Ventilation Ducting: Round Rigid, Flexible, and Rectangular Systems for Kitchen and Bathroom Extract

Why Duct Selection Makes or Breaks an Extract System

An extractor fan rated at 100 m³/h will struggle to move 60 m³/h through an undersized, kinked, or overlong duct run. Inadequate ducting is the most common cause of extract ventilation failures — not the fan. Getting duct diameter, material, length, and fittings right is what separates a compliant, effective system from a call-back waiting to happen.

This guide covers the practical selection and installation of ventilation ducting for kitchen cooker hoods, bathroom/en-suite extraction, and utility room venting under Building Regulations Part F and Document F 2021 edition.

Duct Types at a Glance

Four duct formats dominate domestic and light commercial installations:

• Round rigid (PVC or aluminium): 100mm, 125mm, 150mm diameter. Lowest friction loss. Best for straight or near-straight runs. The installer's first choice where space allows.
• Rectangular rigid (PVC): Typically 110×54mm or 204×60mm. Fits inside standard kitchen units, slim cavities, and behind plinths. Higher friction than round equivalent; factor a length de-rating.
• Flexible aluminium duct: 100mm and 125mm. Useful for final connection between rigid run and fan spigot. Should be kept as short as possible — ideally under 1.5 m — and fully extended (never compressed).
• Semi-rigid flexible: Foil-lined plastic, more shape-stable than pure flexi. Acceptable for short drops or offsets but not as a substitute for rigid runs.

Diameter and Flow Rate: Getting the Sizing Right

The basic rule: match duct diameter to the fan's spigot size and verify the system can achieve the minimum extract rate required under Part F.

Minimum extract rates (Part F 2021, Table 1.1a)

Common duct diameter vs. recommended max flow

Rule of thumb for rectangular duct: Convert to equivalent diameter before comparing friction. A 204×60mm duct has an equivalent diameter of approximately 113mm — fractionally better than 100mm round, significantly less than 125mm round. When switching from rectangular to round for part of a run, step up to 125mm to recover capacity.

Equivalent Lengths and Fitting Resistance

Fittings add resistance equivalent to a length of straight duct. For planning purposes:

• 90° bend (long radius): +0.5–1.0 m equivalent length
• 90° bend (short radius / tight elbow): +1.5–2.5 m equivalent length
• 45° offset bend: +0.3–0.5 m equivalent length
• Duct reducer (1 size): +0.5 m equivalent length
• T-branch: +1.5–3.0 m equivalent length (flow direction dependent)
• Wall grille / external termination cap (louvred): +4–6 m equivalent length (significant — always use low-resistance termination for longer runs)

Manufacturers publish maximum system resistance — typically 60–100 Pa — and a fan curve showing flow vs. pressure. Estimate your total equivalent length, calculate resistance, and verify the fan can still meet Part F minimums at that duty point. Most fan product sheets include a simplified guide; for longer or complex runs, use a duct design calculator.

Material Selection

PVC rigid duct

Standard for most domestic installs. Easy to cut, glue (solvent weld), and clip. White PVC suits surface-run applications. Doesn't corrode. Not suitable for very high-temperature applications (cooker hood extract with grease fire risk — use stainless or aluminium in those cases).

Galvanised steel

Commercial kitchens and high-grease environments. Heavier, requires tin snips and angle grinder for site cutting. Not typical for domestic except where Building Control specifies it for commercial-type kitchens in domestic properties.

Aluminium rigid

Lightweight, corrosion-resistant. Often used externally or in roof voids. Connects with self-tapping screws and aluminium foil tape (not PVC tape — foil tape only for airtightness).

Insulated duct

Where a duct run passes through an unheated void (loft, garage, external wall chase), insulate to prevent condensation forming on the duct surface and eventually dripping back into the fan. Pre-insulated flexible duct is available; alternatively, wrap rigid duct with 25mm pipe lagging or mineral wool and foil-tape all joints.

Condensation: The Real Enemy

Condensation inside duct runs is the leading cause of premature fan failure, water staining, and mould. Prevention measures:

1. Grade to drain: Ensure any horizontal duct run has a slight fall (1:100 minimum) towards the external termination so condensate drains out rather than back to the fan.
2. Insulate cold sections: Any duct running through an unheated space must be insulated.
3. Minimise duct length: Shorter runs mean less time for warm, moist air to cool before leaving the building.
4. Use a condensation trap or condensate drain: For longer runs or where grading is impossible, a condensate drain point lets accumulated water escape without reaching the fan housing.
5. Avoid looping flexible duct: A dip in flexible duct creates a condensate trap. Keep flexi fully extended and supported so it runs straight or on a positive gradient.

Rectangular Duct Runs: In-Unit and Through-Wall

Rectangular ducting was designed for fitting inside standard 150mm–175mm kitchen wall units. Typical route for an island or ceiling-mounted hood: fan spigot → flexi connection adapter → rectangular duct run through overhead unit to wall → rectangular-to-round adapter → 125mm rigid round through external wall → external termination cap.

