Flat Plastic

Frequently asked questions

What size flat ducting do I need?

Match the flat duct to the round spigot it connects to via an adaptor. System 100 (110x54mm) has a free area equivalent to 100mm round duct and suits bathroom and utility extraction up to about 100 m³/hr. Supertube (204x60mm) equates to 125mm round and handles higher flows. Megaduct 220 (220x90mm) equates to 150mm round and is used for kitchen cooker hoods and higher extract rates. Never step down to a smaller free area than your fan spigot, as it chokes airflow and raises noise.

How does flat ducting compare to round for airflow?

For the same free area, round duct has slightly lower resistance because it has no corners, but good-quality rigid flat duct like Domus is smooth-bore and performs well. The real advantage of flat duct is fitting into shallow spaces where round simply will not go. A common approach is round duct for the main run and a flat section only where depth is restricted, joined with a round-to-rectangular adaptor.

How do I join flat rectangular ducting?

Flat duct joins with push-fit straight connectors that bridge two lengths, and changes direction with matching flat bends. The joints are mechanical and need no adhesive, but for airtightness on extract and MVHR systems you should tape or seal each joint. Use a straight connector between lengths rather than pushing bare duct ends together, and a connector with a built-in damper where you need backdraught protection.

Can I connect flat ducting to a round fan or grille?

Yes, using a round-to-rectangular adaptor. We stock adaptors that convert each flat system to its matching round size, for example 110x54 to 100mm, 204x60 to 125mm, and 220x90 to 150mm. Fit the adaptor at the fan spigot or the wall grille so the rest of the run can be flat. Always pair a system with its correct adaptor so the free area is maintained through the transition.

Which system is used for cooker hood extraction?

Most kitchen cooker hoods move 300 to 600 m³/hr and connect to a 150mm spigot, so Megaduct 220 (220x90mm) is the usual flat choice, giving the free area of 150mm round in a low-profile channel that fits above wall units. Supertube (204x60mm) can suit lower-output or compact hoods. Check your hood's extraction rate and spigot size and match the flat system to it.

Can flat ducting run through a cold loft or void?

It can, but it must be insulated to prevent condensation. Warm, moist extract air passing through cold duct in an unheated loft will condense inside and can drip through ceilings. Wrap the flat duct with at least 25mm of duct insulation and a vapour barrier, or keep cold-space runs as short as possible. Building Regulations require insulated ductwork in unheated spaces for both extract and MVHR supply.