Cooker hood quiet & powerful until duct is connected

[SiliconS]

I've nearly finished installing our new kitchen. I've fitted a new chimney extractor above the hob that - despite being the £99 cheapo from B&Q - is quiet and powerful... until I connect it to the ducting through the wall.

Our house is ten years old. The wall duct is offset from the centreline of the hob (builder cock-up, I think) so to connect this new chimney to the existing hole I've had to use two 100mm rectangle right-angles behind the plasterboard together with the circle-to-rectangle adapter above the hood. Joining the hood to the new duct is a 1 foot length of 100mm PVC ducting hose.

On the lowest speed setting the hood can easily hold a piece of A4 paper against the filters, but if the duct is connected then it can't. With the duct connected the hood is noisy, rattly and ineffective. Can a short length of ducting with two right-angles have such a massive effect on the efficiency of the hood? What can I do to overcome this problem?

 

[stephenf8542]

Where does your duct vent to?

 

[SiliconS]

It vents through the (external) wall, through a backdraft flap and then a brick-sized vent. I haven't disturbed those components - the flap and the vent brick are the original ones that the builders used.

I've a feeling my ducting is too small, but to enlarge it I'll have to enlarge the hole in the exterior wall.

 

[stephenf8542]

take the ducting off the vent and see what the air flows like. Is a single speed exractor?

 

[SiliconS]

That's a good idea. That'll show whether it's the duct or the vent that's the biggest restrictor.

It's a 3-speed fan. The performance on the highest setting with the duct in place is worse than the performance at the lowest setting with the duct disconnected. It's noisier too.

Our old Neff extractor was noisy and perhaps the small duct was the cause of that.

What are hoods like on recirculate mode? Worth having?

 

[Steve]

What are hoods like on recirculate mode? Worth having?

Poo and No.

 

[SiliconS]

Thanks. I thought that might be the case.

My wife has told me to connect the bloody thing up and just leave it. The only alternative is to drill a much larger hole through the wall and fit a full-diameter vent, which seems like overkill. I don't have the tools to do this so we'd need to fork out for a builder. Sod it.