Cooker Hood Venting


Firstly, I would like to thank everyone who made suggestions on my recent question regarding putting up plasterboard over my existing kitchen ceiling.
I now have a new problem regarding SWMBO's cooker hood which is of the chimney type. We bought the hood yesterday and find that it requires carbon filters if it is not going to be vented to the outside and they cost GBP20.00 per set and require changing every 3 months (ouch!).
I can run a venting pipe (125mm) along the ceiling to the outside wall (boxed in of course) but the external grill would be just 40cm above the boiler flue pipe exit. I would use a grill with a Back Draft Shutter but is it going to be too close to the boiler flue and breach some regulation or other?
--
Charles

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Hi Charles
I'm no expert on this but my humble opinion is that you run the risk of having the exhaust fumes been blown back into your kitchen whilst the extractor isn't in operation. You say that it'll be a shutter type external grill but these still allow air through and you may run the risk of your boiler fumes getting past the grill. You may also find that your extractor exhaust will be too close to the shutter and you will need to fit a heat shield to stop it melting.
Before I fitted our latest kitchen (the wife is looking at changing it again !) we used to have an extractor and I avoided the manufacturers filters for the same reason you're looking at venting to the outside ... the cost ... which was about the same 20 as yours. Instead I went to our local 1 shop and bought a decorating dust sheet (1800mm x 1500mm) and cut it to the right shape to fit the extractor and used these as the filter .... I've still got some left and we used this option for years without any problems.
Ash
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Forgive me if I'm wrong here, but I'd imagined that the special filter must do something to the CO which would be present from a gas hob. Your method would just blow it round and round the kitchen?
Of course, if you are using an electric hob, no such problems.
JW
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The use of an extractor fan is to remove smells (and if vented outside steam) and not to remove CO. The filter is charcoal based and is supposed to absorb smells when the fan is not venting outside. Correct installation a gas cooker means that there is no CO danger either with or without an extractor fan.
Adam
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Reply to both replies! ;-)
Yes, true. I don't really understand the difference between the two filters though. In my extractor hood, you just use the dirt cheap sheets if externally vented. If internally vented, you use the circular, plastic filter instead (about 20 I suppose).
So I still don't understand how the decorating sheet can outperform the large plastic filter.
JW
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Hi John
As a rough guide
The externally vented filters trap grease and nothing else. The internally vented filters are filled with charcoal and absorb smells (or are supposed to) as well as filling up with grease.
I seldom cook with fat/oils and my extractor is mainly used to expel moisture and smells.
Adam
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The decorating sheet I bought had little perforations which allowed venting and when folded in two or three provided a nice alternative to the manufacturers one ... mine is a large rectangular extractor filter. It probably wouldn't out perform the bespoke one but it worked fine for us .... it's a bit like saying can you tell the difference between "Base Line" products in the supermarket and the "Brand" products .... some you can and some you can't .
Ash
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Hi JW
I thought the filter was just to trap those floating fat particles (or soot in the case of my cooking) from getting into the workings of the extractor fan and not a CO2 scrubber?. If you don't have a gas hob but you do have a gas oven you don't need an extractor so where does the CO2 go from the cooker?
Ash
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Our nextdoor neighbour appears to have their hood vent about 3 bricks above their heating vent. However, it just occurred to me you could surly use say, drain pipe, to effectively move the the external vent to a higher level or even to one side. After all the emissions are not dangerous mainly steam I would think. Does it have to be a 125mm vent pipe? Again our neighbour vent is much smaller than that.
Ron
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Hi Charles
For a fanned flue the usual requirement is that the flue should be no closer that 300mm either above, below or at to the side of an opening, air brick, opening window etc.
The manual for your boiler is where you will find the authoritive answer. If you give me a make and model no for your boiler I may have the manual.
For non fanned flues I have no idea.
An alternative to boxing in ducting there is sometime the option of using flat ducting under the floorboards.
Adam
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writes

Thanks for the responses everyone and like all the best stories, this one gets more complicated.
SWMBO almost removed my bits when I told her what I was going to do with the cooker hood vent and I was told where I could stick the offending article if any part of the ducting or boxing in was visible :o(
Plan "B" had the ventilation pipe running across the kitchen ceiling above the plasterboard and between the joists to the other outside wall, a run of 3 metres, but this morning's removal of a section of the ceiling reveals that the chosen path has 15 x 10 cm noggins every 300cm as the bathroom wall is right over the gap :o(
Plan "C" involves going about 0.8 metre between the joists and then up through the bathroom floor under the bath and out to the fresh air. This now puts the vent about 1 metre to one side of the boiler flue.
I think this solves the problem and it's cheaper as I can only use flexible ducting. Any problems I should look out for?
--
Charles

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