Reduction in final connection to bathroom extract duct?

[SuzieJarvis]

Hi All,
I've just changed a 20 year old bathroom extractor (which did a brilliant job) for a nice shiny new 100mm Xpelair model (which doesn't do anything - paintwork will need re-doing to remove black condensation marks after just 3 weeks!)
To try to overcome this, I've now purchased a 150mm Xpelair (might have to put weights on kids when they are in the shower!) The new (solid PVC) extract duct will go vertically from the fan about 400-500mm to the underside of the roof, but the roof outlet only has approx 100mm connection, so I will need to reduce the duct diameter for the final connection to outside.
Is this going to have a detrimental effect on extractor efficiency and noise levels, given that it is a relatively short run?
Also, existing 100mm vent has a condensation trap, which obviously won't fit the new pipe. Again bearing in mind that it's a relatively short run in a never that cold roof space would I get away with just insulating the duct rather than fitting a new condensation trap?
Thanks.

 

[jj4091]

I believe that reducing the diameter of the ducting will not make any difference & you could therefore put the reducer before your existing condensation trap & utilise the existing duct. But you will get others that will & have in the past disagree with my opinion, so I think the decision will be yours.

 

[SuzieJarvis]

Thanks jj4091, but I have to fit 150mm duct first to allow for the flaps to open. If I try to use the existing condensation trap it would mean having 100mm of solid duct (150 dia) then a reducer, then the condensation trap, then 100mm flexi pipe to the roof. Not sure if that is going to be ideal...

 

[John D v2.0]

Late reply I know, but the condensation trap is needed to stop the steam that condenses going back into the fan and damaging it. And the resistance and noise of 100mm duct is a higher because the speed of the air will have to be higher. 150mm duct has 2.25x the free area of a 100mm duct, and only 1.5x the wall length. So it really is hard to underestimate the benefit of the bigger pipe.
Having said that, for a short run give it a go and see how it is. You can always put 2 tiles in parallel to just about get there.
If it was for an mvhr system or something continuous, it's more important to get it right to avoid excessive power consumption, but for this, don't worry about that.