DIY GUIDE · ELECTRICAL · CHECKED OCTOBER 2026
How to replace an extractor fan yourself
One to two hours. £21–£45 for a 100 mm fan with a timer or humidistat.
Same size, same hole, same wires. Clean the duct first — and nothing new in the bathroom's wiring.
- Every step, start to finish
- Checked October 2026 — sources at the end
- Honest about when to call a pro
FIRST, THE HONEST ANSWER
Do it yourself, or get a fitter?
Do it yourself if…
- There is already a fan, the new one is the same diameter and fits the same hole
- The new fan works the same way as the old — switched with the light, with a timer, or with a humidistat — on the same wiring
- You can isolate the fan's circuit at the consumer unit and prove it dead with a two-pole tester
- You can reach the fan safely from a step-ladder
Get a pro if…
- There is no fan, or the new one needs a new hole through a wall or ceiling
- The new fan needs a third wire, a new isolator or a new fused spur that is not there now
- The fan is inside the shower enclosure and you are not sure of its rating or the wiring
- The room has an open-flued boiler or fire, which a fan can draw fumes from
Safety first
- Isolate the circuit at the consumer unit, tape the breaker off with a note, and prove the fan's terminals dead with a two-pole tester before touching any wiring. The fan maker's instructions warn that the fan and its controls must be isolated during installation and maintenance; HSE's guidance is to isolate securely and check dead.
- A fan wired to the lighting circuit can be live even with the light switch off, because a timer fan has a permanent live. The wall switch is not isolation.
- In a room with an open-flued gas boiler or fire, an extractor fan can pull fumes back down the flue. The fan maker requires enough replacement air for both the fan and the appliance; have a Gas Safe registered engineer check before fitting a more powerful fan.
- Work from a stable step-ladder, not the bath edge or the toilet seat. HSE's ladder guidance applies: three points of contact and no overreaching.
- Wear safety glasses and a dust mask when cleaning an old duct; years of fluff and mould come out at once.
Stop here: Stop here: new wiring in a bathroom, new holes and open-flued appliances
Replacing a fan like for like on its existing wiring is maintenance. In England, a new circuit, or new or altered wiring close to a bath or shower, is notifiable under Part P of the Building Regulations unless a registered competent person does and certifies them, and a new fan must also meet the ventilation requirements in Approved Document F. Wales applies its own Approved Document P, and there work in kitchens and bathrooms is never exempt. Scotland works to building standard 4.5 and the wiring regulations, with a building warrant needed for some work. Northern Ireland has no Part P-style notification for home wiring, but new work still needs a competent electrician. A fan in a room with an open-flued gas appliance needs the appliance checked by a Gas Safe registered engineer, under the 1998 Gas Safety (Installation and Use) Regulations in Great Britain and the 2004 Regulations in Northern Ireland. How to hire an extractor fan installer →
| MATERIAL | COVERAGE | FOR THIS JOB |
|---|---|---|
| Extractor fan | Same diameter, same switching type, zone-rated for its position | One |
| Ducting | Same diameter, only if the old run is damaged | The length of the damaged section |
| Wall plugs and screws | If the old holes do not line up | Three or four |
| Sanitary silicone | To seal the fan's flange to the wall or ceiling | A small tube |
Is the fan big enough for the room
Room air changes per hour = fan airflow in m³/h ÷ room volume in m³. A bathroom 2.5 × 2 × 2.4 m is 12 m³; a fan moving 85 m³/h changes the air about seven times an hour, before duct losses.
Every metre of duct and every bend reduces the real airflow. If the old fan sat on a long duct and the room still steamed up, choose a mixed-flow fan in the same size.
Most bathroom fans are 100 mm, which suits the common duct. Measure the old fan's spigot and the hole. An axial fan is cheapest and suits short, straight runs through a wall. A mixed-flow or centrifugal fan pushes harder and suits longer ducts through a ceiling to a soffit or roof. If the old fan struggled on a long duct, replace it with one designed for longer runs, in the same size.
A standard fan runs only while its switch is on. A timer fan carries on after the light goes off; the one whose instructions we checked has an adjustable overrun of 5 to 30 minutes, and every timer version needs a third, switched live conductor. A humidistat fan switches itself on when the air is damp. Match what your wiring already supports: count the conductors at the old fan.
