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Ducting Size Guide
Plastic Ducting Size Guide
Plastic Ducting Sizes, Dimensions & Technical Drawings - Space Requirements Explained
Planning your ventilation setup? Below you’ll find detailed dimensions, cross-sectional sizes, and technical drawings for both round and rectangular plastic ducting.
Use this guide to understand space requirements, ensure compatibility with fittings, and choose the right size for your installation.
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) 
ROUND DUCTING
Our round plastic ducting pipes are slightly bigger on the inside. That means most fittings (like bends or joins) slide into the pipe. If you need to join two fittings together, you’ll need a short piece of pipe to go around them.
- Pipe is female – slightly wider inside
- Most fittings are male – they slide into the pipe
- To join two fittings: use a short piece of pipe over both ends
- Common for bathroom fans, kitchen vents, and tumble dryers
RECTANGULAR DUCTING
Our rectangular ducting pipes are slightly smaller, so most fittings (like bends or joins) slide over the pipe. If you need to join two fittings together, just use a short piece of ducting to fit inside them.
- Pipe is male – slightly smaller
- Most fittings are female – they slide over the pipe
- To join two fittings: insert a short piece of ducting inside
- Ideal for tight spaces, above cupboards, or in walls
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
Need Plastic Ducting for Your Project? Explore Our Full Range
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Give us a call - our experts are here to help, Monday to Friday, 8:00am to 5:30pm. 📞 Call us on 020 8463 9696
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