Island extractor hood- is this normal.

[Batterme]

So I have lived with what I feel is a very poor performing island extractor hood. It was from Ikea, so most likely Whirlpool in origin.
It is 800 wide, but only has a single intake vent, about 250 square.
It has always struggled to cope with even steam from a boiling pan, it was a new build, so is vented straight up and out.
The build dictated that the top outlet flange went onto standard 100mm flex, which has a 90 turn just above the flange, and then quite a long straight run (about 4m) within the ceiling cavity, to the outside.
I have just replaced a faulty transformer for the lights, and whilst it is in bits, took the opportunity to have another look.
When I split the connection pipe on the top outlet and run the fan, there is a huge amount of air being moved- I was actually taken aback by the force of the flow.
However, on the intake side you can hold a piece of tissue onto the metal filter, and it just about holds it in place, and I mean just.
I only have the external metal grease filter in place, and have now also removed the return air baffle on the top of the hood exhaust, which you could hear opening and closing when the fan was running.
I realise the high movement of air on the outlet I am feeling is constricted down and out of a 100mm opening,but the difference in the perceived airflow from the intake grill just seems wrong?
So question is, is the physics of what I an feeling normal or not, and is the long run length of my exhaust pipe the route of my problem?

Thanks,

John

 

[jj4091]

When you say you are holding a piece of tissue against the filter do you mean a piece that covers & seals the whole thing or just a small piece. You should be able to hold a sheet of newspaper up to the filter & feel it suck it up. Have you washed out the grease filters?

 

[bernardgreen]

is the physics of what I an feeling normal or not, and is the long run length of my exhaust pipe the route of my problem?

Long straight smooth wall ducting has the least resistance to air flow. Flexible ducting with corrogated wall has the greatest resistance to air flow ai it creates tubulance in the air.

 

[Doggit]

Sorry, but you're figures are a little confusing. If it's an opening of 250mm square, then that's an area of 62500 sqmm, and if it finally does down to 100m pipe, then that's only 7855sqmm. But the worst aspect, is that you've got a 90 degree bend, and then goes 4m, so you've not only got a reduced flow, but I doubt the motors powerful enough to push the air that distance. But I suspect that even if you could get to 150mm smooth bore, it'll still struggle over that distance.