Ducting Fittings

Frequently asked questions

What ducting fittings do I need for a standard installation?

At minimum: a straight connector to join duct lengths, at least one bend (90° or 45°) to change direction, and an adaptor to connect the duct to your fan or external grille. For a kitchen cooker hood running through a ceiling void, a typical list would include a round-to-rectangular adaptor (if using flat channel above the cabinets), one or two flat bends, a straight connector, a rectangular-to-round adaptor to transition back, and a wall plate or external grille. Buy one or two spare straight connectors. You will almost certainly cut a duct length and need to rejoin it.

Are round and rectangular fittings interchangeable?

No, round fittings only fit round duct, and rectangular fittings only fit the matching rectangular size. A 100mm round bend will not connect to 110x54mm flat channel. To switch between the two systems in the same run, you need a round-to-rectangular adaptor. Domus make these for 100mm to 110x54mm, and 125mm to 204x60mm. Each system's bends, connectors, and tee pieces are dedicated to that size and will not interchange with other sizes either.

Should I use 45-degree or 90-degree bends?

Use 45° bends wherever you can. Two 45° bends create a gentler direction change than a single 90° and produce less airflow resistance, roughly 0.5m equivalent duct length per 45° bend versus 1m to 1.5m for a 90° bend. For a duct dropping from a ceiling void to a wall grille, two 45° bends are almost always better than one sharp 90°. Use 90° bends only where the space forces a tight right-angle turn. If your duct run has three or more 90° bends, consider whether a different route with fewer direction changes would give better performance.

How do Domus push-fit fittings work?

Domus fittings use a spigot system: the male end of the fitting slides inside the duct or into the female end of the next fitting. No adhesive or solvent welding is needed for a secure mechanical joint. The fit is snug enough to hold during installation, but for best airtightness you should seal every joint with PVC duct tape or mastic sealant. This is especially important on extract systems (where negative pressure draws air through any gap) and essential on MVHR installations where the SAP calculation assumes fully sealed ductwork.

What is a round-to-rectangular adaptor and when do I need one?

A round-to-rectangular adaptor transitions between round duct and flat channel in the same run. You need one whenever you are switching from round to rectangular or vice versa, typically where a round fan spigot connects to flat channel ducting above kitchen cabinets, or where flat channel running through a shallow void transitions back to round for the remainder of the run. Domus make adaptors for 100mm round to 110x54mm flat channel, and 125mm round to 204x60mm flat channel. Always use the correct adaptor, and never force-fit or tape mismatched systems together.

Do I need different fittings for MVHR systems?

The fittings themselves are the same Domus push-fit range, but the installation standard is higher. MVHR systems run at higher static pressures than simple extract fans, so every joint must be sealed with mastic, not just taped. Leaky ductwork on an MVHR system wastes the heat recovery you have paid for and will underperform against the SAP calculation. For MVHR, many installers also prefer semi-rigid ducting systems (like Domus Radial) over standard rigid PVC fittings for the branch runs, because the continuous semi-rigid tube has fewer joints to seal. If you are specifying fittings for an MVHR installation, call us on 0161 399 3825 and we will help you choose the right system.

How do I size ventilation ducting?

Size the duct from the airflow the fan has to move, the length of the run and the number of bends. Never run smaller than the fan spigot, keep bends to a minimum and avoid reducing the diameter part way along the run, because each of these adds resistance, noise and lost airflow. For a fan, check the duct length and bend allowance in its installation instructions, or send us the layout and we will check it.

Is aluminium ducting better than plastic?

It depends on the application. Aluminium ducting offers high-temperature resistance and strength for commercial or kitchen systems. Plastic ducting is lightweight and cost-effective for domestic or low-heat use.

New to ducting? Read our guide: What is Ducting? Types, Sizes & Materials Explained.