Round Plastic Ducting

Frequently asked questions

What size round ducting do I need?

Match the duct diameter to the spigot size on your fan or ventilation unit, and never reduce it. 100mm (4 inch) is the standard for most bathroom extractor fans and utility room extraction, handling airflow rates up to about 80 to 100 m³/hr through runs of 4 to 6 metres. 125mm (5 inch) suits higher-flow bathroom fans like the Vent-Axia Svara and some kitchen applications, handling up to about 150 m³/hr. 150mm (6 inch) is standard for kitchen cooker hoods and higher-flow residential extract; if your cooker hood shifts 400+ m³/hr, 150mm is the minimum. Check your fan's datasheet for the connection size and always match the duct to it.

Why use rigid round PVC instead of flexible ducting?

Performance. The smooth internal bore of rigid PVC creates far less friction than corrugated flexible duct. A 3-metre run of flexible duct creates roughly the same resistance as 9 to 10 metres of rigid, effectively tripling the duct length as far as your fan is concerned. Rigid PVC is also airtight when properly jointed, doesn't sag between supports, and won't collect condensation in low points the way flex does. Use rigid round for the main duct run and flexible only for the last 300 to 500mm connection to the fan or terminal.

How do I join round plastic ducting together?

Domus EasiPipe uses a push-fit spigot system, where the male end of each fitting slides inside the duct or into the next connector. No adhesive is needed for a secure mechanical joint. For best airtightness, seal every joint with PVC duct tape or mastic sealant. This is particularly important on extract systems, where negative pressure can draw air through gaps, and essential for MVHR installations where the SAP calculation assumes sealed ductwork.

How many bends can I have in a duct run?

Each bend adds resistance. A 90° bend in 100mm duct adds roughly 1 to 1.5 metres of equivalent straight duct length; a 45° bend adds about 0.5 metres. As a guide, most domestic axial fans can handle a total equivalent duct length of 4 to 6 metres, so a 2-metre run with two 90° bends and an external grille might already be at the limit. Always use two 45° bends instead of one 90° where space allows, for less resistance and a gentler turn for the airflow. If your run is long or has multiple bends, consider upgrading to an inline mixed-flow fan with higher static pressure, or stepping up a duct size.

Can I use round PVC ducting in a loft?

Yes, but you must insulate it. When warm moist extract air passes through cold ductwork in an unheated loft, the temperature drops below the dew point and condensation forms inside the duct. This pools at low points, grows mould, and can drip through ceilings. Either use pre-insulated ducting for the loft section or wrap standard rigid PVC with at least 25mm of duct insulation and a vapour barrier. This applies to both extract ductwork and MVHR supply ducts running through unheated spaces, and Building Regulations require insulated ducting in these locations.

What's the maximum duct run length for round PVC?

It depends on the fan, duct diameter, and the number of bends. Building Regulations Part F gives maximum equivalent duct lengths for different fan types, typically 4 to 6 metres for a 100mm axial fan on a straight run. Each 90° bend reduces that by about 1 to 1.5 metres, and the external grille adds another 2 to 4 metres equivalent depending on the type. For longer runs you have three options: reduce the number of bends, step up to a larger duct diameter, or use an inline mixed-flow fan that generates higher static pressure. If you are not sure whether your fan can handle your duct configuration, call us on 0161 399 3825 and we will check it for you.