe, quiet, and able to cope with vapours that come from other appliances, not necessarily immediately under the 'hood'. You should not be able to catch your head on it when you bend over to look in your pots. And for us, it needs to hang somehow from the ceiling, because, as you see in the photos, we don't have a conventional splash-back, for there is a window built behind the stove. Just to add to the issues, our ceiling cavity is not accessible, so we needed to avoid any solution that required penetrating the roof space.

Range hood problems

But most hoods, when they are over a busy stove, only function effectively when they are on full chat. Some of them even have two fairly high-powered fans in an attempt to 'capture' everything coming off the stove, before it all escapes pastthat inverted funnel and drifts up to the ceiling, where the hood suction certainly cannot reach. In an open plan, sealed, house like this one, that means cooking odours drift throughout the living area: we wanted to stop that happening. We needed a hood, but clearly, it had to be vastly different from the conventional. That, of course, means unconventional.

Try Plan B?

But have you been shopping for a range hood lately? Or looked at those in your friends' houses? Found any that would meet our requirements? Even most of our requirements? Nor did we.

Short answer - nothing on the market could solve all these problems. We were forced back on our own ingenuity. So what else is new?

Unbridled confidence

Well, we've done some exhaust fan things recently (our shower cubicle project) and in 1970 Peter had installed a second-hand Expelair where he was living at the time and was optimistic that he still could remember how he did it. So, full of ignorant confidence we decided to put the range hood on the list in conjunction with the kitchen upgrade project. It only took three years of scribbling and crumpled paper to get it to the point where we could have some realistic hope that it would be (a) buildable, and (b) function as we intended  . . . . . .


Copyright © 2007 Peter Leon Collins
v1.0, 24 March 2007