90cm Chimney Cooker Hood with long 100mm Ducting Query

[Twinkletoes8]

We're trying to find out what 90cm chimney cooker hood we can use successfully with 4"/100mm flexible ducting (our only option), which is 5-6m long? Does anyone have any experience of a chimney style hood with 100mm ducting (not ideal we know).

We've established 2 Smeg models that have a 120mm outlet, which seems likely to be better suited to our ducting with a reducer than the bigger more standard 150mm outlets. We know all the facts about using smaller ducting voiding the warranty, making extraction more noisy and less efficient, but our previous recirculating hood (which we've been unable to find a connector for to try venting it as it's a strange 20x11cm rectangular outlet) was useless and our smoke alarms were going off all the time, so we are desperate to find one that vents externally to some extent, even if it doesn't work as well as it should! Our kitchen size is approx 22cubic m.

Of the 2 Smeg models the extraction rate of the cheaper one is from 250-350 (noise level 45-58dB, nominal power; 275 watts), and the more expensive 285 to 535 (50-72dB noise level, nominal power 240 watts). Is there any point getting the one with the higher extraction rate or is it just likely to be meeting too much resistance from the 100mm flexible ducting to work any better than the lower spec one (and will also probably be too noisy)? Or is the lower spec one not likely to be powerful enough to be able to vent out of such a long duct (5-6m)?

Any advice gratefully received!

 

[fidom]

If the hood proves ineffective you could add an extra inline fan in your ducting:

http://www.vent-axia.com/range/acm-line-mixed-flow-fans.html

 

[foxhole]

You can reduce any to the 100mm size, very little effect on efficiency.

 

[Twinkletoes8]

Fidom, thanks for that suggestion, tho I'm not sure quite how we could get access to the ducting to put one in as it's within the ceiling void and very difficult just to get the ducting though according to the electrician who fitted it.
Plus now that the ducting's in place the floorboards at the far end are back dodown in place again.
Foxhole, that's reassuring - is this from personal experience, as all the manufacturers are adamant that 100mm just wouldn't be up to the job and so not worth bothering with.

 

[jj4091]

Foxhole, that's reassuring - is this from personal experience, as all the manufacturers are adamant that 100mm just wouldn't be up to the job and so not worth bothering with.

Basic physics determines the flow rate of air & a smaller duct just increases the pressure not the volume so a smaller duct will not be a problem. It's like putting your finger over the end of a hose pipe as you fill a bucket, it still fills as quickly but splashes all over the place because of the increase in pressure.