Ducting Supplies

Frequently asked questions

What size ducting do I need?

Start with the fan's outlet diameter, which sets the minimum. Most domestic installations use 100mm, 125mm or 150mm rigid round duct. Bathroom extractor fans are typically 100mm to 125mm; kitchen extractors and inline mixed flow fans often need 150mm or 200mm. Never reduce below the fan outlet. Going one size up on a long run lowers air velocity, which cuts both pressure loss and noise, so where there is room for a larger duct it is usually worth taking.

How do I size ducting for a given airflow?

Size on airflow, run length, number of bends and the pressure drop you can afford. Higher volumes and longer runs need a wider duct to keep velocity and resistance down. The three most common mistakes are using duct too small for the airflow, fitting more bends than the layout needs, and reducing diameter part way along a run. All three add resistance, which costs airflow, adds noise and pushes the fan off its duty point, and no amount of fan power compensates cleanly for a badly laid out run.

Should I use rigid or flexible ducting?

Rigid plastic duct gives the best airflow and durability and is the right choice for any permanent run. Flexible ducting suits tight spaces, short connections and awkward final links to an outlet. Flexible duct creates considerably more friction than rigid of the same diameter, and that penalty gets worse if it is left slack or compressed, so pull it taut and keep runs short. Many installers use both: a rigid main trunk with flexible only where the geometry demands it.

What is flat channel ducting used for?

Rectangular flat channel, available in 110 x 54mm, 204 x 60mm and 220 x 90mm, routes ventilation along skirting boards, under floors and through shallow voids where round duct will not fit. It is common in retrofit work and whole-house systems where space and appearance matter. Equivalent round adaptors let you switch between flat and round along the same run.

Do I need different fittings for rigid and flexible ducting?

Yes. Rigid duct uses push-fit connectors, bends and tee pieces. Flexible duct needs jubilee clips and adaptors to join onto rigid sections or outlet spigots. We stock Domus adaptors, 45 and 90 degree bends, Y-pieces and jubilee clips across the standard sizes. Match the fitting diameter to the duct exactly, since a near-miss on size is where most air leakage starts.

Can I mix brands of ducting and fittings?

Yes, as long as the diameters match. A 150mm rigid Domus duct will connect to a 150mm Systemair bend or adaptor. Verify the measurement rather than the label, because a nominal size is not always the same external dimension between ranges, and check that ductwork, connectors, flanges and clips are all the correct diameter and material for the system.

Is aluminium ducting better than plastic?

It depends on the duty. Aluminium ducting handles higher temperatures and takes more physical abuse, which suits commercial and kitchen extract. Plastic is lighter, cheaper and easier to seal, which suits domestic and low-temperature work. Temperature is the deciding factor: do not put plastic duct on a duty that will run hot.

When do I need insulated ducting?

Any time duct carrying warm, moist air passes through a cold space, typically a loft or an unheated void. Without insulation the air inside cools, moisture condenses on the duct wall and runs back down to the fan or drips through the ceiling. Insulated ducting is effectively standard practice on bathroom and kitchen extract runs crossing a loft, and on MVHR supply and exhaust duct outside the heated envelope. It also cuts noise transmission along the run.

Do you supply ducting for trade and project work?

Yes. We supply M&E contractors, installers, developers and facilities teams, and hold stock across the standard sizes for next day delivery. Send a drawing, a schedule or just the fan model and run details, and we will list the duct and fittings and price the lot. Call 0161 399 3825 or email hello@efans.co.uk.