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.

Kevin wrote:

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.

Dead right.

snip ducting routing problem

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!

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?

Have you tried http://www.bes.ltd.uk?

David

Kevin wrote:
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.

So far so good.


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.

Very sensible.


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.

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?


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?

http://www.bes.ltd.uk/products/180.asp
http://www.bes.ltd.uk/products/181.asp

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.
http://www.elica.co.uk/master.asp?http://www.elica.co.uk/view_products_parent.asp?Designer%20Hoods

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


--
Grunff

Kevin wrote:

I should avoid using PVC flexible ducting and that

why not

I should not use flat channel pipe

again, why not ?

genuine questions.

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

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.

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


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Grunff wrote:

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.
http://www.elica.co.uk/master.asp?http://www.elica.co.uk/view_products_parent.asp?Designer%20Hoods

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

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.
http://www.elica.co.uk/master.asp?http://www.elica.co.uk/view_products_parent.asp?Designer%20Hoods

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


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.

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.

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.