Bathrooms are divided into zones around the bath and shower. A fan inside the shower enclosure or above the bath needs a rating for that zone. The fan we checked is marked suitable for zones 1 and 2 and rated IPX5. If the old fan was a low-voltage model with a transformer outside the room, replace it with the same type.
Many plastic fans are class II double insulated and have no earth terminal; the maker's instructions for one we checked say it must not be earthed. The earth conductor in the cable is then terminated safely in a connector, not left loose. Read the new fan's wiring diagram before buying.
The fan we checked needs a local isolator that disconnects all poles with a contact gap of at least 3 mm, and a supply fused at no more than 3 amps. If your old fan has that isolator and fused spur, the new fan uses them. If it has neither, adding them is new wiring.
Fans list a noise level and a power draw. Low-watt quiet models cost more but are used more, because people do not switch off a fan that does not annoy them. A fan that is switched off does nothing for the mould.
THE PART THAT DECIDES THE FINISH
Before you start — the fan and the duct you have
Isolate the circuit and prove it dead, then take the old fan's cover off. What you find decides the swap.
Fan through an outside wall
A short, straight sleeve to a grille outside.
- Look through the sleeve to the outside grille; it should be clear and its louvres pointing down
- Clean dust and fluff out of the sleeve with a vacuum and a brush
- Check the sleeve slopes slightly down towards the outside, as the maker requires, so condensate runs away
- Measure the hole to confirm the new fan's spigot fits it
Fan in the ceiling with a duct
A flexible or rigid duct running through the loft to a soffit, roof or wall vent.
- Check the duct in the loft: no kinks, no sags full of water, no crushed sections
- A long run should be insulated in a cold loft and slope away from the fan, with a condensation trap if the maker requires one
- Check the duct is still connected at both ends
- Replace a torn or crushed duct with the same diameter
Two conductors at the fan
Line and neutral only: a standard fan switched with the light or its own switch.
- The new fan must be a standard, non-timer model, or a timer model wired as a basic on-off fan as its instructions allow
- A timer fan proper needs a third conductor that is not there
- Note which conductor goes to L and N
- Check for an earth conductor and how the new fan deals with it
Three conductors at the fan
Permanent line, switched line and neutral: a timer or humidistat fan.
- Label each conductor: L for the permanent line, LS for the switched line, N for neutral
- Check the new fan has the same three terminals
- Check the isolator and fused spur outside the room are working
- Note the timer setting on the old fan if you liked it
THE JOB
How to replace the fan
Set up: isolate, prove dead, open up
15 minSwitch off the fan's circuit at the consumer unit, or the main switch if you are not sure which breaker feeds it, and tape it off with a note. Switch off the fan's isolator too, if there is one. Set the step-ladder up under the fan. Unclip the old fan's grille, test the terminals with a two-pole tester checked on a live supply before and after, and then read the cards above for what you find.
- Test between every pair of terminals: line, switched line, neutral
- Photograph the terminals before disconnecting
- Lay a dust sheet; old fans drop fluff
Mistake to avoid: Trusting the light switch. A timer fan has a permanent live, and it is live with the light off.
Clean the duct and grille
20 minThis is the prep from your card above. Vacuum the sleeve or the duct as far as you can reach, and brush the outside grille clear. In the loft, follow the duct and straighten kinks and sags. Many fans that seem dead are pushing against a blocked grille or a crushed duct. Before you replace the fan, clean the old one's impeller and run it: if it works well now, you are done.
- Outside grilles collect spider webs and leaves that stop the shutters opening
- A duct that sags in the loft fills with condensate and stops the airflow
- Wear a mask; old fluff is full of mould spores
Mistake to avoid: Replacing a fan whose duct is the real fault. The new fan is just as weak through a blocked or crushed duct.
Disconnect the old fan
10 minWith the circuit proved dead, label each conductor and undo it from the old fan's terminals. Undo the screws holding the fan to the wall or ceiling and ease it off its spigot. Check the condition of the cable sheath and the conductors where they enter the fan.
