Cooker hood ducting - 125mm to 100, or bigger hole?

[MDA]

Hi all,
A couple of related questions I'd appreciate some advice on:

I need to replace an old cooker hood (circa 190 m3/hr) that has a 100mm duct. The duct routing is very tight - two 90 degree bends at 90 degree orientation to each other. On the positive side the ducting is only around 0.5m in total length to the outside vent.

I'd like to replace it with a more powerful unit (circa 400 m3/hr). Understandably that has a larger duct requirement - 125mm.

Two options of course:
make the hole in the wall bigger. The extra difficulty here is that due to the tightness of the routing I'd have to make the new hole offset to the existing - ie generate the extra 25mm all on one side. I'd therefore only be able to stitch drill part of this. Any ideas on the least painful way to do this?

The easier option - a 125 to 100mm reducer and go with the existing hole. A bit of maths says that's about a 35% reduction in area. Does that directly equate to a reduction in airflow by 35%, or something less but with an increase in noise?
More importantly, am I likely to damage the hood running it with a reduced duct?

What would you do? Any advice would be most appreciated.

Thanks,

MDA

 

[jj4091]

Just use a reducer you will get the same amount of airflow , it will just go through the hole faster & you will not build up enough back pressure that might affect the fan.

 

[wsts]

Just use a reducer you will get the same amount of airflow , it will just go through the hole faster & you will not build up enough back pressure that might affect the fan.

That's incorrect, it will not go through faster, it will also likely make the fan noisier and possibly overheat the motor which more crucially invalidates the warranty.

It says 125mm for a reason, I get called out to endless bodge installations where the customer is complaining its noisy or not extracting correctly always to find the same thing, the extraction tube is not the correct size/construction.

 

[jj4091]

Just use a reducer you will get the same amount of airflow , it will just go through the hole faster & you will not build up enough back pressure that might affect the fan.

That's incorrect, it will not go through faster, it will also likely make the fan noisier and possibly overheat the motor which more crucially invalidates the warranty.

It says 125mm for a reason, I get called out to endless bodge installations where the customer is complaining its noisy or not extracting correctly always to find the same thing, the extraction tube is not the correct size/construction.

That is a load of b*****s, apart from the bit about being noisier & even that will be hardly noticeable in a cooker hood. They usally have a fan bigger than 4 inch inside but also a manifold that reduces the outlet to 4 inches. Please show me where it says a 6 inch fan has to have 6 inch ducting. I think you should brush up on your basic physics.