Rectangular Plastic Ducting

Frequently asked questions

Which size rectangular ducting do I need?

Match the flat channel size to your fan or cooker hood's round connection size using a round-to-rectangular adaptor. 110mm x 54mm is the standard for 100mm bathroom extractor fan runs where the duct passes through a shallow void or above kitchen cabinets. 204mm x 60mm suits 125mm connections and higher-airflow applications including most domestic cooker hoods up to about 400 m³/hr, which is the Domus Supertube 125mm system. 220mm x 90mm is for 150mm connections and high-output cooker hoods. Always match the duct to the fan connection size and never reduce it.

When should I use rectangular ducting instead of round?

Only where the space genuinely requires it. Flat channel has a higher perimeter-to-area ratio than round duct, which means slightly more airflow resistance per metre. For short runs under 3 metres with one or two bends, the difference is negligible. For longer runs, round duct will always perform better, so use flat channel for the tight section (above kitchen units, through a stud wall) and transition back to round for the rest of the run using a round-to-rectangular adaptor. This gives you the best of both.

Can I mix rectangular and round ducting in the same run?

Yes, and this is common and recommended for longer installations. Use flat channel where the ceiling void or wall cavity is too shallow for round, then switch to round PVC for the remainder of the run. Domus make round-to-rectangular adaptors for all three sizes: 100mm to 110x54mm, 125mm to 204x60mm, and 150mm to 220x90mm. Keep the flat section as short as practical to minimise resistance.

Are Domus fittings interchangeable between sizes?

No. The 110x54mm, 204x60mm and 220x90mm ranges each use their own dedicated bends, connectors and adaptors, and they don't interchange. Make sure all fittings match the same system size before ordering. If you're unsure which system you need, call us on 0161 399 3825 and we'll help you spec the right components.

Do I need to seal the joints?

Domus push-fit connections are reasonably airtight, but for best performance, particularly on extract systems where negative pressure can pull air through gaps, seal each joint with duct mastic or PVC tape. This is especially important for MVHR systems where the SAP calculation assumes sealed ductwork.

What's the maximum duct run length for flat channel?

Because flat channel creates slightly more resistance per metre than round, your effective maximum run length is shorter. As a working guide, treat each metre of 110x54mm flat channel as roughly 1.5m of equivalent 100mm round duct when calculating total system resistance. Building Regulations Part F sets maximum equivalent duct lengths for different fan types, so check your fan's datasheet for its performance at the required static pressure. Not sure? Call us on 0161 399 3825 and we will match your duct configuration to the right fan.