Extracter hood, ducting and vent

[Imgoingout333]

Need advice. So we have a new extracter hood installed. It says on the booklet that it requires a 150mm round rigid duct vent. The problem is we have a older property and was fitted with a 100mm flexible ducting. It will be hard to change and runs up above across the cieling void on to back wall. So effectively across the entire length of the kitchen ( medium size)
My question is. Is it okay to use a ducting reducer and use the existing 100mm ducting. I know it will be noisier and fan extraction will not be as effective. But we really have no choice. Is it safe to have it on 100mm flexible ducting?

Thank you in advance.

It is safe but as you say it will be noisier and less effective.

If you ever fit new ducting, the rigid pipe stays cleaner, less likely to trap condensation or fluff and, being thicker, more resistant to noise and rodents.

Thank you piglet John . At least it put my mind at rest. I had a bit of a worry that the fan being a more powerful for the smaller ducting would cause safety problems in the long run. Im thinking it could be a expensive and messy job to change it to a 150mm ducting . I've already just had the cieling replastered . So really dont want them to have to put holes in again chasing the old ducting from the above the cieling joist. In hindsight we should have had it done first before anything else.