Kitchen hood extract shutter noise

[oldman2]

I put in a new 6" duct from a 900mm cooker hood to outside and fitted a multi flapped wall vent cover like this one http://www.tlc-direct.co.uk/Products/BGBD6W.html

It flapped like mad in any sort of wind at all, I tried damping it with draught foam and although that made the sound less obtrusive, it wasnt a fix.
6" duct into a big hood space is amplifying any sound, even road noise when its on the side at the back of the house 100ft from any traffic.

I then bought a 6" Cowled vent like this one http://www.screwfix.com/prods/14789/Heating-Cooling/Ducting/Manrose-Square-Brown-140mm-Cowl-Vent

But that was worse, the wind gets under the single large flap and it then acts like a reed in a kids whistle!
I tried damping it down with a weight and even tried angling the flap so the weight pushed down rather than just adding damping. Still flaps when the wind is just gentle.

So my last attempt will be a 6" inline backdraught shutter like this one http://www.alertelectrical.com/Fans...le-Ducting/150mm-Backdraught-Shutter-1232.asp mounted in the 300mm of duct from the hood. I will keep the cowled external part that I have and remove the flap from it.

Has anyone else managed to crack the problem?

I am hoping that inserting the flap inside the tube will stop the wind from getting under the lips and flapping it. I have to assume the spings on these are set so on low settings the fan can still open the flaps.

 

[foxhole]

Did you try original vent with cowl on top and cowl flap removed so it just acts as a baffle for the original vent.

 

[oldman2]

Spoke to manrose this morning and they suggest the in line backdraught shutter will fix the problem with just the hood on the outside.

I have in the meantime fixed a magnetic strip to the flap on the cowled version I have here which may just be the answer. Have to wait for a windy day to find out now.
The magnetic strip doesnt need a lot of force to open the flap but its more likely to stay shut in wind than it was before.

If this doesnt do the trick I will bite the bullet and spend another £17 on the inline shutter.

 

[oldman2]

Answering my own post here as it may be useful to others.

It works!

The magnetic strip I used was I think originally from inside a fridge freezer door seal, so wasnt over powerful, but wont rust being impregnated plastic design. Stuck that on the inside of the bottom part of the 6" flap, stuck a bit of galvanised L shaped metal to the inner frame so they meet when the flap is shut firmly.

The wind yesterday/today cant open the flap against the magnet but the fan itself can and does. A nice mechanical clonk now when the flap either opens or closes.