- Label L, LS and N with tape before anything comes off
- Look for heat discoloration on the old terminals
- Check the cable enters through a grommet and is not chafed
Mistake to avoid: Disconnecting all the conductors and then guessing which is the switched line. Get it wrong and the timer never starts, or the fan never stops.
Prepare the conductors
10 minTrim any damaged or blackened ends and strip fresh insulation to the length the new fan's terminals need. The fan maker we checked gives a cable size of 0.75 to 1.5 mm². If the new fan is double insulated and must not be earthed, terminate the earth conductor in a suitable connector rather than leaving it loose.
- Strip with wire strippers, not a knife
- Feed the cable through the new fan's grommet before connecting
- Keep bare copper inside the terminal
Mistake to avoid: Cutting the earth conductor off because the fan has no earth terminal. It must stay, terminated safely, for the rest of the circuit.
Fix and connect the new fan
20 minPush the new fan's spigot into the duct or sleeve and mark its fixing holes. Fix it with plugs and screws. Connect the conductors to the terminals exactly as the new fan's wiring diagram shows: permanent line to L, switched line to LS on a timer or humidistat fan, neutral to N. Tighten each terminal and tug each conductor.
- Use the maker's wiring diagram for your model, not the old fan's
- A timer fan wired with only two conductors works as a plain fan, without the overrun
- Make sure the shutters and impeller turn freely after fixing
Mistake to avoid: Swapping L and LS on a timer fan. The fan then runs all the time and the switch does nothing.
Seal and refit the grille
10 minRun a thin bead of sanitary silicone round the fan's flange where it meets the wall or ceiling, leaving the bottom edge open on a wall fan so any condensate can drain. Clip the grille on, checking every clip engages as the maker requires.
- Silicone keeps steam from getting behind the fan and into the wall
- Keep sealant off the shutters and the grille clips
- Check the grille sits flat and square
Mistake to avoid: Sealing the whole flange of a wall fan. Condensate then has nowhere to go and runs back into the fan.
Power on, set and test
15 minRemove the tape, switch the circuit and the isolator back on. Switch the fan on and check it runs and the shutters open. Switch it off and time the overrun on a timer fan; adjust it with a small screwdriver as the maker's instructions show, clockwise to lengthen on the model we checked. Hold a sheet of toilet paper to the grille: it should be held against it.
- Set the overrun long enough to clear the steam after a shower
- On a humidistat, set the sensitivity so it starts in a steamy room but not in normal air
- Check the outside grille's flaps open when the fan runs
Mistake to avoid: Setting the overrun to its minimum to save noise. The fan then stops before the room is dry, and the mould comes back.
Check after a week
5 minAfter a week of showers, look at the ceiling and the window. A room that clears within minutes of the shower means the fan and duct are working. Check the fan is not noisy and the grille is still clean.
- Clean the grille every few months with the power isolated
- A fan that gets louder over time is collecting dust on the impeller
- Mould returning means the overrun is too short or the duct is restricted
Mistake to avoid: Never cleaning the grille. Fluff builds on the inlet and the fan's flow falls month by month.
