125mm Round PVC Ducting Pipe

125mm round PVC ducting pipe — also called 5 inch ducting, 125mm plastic pipe, 125mm plastic ducting, or Domus EasiPipe System 125 — is rigid round ductwork for 125mm extract fans, mid-size cooker hoods, MVHR branch runs, in-line fan ducting and commercial low-pressure extract. It's the middle size in the Domus round range, sitting between 100mm and 150mm, and is specified wherever a standard 100mm duct would be undersized — typically larger kitchen hoods, in-line extract fans, whole-house MVHR branches and longer domestic runs where lower pressure loss matters.

We stock the full Domus EasiPipe range in 125mm: 2m, 1m and 350mm straight lengths, 90° bends, T-pieces, Y-pieces, straight connectors, connectors with integral damper, spigot adaptors stepping up (125mm to 150mm) or down (100mm to 125mm), round-to-rectangular adaptors (204x60 to 125mm), hose connectors, pipe clips, a telescopic assembly section, plus thermal insulated bend for MVHR cold-roof or external runs. All parts are push-fit with gasketed sockets. In stock with next-day UK delivery. 

All Round PVC Ducting / 100mm Round / 150mm Round / 204x60 Flat Equivalent / 125mm Flexible

What is 125mm round ducting?

125mm round ducting is rigid circular PVC ductwork with a 125mm internal diameter — the UK standard mid-size for domestic and light-commercial extract. It's sometimes called 5 inch ducting (125mm is the metric equivalent of the imperial 5 inch size), and in the Domus system it's branded EasiPipe System 125. The 125mm diameter carries roughly 1.5x the airflow of 100mm at the same velocity with significantly lower pressure loss per metre, so it's specified wherever 100mm would struggle — typically larger cooker hoods, in-line extract fans, MVHR branch runs and extract duties over 150 m³/h.

What is 125mm round ducting used for?

Mid-size domestic cooker hoods (most hoods above 600mm width have a 125mm spigot), in-line extract fans such as the S&P TD-Silent, Domus and Vent-Axia inline range, whole-house MVHR branch runs between the main trunk and room terminals, utility and laundry room extract on higher-duty axial or centrifugal fans, tumble dryer venting where the run exceeds the dryer's 100mm recommendation, and light-commercial extract (small offices, treatment rooms, WCs served by a twin-fan unit). For smaller bathroom and toilet extract stay with 100mm; for MVHR main trunks and whole-house supply go to 150mm.

Is 125mm round the same as 5 inch ducting?

Yes — 125mm is metric, 5 inch is imperial, same nominal duct size. UK ducting is manufactured in metric (125mm internal diameter) but older specifications and some hood manufacturers still use the 5 inch label. Any 5 inch branded extract fan, cooker hood or wall kit will connect to standard 125mm round EasiPipe ducting. The same equivalence applies at 100mm/4 inch and 150mm/6 inch.

What's in the Domus EasiPipe 125mm range?

The full EasiPipe System 125 round range: 2m straight lengths (1200-5), 1m straights (1100-5 and 2100-5), 350mm short straight (135-5) for tight sections, telescopic assembly section (130-5) adjustable from 250–450mm, 90° right-angle bends (590), T-pieces (592) for branched runs, Y-pieces (599M) for split runs, straight connectors (593), straight connectors with integral backdraught damper (594), spigot connectors with damper and wall plate (595), pipe clips and fasteners (596), round hose connectors (580), rigid 100mm-to-125mm adaptor (544), spigot adaptors 100mm-to-125mm (119) and 125mm-to-150mm (118), and round-to-rectangular adaptor 204x60 to 125mm (570). Thermal insulated 90° bend (TS590) available for MVHR cold-roof or external runs.

Should I use 125mm round or 204x60 flat ducting?

Airflow-equivalent, the choice depends on void depth and system compatibility. Use 125mm round where there's adequate depth (125mm+) in the ceiling void, loft or chase — it's cheaper per metre, has the lowest pressure loss, and connects directly to any 125mm fan or cooker hood spigot. Use 204x60 flat where depth is restricted — typically above kitchen wall units, through shallow ceiling voids, or concealed in stud walls. Most domestic kitchens route a 125mm cooker hood in flat 204x60 behind the wall units and transition to round only after reaching the external wall or loft void. Connection between the two is via a Domus 570 round-to-rectangular adaptor.

Should I use 125mm round or 125mm flexible ducting?

Rigid EasiPipe is the right specification for the main run in any serious extract or MVHR system — it has lower pressure loss, airtight push-fit joints, long service life, doesn't sag and doesn't trap grease or dust in corrugations. Flexible 125mm hose has its place for short final connections (between the fan and rigid duct, or to absorb misalignment) and for temporary or access-restricted installs, but keep flexible sections under 1m. For MVHR, Building Regs Part F-compliant extract and cooker hoods specifying BS EN 13141, rigid EasiPipe is effectively required — flexible hose won't meet the pressure-loss calculation for longer runs.

How do I join 125mm EasiPipe sections?

EasiPipe uses push-fit joints with integral neoprene gaskets. Slide the plain end of one duct into the socketed end of the next, or use a straight connector (Domus 593) to join two plain ends. For airtight joints — essential on MVHR and Building Regs extract — apply duct sealant inside the socket before pushing home, then foil-tape over the joint externally. Fix horizontal runs every 600–800mm with pipe clips (Domus 596) to prevent sag and noise transmission. Bends, T-pieces and Y-pieces have sockets at each leg so they push between standard duct sections.

Do I need to insulate 125mm ducting?

Yes, for any run through a cold space — unheated loft, cold roof void, external, or through an uninsulated garage or outbuilding. Warm moist extract air hitting a cold duct wall causes condensation inside the duct, which pools, drips back to the fan and in severe cases drains into the ceiling below. MVHR supply runs through cold spaces must also be insulated to prevent condensation on the outside of the duct. For simple condensation prevention, wrap the duct in site-applied duct insulation. For MVHR where thermal performance matters, use the Domus thermal EasiPipe range (TS590 bend in 125mm) which has a moulded insulation jacket integrated with the fitting.

What's the maximum run length for 125mm ducting?

125mm handles significantly longer runs than 100mm because pressure loss per metre is roughly half. Typical allowances: axial in-line fans 10–20m total equivalent length, centrifugal in-line fans 20–30m, MVHR branch runs per manufacturer's commissioning spreadsheet. Each 90° bend adds approximately 1.2m equivalent length, each 45° bend approximately 0.6m. Keep runs as short and straight as practical. For runs approaching the fan's limit, specify a higher-duty fan, reduce the number of bends, or step up to 150mm round.

Can I use 125mm round ducting outside?

Short external terminations are fine — external wall kits and cowls are designed to connect to 125mm round spigots. For longer external runs, switch to aluminium or galvanised spiral: prolonged UV exposure will eventually degrade PVC. Terminate with a proper cowl or external grille with an integral backdraught shutter, never leave a duct end open. Any external or cold-loft run should be insulated — either with site-applied duct wrap or by specifying the Domus thermal EasiPipe range for branch runs that matter thermally.