Reduction in ventilation ducting for cooker hood

[Paul205]

Dear All,

I have an Elica Plaza cooker hood which has been installed for around 5 years. It currently vents through the kitchen wall to the open air via 150mm ducting.

I am considering re-routing the ducting due to fitment of a new extension. To allow this the cooker hood ducting will need diverting to allow it to vent to the open air via the soffit. Unfortunately, the soffit is small so cannot accommodate the fitment of a 150mm grille. I am therefore considering fitment of a 150mm to 100mm reducer in the last 300mm of ventilation before the soffit. 95% of the ducting would be in 150mm and the last 5% in 100mm.

I would appreciate your thoughts on whether this would put any stress on the fan and also whether there would be any real measurable reduction in performance.

Kind Regards




Paul

 

[foxhole]

150mm duct can be split ,extract to twin 100mm outlets via "Y' joint. No loss of performance.

 

[Paul205]

thanks fox, although I'd rather not use 2 x grilles as the soffit may look unusually busy. I would be looking for a single 100mm grille

 

[Deleted member 174758]

It might be worth asking the original manfacturer of the extractor fan what their requirements are for their fans. If you reduce the pipework at the end you'll restrict the airflow quite drastically because the cross sectional areas of 150mm diameter pipe and 100mm pipe are 17,678mm sq and 7,857 mm sq respectively - in other words a 100mm pipe is a lot less than half the cross section of a 150mm one. Expect the performance to be drastically reduced (especially if it's a long run) and the fan motors to have a shorter life if you reduce to 100mm. Have you considered going to a similar cross sectional area rectanguler final pipe and soffit vent cover?

 

[jj4091]

Basic physics tells us that reducing the ducting to 100mm will have no effect on the performance or the life of the fan. The only difference you may notice is a slight increase in noise due to the increased speed & turbulence of the airflow through the last section.

 

[Paul205]

The fan manufacturer basically said what you said Job and Knock. I am perhaps considering splitting to two x 4 inch vents

 

[jj4091]

The fan manufacturer basically said what you said Job and Knock. I am perhaps considering splitting to two x 4 inch vents

Why? Do your own research via google you will find it is a load of bull!

 

[Newboy]

Must be some new laws of physics that only you have access too!

 

[Paul205]

People, what I would say is it is only about the last 10% to 20% which would be in the 4 inch size

 

[phatboy]

Doesn't matter, restricting the pipe at any point has the same result.

Can you go to flat ducting for the final section? You can get some thats equivalent to 6" round.

Like this range: http://www.tlc-direct.co.uk/Main_Index/Ventilation_Index/Ducting_Flat_6/index.html

Then you just need a suitable grill for the soffit.

 

[jj4091]

Must be some new laws of physics that only you have access too!

Ever heard of "conservation of mass" ?

 

[Paul205]

Cheers gents, it would appear I may be able to route the piping in a different way and retain the 150mm ducting!! cheers

 

[Newboy]

Must be some new laws of physics that only you have access too!

Ever heard of "conservation of mass" ?

Certainly have - do you understand the difference between an open & a closed system?

 

[jj4091]

Yes, & I would regard this system as open, so from the ops & any one else in this situations perspective, applicable. That is only the pressure at the exit of the duct is affected & not the volume of the airflow.
Seems to be irrelevant now though.