The mistakes that show
- Treating the light switch as isolation on a timer fan
- Replacing a fan when the duct or grille was blocked
- Swapping the permanent and switched lines
- Fitting a fan without the zone rating its position needs
- Fitting a more powerful fan in a room with an open-flued appliance without a check
Stop here and get an electrician
- There is no fan now, or the new one needs a new hole through a wall or ceiling
- The new fan needs a third conductor, an isolator or a fused spur that is not there
- The fan is in the shower enclosure and you cannot confirm its zone rating and wiring
- The room has an open-flued boiler or fire
| WHAT YOU SEE | WHY, AND THE FIX |
|---|---|
| Extractor fan runs all the time | The permanent and switched lines are swapped, or the humidistat is set too sensitive. Isolate, prove dead and check the wiring against the diagram; turn the humidity setting up. |
| Fan does not run on after the light goes off | The timer has no permanent live — only two conductors are connected — or the overrun is set to its minimum. Check the wiring and the adjuster. |
| Fan runs but the room still steams up | The duct or grille is restricted, the duct is too long for an axial fan, or the overrun is too short. Clean the duct, lengthen the overrun, or fit a mixed-flow fan. |
| Fan is noisy | Dust on the impeller, a loose fixing, or a duct vibrating in the loft. Isolate and clean the impeller, tighten the screws, and clip the duct. |
| Water dripping from the fan | Condensate in a duct that slopes back to the fan. Re-run the duct with a fall away from the fan, insulate it, and fit a condensation trap if needed. |
| Breaker trips when the fan starts | A conductor is touching the wrong terminal, or the fan is faulty. Isolate, prove dead, and check the wiring; if correct, return the fan. |
| Shutters do not open | Something is catching them, or the outside grille is blocked. Clean the grille and check the fan sits square in the hole. |
| Draught through the fan when it is off | No backdraught shutter, or a grille with stuck flaps. Fit a shuttered grille matching the duct size. |
WHAT YOU SAVE
What it costs to do it yourself
| 100 mm extractor fan with timer or humidistat | £21–£45 |
|---|---|
| Semi-flexible ducting, if needed | £9–£27 |
| Two-pole voltage tester, once | £25–£46 |
| Total, fan only | £21–£45 |
| DIY total | £21–£45 |
Guide prices compiled by the Locufy team from published Irish and UK provider price lists and 2026/27 cost guides. Updated 2026.
An electrician's guide price per fan usually includes cutting the hole and the wiring. A like-for-like swap into an existing hole and wiring costs the fan, and a tester you keep.
RATHER NOT?
Extractor fan installers near you, with prices shown
The ones who cover your area, nearest first, with one score from every review. No account needed to look.
Or post the job free and let them come to you.
With the power isolated, wipe the grille and inlets with a damp cloth every few months, as the maker recommends; the bearings need no oil.
Let the fan run on for long enough to clear the room after every shower. Mould is cheaper to prevent than to treat.
An old fan is electrical waste. The maker's own instructions say not to put it in household waste; take it to the electrical recycling at your council's household waste recycling centre. Find your centre on the council website or the recycling pages for your nation.
If the fan has an isolator switch outside the room, label it 'extractor fan' so the next person knows what it does.
Replacing an extractor fan like for like: questions people ask
Replacing a fan like for like on the same wiring is not notifiable in England, so a competent householder can do it after isolating and proving dead. New wiring, a new spur, or a new fan near the bath or shower is notifiable work, and Wales, Scotland and Northern Ireland have their own rules.
A timer fan needs a permanent live as well as the switched live. If only two conductors are connected, it works as a plain fan. Check the wiring, and check the overrun adjuster.
Long enough to clear the steam. Timer fans are adjustable; the one we checked runs on for 5 to 30 minutes. Set it towards the longer end.
The fan we checked requires a local isolator disconnecting all poles with a contact gap of at least 3 mm. If your fan has none, fitting one is new wiring and a job for an electrician.
Usually a blocked grille, a crushed or sagging duct, or an axial fan on a long duct. Clean the duct and grille first; replace with a mixed-flow fan if the run is long.
Locufy's UK price guide puts extractor fan installation at £150–£500 per fan, typically £300 — the same three figures as the strip at the top of this page.
MORE GUIDES
More DIY guides
Sources — checked October 2026
- Manrose — Quiet Fan Conceal installation and wiring instructions: zones, isolation, fusing, timer, ducting
- HSE — electrical safety: frequently asked questions (isolate and check dead)
- HSE — ladders and stepladders
- HSE — carbon monoxide awareness
- gov.uk — Approved Document P: electrical safety in dwellings (England)
- gov.uk — Approved Document F: ventilation (England)
- gov.uk — building work, replacements and repairs to your home (England)
- gov.wales — building regulations: electrics
- gov.scot — Building Standards Technical Handbook 2022 (domestic), standard 4.5 electrical safety
- Gas Safety (Installation and Use) Regulations 1998, regulation 3
- gov.uk — recycling collections: find your council (England and Wales)
- mygov.scot — find your council's recycling centre (Scotland)
- nidirect — recycling and reusing (Northern Ireland)
- B&Q — price check: extractor fans
- B&Q — price check: ducting
- B&Q — price check: two-pole voltage testers