How to Block Draught from Kitchen Extractor Hood

Absolutely nothing to do with engineering per se, but someone may be able to help.

We have a fairly typical kitchen extractor hood, but when turned off, it lets in a large amount of cold air. The outlet vents into the driveway, which acts like a funnel and blasts the air in. I have tried various types of outside covers, flap covers and shrouds, but none works. I have fitted a flap valve in the pipe, which has helped, but only a little.

Re-routing the outlet pipe is not practicable or viable.

I have seen a range of electrically-controlled shutters from Helios which can be linked to the on and off switch for the extractor. However, at the best part of £60.00, I am a bit wary of wasting more money.

Has anyone else had this problem and, if so, has anyone successfully overcome it?

Thank you.

James.

Edited By James Alford on 20/02/2016 09:21:02

You could look at the venturi ventilators on railway carriages and get the 'funnel' in your drive to make it extract only but that still give a draught. Or as an engineer you can make your own butterfly valve or copy other carburettor designs. Use a Bowden cable to control it and arrange a physical or microswitch interlock with the fan power.

One problem with modern solid fuel stoves is the regulations requiring enormous air holes without regard to situation which creates just this sort of situation.

I recall seeing a flap device with a threaded rod and heavy nut down the face of it and rotating around a central spindle.. This allowed the balance of the flap to be adjusted as necessary for the average conditions (a few days of fiddling in different weather) so that the draught would mostly allow one way function. Not perfect perhaps but a cheap DIY, simple mechanical option

I have an outlet grill on mine that only opens when the extractor runs and blows it open. This week I fitted a new extractor which has a sort of non return flap that opens when running it also forms the connector to the outlet pipe. Have a rummage in B&Q or similar for the outer grill with the moveable flaps as that is easiest to change.

Chris.

Are the flaps lifted just by the air coming out fromthe extractor? If so, do the flaps stay shut when not in use and a wind is blowing? I currently have one which I think came from Screwfix, but the flaps lift in wind.

Jame.

My outlet is a horizontal 'Y' shape , with two rectangular outlets , approx. 3"wide x 7"deep.

I hung 2mm thick neoprene over the outlets with lead weights on the end.

Been working 6 years now.

Had the same problem which I over came by using a thin piece of rigid plastic slightly larger than the kitchen side extractor grid, attaching thin magnets as needed around the edge and draught excluded tape to seal between the magnets. Handle in centre. Works well but needs to be removed and replaced before and after use. Of course only works on a metal faced unit.

In the winter, to stop hot air leaving the kitchen, and when the extractor is not needed, mine gets blocked off with a folded up news paper.

Ian S C

How about 281309044004 or 250875511675 for a couple of ideas on you know which on-line auction site.

Dave
The Emerald Isle

Edited By steamdave on 20/02/2016 10:33:25

Thank you all for the suggestions.

Dave: thank you for the e-bay links. I tried one of the hoods, with little success and have one of the flap valves in place. This has helped a bit.

I rather like the idea of hanging magnetised flap over the vent and shall have a look at this. I have also just found these **LINK** and wondered about replacing the threaded rod with a piece of rod and using a coil spring to shut the valve, relying on the air from the extractor to open it when in use.

Regards,

James.

Try adding weights to the existing flap first.

James, I don't think weighting the flaps and relying on gravity will help much. If the flaps are proof against the wind then almost certainly the fan won't generate enough force to lift them either.

I've got an old Ventaxia unit installed in the bathroom, old in that it was in place when I bought the house 27 years ago - and wasn't new then, the system on that would be worth copying. The blanking flaps on mine are on the inside of the fan - so in the bathroom, but that isn't important from an operational point of view. The pull cord works by opening the flaps which then in turn trip the switch for the fan. Provided that you could properly protect the switch from the weather then the blanking flaps could be on the outside. If your fan is started by an automatic humidity or thermal switch then using that to open the flaps, through some sort of servo system, which then start the motor through another switch would probably be the way to go.

Extractor fans generally don't generate the sort of pressure head you'd need to shift blanking flaps that the wind can't move.

Keith

Do like they do on dust extraction systems fit an automatic blast gate into the ducting that is wired into the extractor so it opens when you switch the extractor on.

The louvers on our bathroom extractor are operated by a bi metal strip that has a small heating element that is powered when the fan is on. They take several seconds to open and close but they do the job.

Les.

I installed one of the usual Venetian blind types that just swing out when the extractor is on. On an exposed wall in West Wales. No problems with significant cold air coming in but sometime when it's rather windy with high gusts and it does get really windy there the blinds move and make the odd near bang type noise from time to time. Not often enough to be annoying. It does shut correctly The property is only a mile or so from the coast, less from a rather large estuary.

It's much like this one but the slats may be more recessed. Hard to tell from the photo

**LINK**

John

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Edited By Ajohnw on 20/02/2016 16:09:37

Our conservatory gets very warm in summer, so I fitted a small extractor fan (4" on a thermostat. On the outside is a simple shroud with a flap valve in it, and I've never had any problems of the wind getting in. I think the angled cowl helps.

Reviews suggest some people do, so It guess it depends on your situation.

Neil

www.wickes.co.uk/Wickes-External-Cowled-Vent/p/156267

Expelair fans used to have a solenoid to drive the vanes open against a spring. Nowadays, they seem to use "gravity grills" for tumble drier and cooker hood terminals.

The description in the linked item refers to it being "compliant with the Building Regs Part F (ventilation)". Is it possible that Part F actually prohibits anything beyond a simple gravity type flap, hence the lack of choice? Don't have time to ream through it right now.

Murray

Modified my cooker hood in the following way:- The outlet was a 4" pipe through the wall, This then had a 5" diam piece of plastic about 3" away from it mounted on SS studs. I made a sleeve that was a nice fit inside the pipe about 4" long and 3 3/4" diam (?). So with the sleeve rearwards it sealed the pipe and with it pushed forward the air pipe was open (ish). The sleeve was pushed by a mechanism and 6mm studding right from the extractors main cavity.

The problem I had was that when the hood was closed the flap went forward and so a direct push on the studding would have closed the air pipe. So I used a system of levers to reverse the action, which has now worked OK for over 10 years.

I also have a home made motor/gearbox/rack system* for the toilet extractor fan. At 750' in Wensleydale it gets very windy!

* satellite polarisation unit from skip

Frank

Extract from Regulations General Guidance 3.1 shows you can have manual control of the vent

Intermittent operation is where a mechanical ventilator does not run all the time, usually running only when there is a particular need to remove pollutants or water vapour (e.g. during cooking or bathing). Intermittent operation may be under either manual control or automatic control . Manual control is where a ventilation device is opened and closed, or switched on and off, or its performance is adjusted by the occupants of a room or building (see automatic control )