Helpful video reference. We use Charlie DIYte's tutorial "Bathroom Extractor Fans - a DIY Guide" as the video reference here. Charlie is a UK-based DIY channel and covers axial versus inline fans, the IP zone rules, and the duct connection in enough detail to follow on site. Worth watching end-to-end before you buy anything.
1. Check the bathroom zone and choose the right IP rating
BS 7671 Section 701 divides bathrooms into zones based on proximity to water. Zone 0 is inside the bath or shower tray. Zone 1 covers the volume directly above the bath to a height of 2.25 m. Zone 2 extends 0.6 m outward from zone 1 and from floor to 2.25 m.
Most ceiling-mounted extractor fans sit in zone 1 or zone 2. Both zones require at least IP44 - meaning the fan is protected against water splashes from any direction. Most standard extractor fans carry IP44 or higher, but check the data sheet for the specific model you are buying before paying for it.
If your fan is in zone 1, it must also be fed from a circuit with RCD protection. If you are not certain whether the existing circuit has an RCD, check the consumer unit: a modern RCBO on the relevant MCB, or an RCD protecting the whole circuit, counts. If there is no RCD and the fan is in zone 1, the replacement should go hand in hand with adding RCD protection - which makes this a job for an electrician.
2. Work out the extraction rate you need
Approved Document F (ventilation, England and Wales) sets a minimum of 15 litres per second (54 m³/hour) for an intermittent fan in a bathroom. For a combined bathroom and WC room, the minimum rises to 30 litres per second (108 m³/hour).
A rough room-volume calculation helps confirm the fan is adequate. Multiply the room length by width by ceiling height to get the volume in cubic metres. A typical small bathroom of 2 m x 1.5 m x 2.4 m gives 7.2 m³. At 54 m³/hour that is around seven air changes per hour, which meets the minimum.
If the room is poorly ventilated, has no window, or the previous fan left persistent condensation, choose a model with a higher extraction rate or a built-in humidistat that keeps the fan running until humidity drops. These cost more but make a real difference in practice.
Note the duct diameter of your existing installation before buying. Standard UK domestic fans use 100 mm (4 inch) or 150 mm (6 inch) ducting. A like-for-like swap means matching that diameter so you do not need to rework the duct run through the wall or ceiling.
3. Isolate the circuit at the consumer unit
Find the MCB for the lighting circuit that serves the bathroom - or the dedicated extractor circuit if one exists - and switch it off. In older homes with a rewireable fuse box, pull the relevant fuse carrier and put it in your pocket.
Test at the existing fan: remove the cover, hold your voltage tester probes to the live and neutral terminals. Both positions should read dead. If you see any voltage, double check which circuit is off and test again. Do not proceed until the circuit is confirmed dead.
4. Remove the old fan
Unclip the fan cover - most snap off by pressing two tabs on opposite sides. Take a clear photograph of the wiring before disconnecting anything: note which terminal carries the brown live, which the blue neutral, which the green-and-yellow earth, and whether there is a separate switched live feeding the timer.
Loosen each terminal screw, pull the conductors free, and note their lengths. If any conductor is too short to reach the matching terminal on the new fan, you will need to join an extension using a junction box - which is still fine for a like-for-like replacement, just adds a step.
Unscrew the fan body from the wall or ceiling. On a surface-mounted fan this is usually two or four screws. On a recessed fan, the body may be held by clips from inside the duct aperture. Ease the body out without pulling the duct connection free until you are ready.
5. Fit the new fan body and ducting
Offer the new fan up to the existing fixing points. Ideally the new body will line up with the old fixings - most 100 mm fans share standard mounting hole spacings. If the new fan is a different footprint, you will need to make good the wall or ceiling surface around it before fitting.
Connect the duct spigot to the existing wall duct before fixing the fan body permanently. Push the spigot firmly into the duct sleeve. If the duct sleeve has worked loose from the wall, apply a bead of decorator's sealant or duct tape rated for ventilation use around the joint before pushing together. A duct that is not properly connected vents warm, humid air into the void - exactly the opposite of what you want.
Fix the fan body to the wall or ceiling with the screws provided, or reuse the original fixings if the diameter matches.
6. Connect the wiring
Working from your photograph, transfer each conductor to the matching terminal on the new fan: L (live) to L, N (neutral) to N, earth to the earth terminal. Tighten each terminal screw firmly. If the new fan has a switched-live timer terminal (often marked SL or T), connect the brown switched live from the light switch to that terminal - this is what tells the fan to keep running for the programmed overrun period after the switch goes off.
Check the earth path. The fan body must be earthed, not just the internal components. If the fan has a metal body or is in a metal backbox, there should be a flying lead to a dedicated earth terminal on the mounting plate.
For more detail on the wiring arrangement - including light-switch trigger, humidistat and timer configurations - see our bathroom extractor fan wiring guide.
7. Set the over-run timer and test
Most extractor fans have a small adjustment dial for the over-run period, usually accessible without a tool through a small hole in the housing. Set it to between 5 and 15 minutes - 10 minutes is a reasonable default for a bathroom. Check the fan's own instructions, as the dial positions vary between manufacturers.
Refit the cover. Return to the consumer unit and switch the circuit back on. The fan should not run immediately if it is wired to the light switch - only when the switch is on. Switch the light on and confirm the fan starts. Switch the light off and time how long the fan continues before stopping. If it does not stop, or runs immediately with the light off, recheck the switched-live connection.
Check for extraction: hold a tissue near the cover when the fan is running. The tissue should be pulled against the grille. If it is not, the duct connection or the duct run may be blocked - do not assume the fan is at fault until you have checked the duct outlet outside the building.
When to call us
Richard fits extractor fans in Sandwich and east Kent on the standard small-job rate. If the circuit needs an RCD adding, or the duct run needs reworking, that is still a straightforward job - just one that needs the right test equipment and Part P sign-off.
Bathroom work in Sandwich?
Richard carries the common extractor fan sizes and can do a like-for-like swap, add RCD protection, or sort a duct that has never worked properly.
Contact Richard