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Long cooker hood duct ?

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Evening all,
Currently 1st fixing new extension however the existing cooker hood duct which was on outside wall will now be part of new utility room and will need extending. Two issues I have .
1 The original duct is 4” flexi and already extends across kitchen for about 2.5metres.
2. The new utility room would make the total length of duct 5.5 metres and I thought the max was 5 .

Does anyone know what best practice is for this . I would normally use flat duct 6” or equivalent for cooker hoods. Do I have a I change all the duct to a suitable size ? The existing kitchen is not being touched so I know they would rather I didn’t take sections of ceiling down.Taking floor up is not an option. If I have to change exsisting that is what I will do. I am waiting on building control to get back to me but was keen to find out what other sparks have done. I have heard about fitting continuous fan with a boost option like the green vac could be an Option.

Hopefully you have a straight run. The most efficient duct to allow the passage of air is smooth walled and straight and I would use as long a length of 4" soil pipe as I could. This should be installed with a slight run downhill towards the grille

Cooker hoods usually have larger than 4" connectors so has the size been reduced down at the hood? It may be that you will be better off using a 5 or 6" aluminium flexi duct (whichever size your hood is) but make sure that any flexi is pulled reasonably taut. I don't know how 5" taut flexiduct performs in relation to 4" soil pipe.

On longer runs you need to be careful that you dont have any troughs that any condensing moisture may collect in, insulating the duct is a good idea
Cheap plastic flex ducting shouldn't be used for a kitchen hob extractor, at least not close to the hob. The first time there's a 'flare up' on the hob, it will cease to exist.
Always better to use smooth bore rigid pipe, and on the occasions I've had to exceed the recommended length, I've upped the diameter once away from the extractor.
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