Cooker hood ventilation - best compromise.

[Kevin]

At the risk of receiving a lot of conflicting advice, I am seeking opinions on the best compromise for installing ventilation ducting for my Zanussi ZHC925 cooker hood. The installation instructions state that I should use 150mm or 125mm ducting and that the maximum length should not exceed 3metres, to be reduced by 1m for each 90degree bend, otherwise efficiency could be impaired. I have read various threads on this newsgroup suggesting I should avoid using PVC flexible ducting and that I should not use flat channel pipe, but those threads don't take into account the specific problems I have.

One of the main reasons for using a cooker hood is to avoid a build-up of steam in the kitchen. For this reason alone, I don't believe using the hood in recirculation mode would be effective in any way.

The best installation would be to take the ducting straight up through the hood chimney(approx 0.5m) , turn through one 90 degree bend and then run it horizontally between the 1st floor joists to the outside wall (approx 2.4m). Unfortunately I cannot actually get through the wall at this point because there is a large steel lintel in the wall at this point and it would actually end up venting into the conservatory rather than outside. In order to reach a point where I can vent externally, the route I would need to take is up 0.5m, 90degree bend to bring it under the floor joist, another 90degree bend to turn between the floor joists and then the 2.4m run to the external wall.

The advantage of using flat channel is that it is only 60mm high so the visible impact in the kitchen, as it passes beneath the joist is much less than if I was to use 125mm round pipe.

The advantage of using flexible ducting is that I should be able to form much shallower bends, although I still have concerns about the visible impact in the room.

I can of course use any combination of the three, the most obvious being round pipe for the vertical run, round pipe for the horizontal run and flexible ducting to link the two.

Finally, where can I buy a decent length of 150mm round pipe? Screwfix only seem to do 350mm lengths of 120mm and I would assume that each connection itself has some impact upon the airflow, or am I wrong?

Thanks.

[Lobster]

IMHO it's very hard to give an accurate answer, as it's quite subjective. Yes, you'll get lower flow if you introduce a bend; introduce another bend; reduce the CSA of the duct; have multiple changes in duct section. The question is, will you still have what you will consider to be adequate flow in your proposed setup? I don't know!

Have you tried

[Grunff]

So far so good.

I would say avoid the rectangular duct at all costs. How about using round aluminium for the bends, which would allow you to form them as gently as possible?

You could really do with a more powerful extractor. Your ZHC925 is rated at 470m^3/h. You can get 750m^3/h and more, e.g.

Having said that, I'm sure you'll get a usable result if you use round duct (125 or 150) with the ZHC925.

[.]

why not

again, why not ?

genuine questions.

[Owain]

In other words, this fan simply isn't designed for long ducting. I would suggest changing to a centrifugal fan designed for long ducting.

Also, could you go straight up the wall, through the first floor, and up through the roof? This would enable you to have a straight, wide, duct, which woudl be be better even though it would be longer.

Owain

[David Hansen]

On Tue, 15 Nov 2005 20:33:31 +0000 someone who may be Owain wrote this:-

It would need a condensation trap. Otherwise an expensive blue flash and bang is possible.

[Mathew Newton]

There is more to it than comparing the *free space* airflow figures - sure you'll likely be heading in the right direction but there are other factors such as the design of the fan itself as to how well it performs under (back) pressure (e.g. axial vs centrifugal).

You should be able to get the fan performance graphs for varying back pressures and do the sums without too much difficulty in order to ascertain expected performance with different ducting size/configurations.

Mathew

[Kevin]

Unfortunately we've already bought the hood, at a very good price I might add, as much because SWMBO liked the look of it as anything else. It's only now that I'm starting to find out the possible drawbacks of trying to install the damn thing.

[Kevin]

This seems to be general opinion from other threads. If I understand correctly then it's to do with the fact that the airflow is better through round pipe than through square, less drag I guess. Same thing goes for the flexible PVC ducting, opinion seems to be that the coiled wire in it interrupts airflow.

[Kevin]

Not an option anyway, SWMBO would be very upset if she had to put up with a ventilation duct coming up through the middle of her bath.