unconventional extractor fan ducting

[Redeye]

so me and the missus were having a conversation last night about how we want to do the kitchen when we can afford it. she talked, i listened.

anyway, we have a bit of a problem in this kitchen at the moment - condensation. the building is about 250 years old, converted about 100 years ago from a hay barn into a house. so, one of our main priorities will be keep it ventilated a bit better than it is at the moment. we'll probably end up having a hood over the hob and a humidistat controlled extractor fan.

the problem comes with getting this to the outside world. because of an awesome arrangement of window/drains/boiler there is absolutely no way to get through to the outside world above about 1m from the floor. so, can we duct a combined extraction from the hood and the extractor fan down the wall at some point so that we can get it through the wall beneath the window somewhere?

is there any good reason not to do this other than it sounding like a daft idea?

[meow2222]

nowt wrong with doing that but dont combine them, if you do your air will go into the cooker hood and come outnthe ext fan or vice versa, partially.

NT

[The Natural Philosopher]

Its OK in theory, but in practice airflow goes down rapidly with pipe length..you may need a fairly meaty fan and fairly large bore pipework.

[The Natural Philosopher]

Very good point. Its always wise to make ducts an inverted U, rather than U.

NEVER slope UP to the exit.

[Christian McArdle]

You can if you incorporate a drain. However, this should only be done by necessity, not out of choice.

Christian.

[Redeye]

Thanks folks.

The ducting would end up as an inverted U, although more like an inverted J to be honest as the outlet would be a lot lower than either inlet. With a bit of planning it sounds like I'd probably be able to make this work.

Thanks again for your help.

[tinnews]

One issue might be condensation in the duct building up and unable to get out, you might need some sort of clever drain.

[David Hansen]

On 5 Jul 2006 05:06:21 -0700 someone who may be "Redeye" wrote this:-

Most of the water vapour in a kitchen comes from the hob. Extracting from it should reduce condensation dramatically. You might not need anything else, unless there are some particular circumstances.

If the hob is also near where you put down dishes from the oven then the hood can also extract the steam when the lid is removed.

To add to what others have said, where a duct exits a building "horizontally" it should slope down gently. Not only does this allow condensation to drain away, it also stops rain penetrating too far.

If the duct goes up from the hood only slightly and then downwards there should be no great condensation problem at the motor and no need for any more drainage.

Junctions are available, with an airflow operated damper that will allow air to flow from either or both ducts into a common duct. They are reasonably effective, but not perfect.

Remember that you don't want the fumes coming back in through the window when it is open. As a very minimum I would fit an outlet that will divert the fumes away from the window, either downwards, sideways or a combination of both as best suits the location.

[The Natural Philosopher]

Random points

DO understand how long runs of pipework reduce flow rates DRAMATICALLY and use MUCH larger diameter than you might think necessary.

DO consider that if the weather is suitable for open windows you probably won't use the fan..

DO consider that gas cookers are the primary source of steam in a kitchen. WE have oil Aga and electric hobs, and never get steamed up.

DO consider that steam is not the only thing you want to get out of a kitchen. Fried mackerel is another :-) Humidistats are NOT recommended.

[Guy King]

The message from The Natural Philosopher contains these words:

And smoother. Crinkly flexiduct is very draggy.

[Mark]

Avoid 90degree bends too.

Mark