Ducting for kitchen extractor fan

[aroominyork]

We moved into a house with a gas Aga (I think converted from oil). The extraction pipe is fitted against and runs through an external wall, then up the outside of the external wall. The ducting pipe circumference is about 32cm, so about 10cm diameter. There is no extractor fan in the kitchen. We want to replace the Aga with a range and fit an extractor hood above it, ducting out of the same pipework that currently takes the Aga fumes. Will the Aga ducting be OK or do we need to fit wider ducting - I have read 150mm diameter is often needed for extractors and we have a large kitchen so would need a strong extractor.

 

[gmartine]

No matter the size of kitchen, the extractor hood manufactuer will dictate the size of ducting.

 

[aroominyork]

Thanks gmartine. There are some issues about the order of work. Let's say we are looking at a NEFF N50 D65IHM1S2B whose product specs (on ao.com) says "Ducting diameter: 120mm / 150mm". My first question is what do those two different measurements mean?

The range and extractor will be fitted as part of a kitchen refurb in a few months. In the meantime, we need some roofing work done urgently and while the scaffolding is up we want the external flue replaced because it is rusty. Can that external work sensibly be done before the extractor is fitted, and if so what size external flue? Currently the external flue is about 42cm circumference.

 

[snowhead]

120 / 150mm, means 120mm minimum, 150mm maximum diameter of the ducting from the fan.
150mm is the hole size on the extract hood, 120mm would use a reducer to connect to it.
The longer the discharge pipe is, the greater the resistance so the diameter needs to be larger.

Domestic kitchen extracts normally discharge through an external wall, they don't need to rise to roof level and I doubt that the small fan in the hood would push air that far, you'd probably need an external boost fan in the external stack if you need to go to roof level.

 

[gmartine]

42cm circumference = 13.37cm diameter or 133.70mm so a 125.00 mm diameter replacment will be in spec.

However as above, what's the flue termination height and will the motor be able to overcome that as it's more usual to terminate flue horizontally and out of the nearest wall. Again consult manufacturer.