Kitchen & Bathroom Exhaust: Sizing and Placement Guide

Requirements:

• Use a dedicated cooker hood or inline extractor for cooking; hood capture is best for smells and grease.
• Typical ventilation rates: rangehoods 300–700 m³/h (≈83–194 L/s) depending on hob power; if providing whole-room extract, aim for ~10–30 L/s (36–108 m³/h) as a minimum for general kitchen ventilation.
• Grease-capable ducting (smooth, accessible) and an external termination with a backdraught shutter are essential.
• Keep the extractor route as short and straight as possible; use larger diameter ducts if the run or bends are unavoidable to reduce noise and pressure drop.

Requirements:

• Typical extraction rates: 6–15 L/s (22–54 m³/h) depending on room size and fixtures (use higher end for en-suites or combined wc/shower rooms).
• Place the fan or grille close to the moisture source (ceiling near shower or on the wall above the shower head).
• Provide timed or humidity-controlled operation so the fan runs 15–30 minutes after use to remove residual moisture.
• Terminate to outside (not into loft) with a non-return/backdraught valve; ensure the run is as short and straight as possible.

Requirements:

• Aim for low air velocity to reduce noise: target roughly 3–5 m/s in domestic ducts if possible.
• Common duct diameter guidance (rule-of-thumb): 100 mm for up to ~45 L/s (≈160 m³/h), 125 mm for up to ~90 L/s (≈320 m³/h), 150 mm for higher flows up to ~180 L/s (≈650 m³/h).
• Use rigid, smooth ducting where possible; minimise bends and long horizontal runs. Each bend or flexible section increases required fan power and noise.
• Slope horizontal runs slightly toward the outside termination to avoid condensate pooling.

Requirements:

• Put extraction as close to the source as you can: over the hob for kitchens; ceiling or high wall near shower for bathrooms.
• Route ducts to exit directly outdoors — avoid routing through unheated or habitable spaces; if passing through fire-separating elements, provide correct fire-stopping.
• Locate inline fans where they are accessible for maintenance; avoid placing fans inside insulated voids without access.
• Provide transfer paths (airbricks or undercut doors) so the house has a supply path into rooms with extract fans, preventing negative pressure.

Requirements:

• Choose fans with declared dB(A) at your design flow and allow a margin: quieter results come from lower velocities and larger ducts, not just a 'quiet' fan spec.
• Use humidity sensors or timers in bathrooms to ensure adequate post-use run-on and save energy.
• Use anti-vibration mounts and flexible connectors to reduce structure-borne noise.
• Consider an inline fan for noise-sensitive rooms — place the fan away from the living space and use insulated duct runs where needed.

Requirements:

• Check interactions with gas appliances: ensure extract does not cause unsafe depressurisation of combustion appliances — consult gas-safety regulations and a competent installer.
• Ensure electrical installations meet local wiring regulations (IP ratings in wet areas, switched isolation as required).
• Provide backdraught shutters and condensate management on long runs; ensure external terminations are bird/insect-proof.
• Plan for inspection and sign-off as required by local building control before concealment of ducts.