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Ducting Size Guide
Ducting Size Guide
Here are the sizes of ducting systems and space requirements.
Ducting System
Actual dimensions (mm)
Space requirements (mm)
Airflow mm²
For 4" (100mm) extractor fans or for low power cooker hoods up to 250m³/hour
For medium extraction rates or if available depth is limited up to 500m³/hour
For 6" (150mm) extractor fans or for high power cooker hoods up to 500m³/hour & above
For a shallow space, such as ceiling voids or under floor, where high extraction rates are not needed:
System 100
Round/Rectangle
System 125
Round/Rectangle
System 150
Round/Rectangle
System 204
System 220
System 225
System 300
Actual Dimensions (mm) 
Space Requirements (mm) 
Note: Round Ducting: Our straight lengths of plastic duct round tubes are ‘female’ i.e. duct fittings such as bends and reducers are ‘male’ and generally slip inside the duct tube. To connect two ‘male’ duct ends together use a ‘female’ cut length of straight duct / tube.
Note: Rectangular Ducting: Our straight lengths of rectangular plastic ducting are ‘male’ i.e. duct fittings such as bends and reducers are ‘female’ and generally slip over the straight duct. To connect two ‘female’ duct ends together use a ‘male’ cut length of straight ducting.
GOOD TO KNOW
- The length of ducting and number of bends will affect performance and should therefore be kept to a minimum.
- Long runs of flexible ducting are not recommended for use on cooker hoods – rigid ducting should be used.
- Plastic Ducting components have a simple push fit connection. Flat channel can be easily cut to length using a hacksaw.
- When installing a rectangular to round adaptor, it is important that the flat side is fixed uppermost. The flap operation of a grille/fan would otherwise be obstructed.
- When installing a circular through-the-wall duct a core drill can be used.
- Ducting passing through an unheated areas such as roof voids should be insulated to minimise condensate formation and must include a condensation trap and means of running off condensate.
- Vertical ducts should be fitted with a condensation trap for low power systems and a weatherproof cowl of sufficient free area for the air volume.
- Horizontal ducts should fall away from the fan unit.
- Choice of the right system will ensure quiet and efficient operation of fans and or cooker hoods
- All internal system parts manufactured with self-extinguishing materials to meet fire retardancy standard UL94V2
- Plastic ductwork materials do not corrode leading to a clean and long lasting life
- Wall outlets are made with UV stable materials to delay colour fade
- The choice of ducting system will depend on a number of factors including: the power of the cooker hood or fan to be connected to, its location, the configuration of the duct run, the space available and others.
- Maximum working temperature for rigid PVC ducting is 60oC