The adapter from rectangular to round is a friction point. Use a quality swept adapter (not a sharp-edged reducer) and count it as 1.0 m equivalent length in your calculation. Avoid proprietary adapters that constrain the flow area below the target duct's cross section — they'll cause turbulence and noise at higher fan speeds.

External Wall Terminations

All duct runs must terminate externally. Options:

• Wall outlet cowl (louvred): Most common. Fit with the louvres pointing downward. Inspect annually — louvres can jam with grease or insect debris.
• Eaves termination: Routes through soffit into void and out via a louvred tile or dedicated eaves vent. Keeps the external wall tidy. Risk: if the duct run in the loft isn't insulated, condensation is inevitable.
• Roof tile vent: For bathroom fans in rooms below a pitched roof. Use a matching profile tile vent with a 100mm/125mm spigot. Airtight seal to tile critical.
• Anti-backdraft shutter (gravity or spring): Prevents cold air, insects, and wind-driven rain ingress when the fan is off. Better than louvres alone for higher-exposure locations.

Minimum separation from adjacent openings (Part F): external termination should be at least 500mm from any openable window, and should not discharge into a cavity, enclosed porch, or under a balcony where recirculation can occur.

Airtightness at Joints

For new builds and refurbishments targeting Part L airtightness standards (permeability ≤5 m³/h/m² at 50 Pa), every duct joint must be sealed:

• PVC duct joints: solvent weld cement or push-fit with EPDM gasket seals
• Aluminium rigid duct: aluminium foil tape at every joint (not duct tape / PVC tape — these fail over time)
• Flexi-to-rigid connections: stainless steel worm-drive clip over the spigot + foil tape over the clip

Building inspectors testing airtightness will identify leaking duct joints as a major failure point. Seal as you go — it's much harder to remedy after plasterboard or ceiling tiles are in place.

In-Line Fans and Duct Fans

Where a bathroom has no external wall, an in-line duct fan mounted in the loft or ceiling void draws air through a longer duct run (up to ~6–8 m) than a standard axial wall/ceiling fan can manage. Key selection criteria:

• Choose a fan rated for the total system resistance of your duct run (typically 100–150 Pa for longer runs)
• In-line fans are usually rated IP44 or IP45 — sufficient for the fan body, which sits outside the wet zone, though the inlet grille/duct connection may be inside the bathroom (check IP zone requirements per BS 7671 and our IP ratings guide)
• Wire with a standard 3-core flex to the lighting circuit (switched live + permanent live for timed run-on) or via a dedicated fused spur
• Support the fan body independently — don't let duct runs bear the fan's weight

MVHR Integration

Where a property has a Mechanical Ventilation with Heat Recovery (MVHR) system, individual room extract fans are usually eliminated. The MVHR unit handles balanced ventilation via a central duct network. Rigid round duct (typically 125mm or 160mm main runs, reducing to 100mm at terminal valves) is the preferred material; rectangular spiro duct is used in commercial settings. For MVHR, all duct joints must be airtight to maintain heat recovery efficiency — a leaky MVHR duct network wastes much of the energy saving it was installed to achieve.

Common Mistakes and Call-Backs

• Flexi duct left kinked or compressed: A 100mm flex duct kinked 90° can reduce effective diameter to 60mm. Always run fully extended, supported every 500–600mm.
• No insulation in the loft: Guaranteed condensation and drip-back within months in winter.
• Using 100mm duct with a cooker hood: Most hoods extract at 150–350 m³/h — a 100mm duct chokes them to 60–70% of rated flow and causes noise. Minimum 125mm for any kitchen hood; 150mm for high-output models.
• Joining round to rectangular with tape and hope: Use a proper swept adapter with gasket seals. Bodged transitions create both leakage and turbulence (noise).
• Running duct below the fan: If the duct exits the fan body downward before going up and out, you've created a condensate trap at the fan itself. Route horizontally or upward from the fan where possible.
• PVC duct in fire zones: Standard grey/white PVC duct is not fire-rated. Where ducts pass through fire-compartment walls or floors, use fire-rated duct, intumescent collars, or fire dampers per the specification. Check your Building Regulations Part B compliance.

Quick Reference: Standard Duct Connections

Relevant APM Products

APM Electricals stocks a full range of ventilation ducting components for domestic and light commercial installations:

Knightsbridge 100mm Aluminium Ducting Kit (£16.82)

125mm Flexible PVC Hose Ducting 3m (£10.99)

150mm Flexible PVC Hose 3m (£13.99)

Aluminium Foil Duct Tape 50mm (45.7m) (£7.99)

6" External Gravity Flap Shutter (£6.99)

APM Electricals
24 Western Avenue, Acton, London W3 7TZ
Phone: 020 8702 8080
Web: www.apmi.uk
Same-day collection available for West London trades.