Duct pipe for kitchen hood

[TheGlobetrotter]

Hi,

First time ever I post here, bear with me if it's not the right forum.

As part of a big house renovation, I've got a new kitchen with a powerful Rangemaster hood (I like cooking smelly stuff, I need a mighty extractor). It's fitted in extraction (as opposed to recirculation) mode. Builder, DIY friend and myself all checked and it's installed properly. The setup is: hood => vertical 150mm pipe about 350mm long => 90 degree angle (inevitable) => flat pipe for about 2m into the outer wall.

Problem is, the air is blown back into the kitchen, and the smell is horrible. The culprit seems to be the pipe which is not big enough. The only thing that can be changed about this setup is the flat pipe bit. As it is it doesn't work so I don't have much to lose.

Long story short, I need a new pipe, and it has to be visible. To avoid destroying the aesthetics of my brand new kitchen, it has to be black and look reasonable. Builder suggested a soil pipe but no way. I love those nice steel enamel stove pipes. I checked with a chimney supplier and he came up with an extortionate price (£60 a metre, £70 for a 90 degree bend!). Also, I don't need the top fire protection offered by those pipes: it's just a hood, not a stove. But I really like the effect.

Does anyone have any idea what I could use? Perhaps some aluminium pipes that I could spray paint black? Where could I source them? Also, do you think this will solve my problem? There really doesn't seem to be anything wrong with the extractor itself.

Thanks very much
Andy

 

[KenGMac]

TheGlobetrotter, Hi.

What might be happening, a long shot? is that the extract seems to start as 150.mm but if the outlet diameter is reduced by this Quote, "flat pipe? which I assume is a standard B&Q rectangular vent duct ? then what is probably happening is that, the air flow is happy to pass out through the 350.mm. long 150.mm wide section then it hits a much reduced rectangular section where the restriction in Square area takes over and any air being vented simply cannot pass from 150.mm diameter, to what is probably? 100.mm by 50.mm then there is a large restriction, if I have this correct?

At this point, because the air simply cannot pass from a 150.MM Dia. to a lesser duct size the air that cannot get into the lesser sized duct has only one way to go, back into the kitchen?

As a very, very long shot? is the fan in the hood actually extracting? meaning I have seen, generally on a three phase motor the fan running the wrong way around, instead of sucking air out it was actually pumping air back into the property, as I say a very, very long shot?

As for cost? there are a load of sites, including "Machine mart" for one of many, just to keep this within the rules of any forum as regards so called "Advertising" suggest you Google - Vitreous Enamelled flue pipes.

Hope this in some way assists?

Ken

 

[TheGlobetrotter]

Thanks Ken. I can confirm that the extractor itself is working fine as it passes the "light piece of paper test". Also, after I disconnect the pipe it doesn't blow air into the kitchen and you can feel a minor storm coming out of it on the other end.

I think you're right: the combination of the bend and the reduction in diameter must be to blame. I'll google what you suggest.

Thanks very much
Andy