Extractor fan - diameter and pipe type

[R D S]

Hello, I thought i'd fit the extractor fan we bought last year as part of a new kitchen, so it's now finally attached to the wall. Naturally I cannot use the vent hole i made for the previous one (i'm only about an inch and a half out though, bastard!), so a new hole is going to be necessary.

The extractor wants 125mm duct, I thought i'd be OK because I have a

127mm core bit but no, I need nearer 130 so must be i/d.

So shall I reduce down to 120mm? Depending what I read it would appear 125mm has up to 50% more airflow than 100 but along a run of about 1/2 metre is this realistic? I might buy a larger core and stick to 5 inch.

And i'm compelled to use rigid pipe instead of that nasty flexible s**te, opinions?

[ARW]

Just reduce the ducting by any way possibly.

If than means flexi duct and gaffer tape then do it.

It will be fine.

[Adam Funk]

I think cable ties are the usual way to secure flexible ducting to the rigid parts (fan, adapters, outlet, etc.).

Duct tape can lose adhesion over time and work loose (especially in sunlight --- I'm not sure how long it takes in a dark space). But anyway, they now tell people in the US *not* to use this misnamed product on HVAC ducts because it doesn't make durable airtight seals.

[Nick Cat]

Do you mean duct size? It's gonna make very little difference, 120, 125, or 130.

Flexible is terrible for airflow. Whatever size/shape will do if you maintain approx the same cross section. If you block holes with tape it'll fall off after a while.