I'm fitting a new cooker hood with a 150mm duct outlet. I want the fumes to be vented outside the house and there is an available 100mm vent in the wall about 4 metres away via the loft space. My question is about the best way of reducing the size of the ducting between the cooker hood and the vent. Given that it has to transition from 150mm down to 100mm, where is the best place to install the reducer? Should it be near to the fan at the cooker hood end, or near to the vent at the external wall? Does it make much difference? I'm thinking that if it is near to the fan then it will be noisier, but if it is away from the power of the fan it may be less efficient in shifting the air. Anyone here know about these things?