diameter of holes for extractor fans?

[Stephen]

Hello,

My hob has a cooker hood above it but it has been installed in re-circulation mode. If I change the carbon filter it might absorb smells but it will not do anything about steam, so I would like to convert it to extract outside. That would be straightforward: the hood is about 6.5' or 2m ish from the wall. I could run ducting along the top of the cupboard units.

What ducting is best: circular or rectangular? I know the oblong one can be hidden behind a pelmet but I'm happy to have unsightly circular if that is better - does one shape offer less resistance to the air?

The thing that may be a problem is that the wall is only 2' wide. Then there is a french door in the wall and I'm not sure whether there is a lintel above it.

I'm just wondering whether I can cut a 6" hole in the wall for the extractor fan if the wall is only 2' wide; that's a quarter of the width of the wall, which sounds a lot.

OTOH I live in a semi, so although I only have 2' on my side, there is another 2' on my neighbours side which mirrors mine.

What do you all think?

Thanks.

[newshound]

Modern house presumably? Cavity wall with insulation?

*Probably* not a lintel at the place you would want to make the hole, which is presumably at the junction of the party wall and one side of your 2 foot wall.

Not sure what if any implications there are for putting vent close to your neighbour.

With a 2 metre straight run I would not worry about pressure drop in rectangular duct.

Can't see a 6 inch hole having structural implications, especially as it is probably nearer 4 inches.

[ARW]

If using rectangular ducting then you would be better off with a brick grille.

[ARW]

Another option that I have used before is to install the ducting between the joists esp with the chimney hoods.

[Jeff Layman]

The OP needs to watch out if the cavity wall insulation is old polystyrene beads which no longer stick to each other. When the hole saw is removed the stuff will just pour out (especially if it's a standard two-storey house and the hole is at the top of the ground floor). I would advise him to have a vacuum cleaner ready and to shove the ducting in a soon as possible after the hole saw is pulled out.

[Harry Bloomfield]

Stephen formulated on Sunday :

Check which way the joists in the floor/ ceiling above run, you might be able to fit a suitable duct between joists. That is how I did my hob extract. Rigid plastic pipe up from hob into ceiling, then duct between joists, terminating in a gravity closing terminal on the outside wall.

[tabbypurr]

the big difference is between smooth walled & corrugated flexible, The latter is bad news for airflow, especially with axial fans which only deliver low pressure.

6" in 4' should not be a problem

Drill an 8mm test hole. You can block that trivially easily if it rains beads.

NT

[billyorange007]

Yes. Flexible hose kills the efficiensy of the air flow handling . Also get the largest size recommended by the extractor manufacturer. SWMBO got a Miele fan and it could cope with 6" pipe. I used smooth wall plastic pipe. It works perfectly. Even frying fish laves no residual odours in the kiytchen and no steam at all

[Michael Chare]

Are you able to download installation instructions for the hood?

I was able to buy a part that connects the round pipe inside the hood to a square pipe that runs along the top of my cupboards. The square pipe was a standard part which terminates in a brick coloured plastic vent in my outside wall.

[newshound]

IIRC Wickes has various "standard" adaptors, like a round to rectangular elbow, that may well fit generic hoods. You might have to apply a few turns of duct or PVC tape to make them a push fit instead of a loose one.

[Stephen]

1930s. Cavity but no insulation (must get round to that...)

I don't think there is a lintel above the door they retrofitted.

They don't have any windows or doors nearby, so I think I'm ok?

Thanks,

[Stephen]

No insulation in the cavity but I didn't know that. I'll remember that if I ever move house and have to do it again.

[Stephen]

I wondered that but what are the chances of the brick being in perfect alignment to where I want the duct!?

[ARW]

Flexible rectangular ducting for the last bit?