Ducting Fittings

Browse our complete range of PVC, aluminium and insulated ducting. We stock rigid plastic ducting, flexible ducting hose, flat channel systems, round duct, and all the connectors, bends, adaptors and tee pieces you need for a professional installation. Available in 100mm, 125mm, 150mm, and 200mm diameters from leading brands including Domus and Systemair. Whether you're fitting a bathroom extractor fan, kitchen cooker hood, or full MVHR system, we have the ducting to match.

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'll 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 won't 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 won't 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 1–1.5m 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's 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're 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 — 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've 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're specifying fittings for an MVHR installation, call us on 0161 399 3825 and we'll help you choose the right system.

How to size ventilation ducting?

Size ducting according to required airflow (m³/h or l/s), duct length, bends, and acceptable pressure drop. Larger volumes or longer runs need wider ducts to maintain efficiency and reduce noise. Correct sizing ensures balanced airflow and system reliability.

What is a duct in a house for ventilation?

A ventilation duct carries stale air out and fresh air in. In homes, it connects to extractor fans or whole-house systems, improving air quality and moisture control by moving air between rooms and exterior points.

What are common duct sizing mistakes?

Using ducts too small for the airflow, adding too many bends, or reducing diameter mid-run are frequent errors. These increase resistance, noise, and energy use, reducing fan efficiency and overall airflow performance.

What are the three types of ducts?

• Rigid ducting: Best airflow and durability.

• Flexible ducting: Ideal for tight spaces.

• Semi-rigid ducting: Balanced performance and easy installation.
  Choose based on airflow needs and available space.

How to calculate ventilation duct size?

Calculate using required airflow, air velocity, and pressure loss. Larger or high-volume systems need wider ducts to maintain efficiency and comfort while keeping noise low.

Is aluminium ducting better than plastic?

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.