Bathroom Extractor Fan Upgrade: What to Fit and Why

A bathroom extractor fan upgrade works when the new fan suits the room, the duct run and the way the bathroom is used. It does not work when an old fan is swapped for the same model that was already failing. Size, fan type, duct route, controls and electrical safety all decide whether steam clears, so each is covered below.

TL;DR

• A like-for-like swap often repeats the original problem. Check the duct length, the outlet and the controls before buying anything.
• Part F guidance sets 15 litres per second as the intermittent extract rate for a bathroom. Treat that as the floor, not the target.
• Bathrooms more than 1.5m from an outside wall usually need a centrifugal or inline fan, not a standard axial unit.
• Fans in Zone 1 and Zone 2 need an IPX4 rating at minimum, and the circuit may need work too.
• If mould keeps returning after a new fan, the fan is probably not the cause. A survey finds out what is.

Signs the fan is the problem

Not every damp bathroom needs a new fan. Look for these signs first:

• The mirror stays fogged long after a shower.
• Condensation runs down the walls or window and pools on the sill.
• Black mould appears on the ceiling, around the window reveal or along sealant lines.
• The fan is noisy but the air barely moves. A tissue held to the grille will not stay in place.
• The fan is over about ten years old. One maintenance guide puts typical bathroom fan life at roughly ten years, though lifespan depends on the model and on how well it has been cleaned.

Cleaning comes before replacing. Dust on the impeller and grille cuts airflow, and fans need cleaning at least annually. Isolate the fan at the switch, remove the cover, wipe the blades and check that the outlet flap opens freely. If airflow does not recover, the unit is worn or the wrong type for the room.

Why a like-for-like swap often fails

Most older bathrooms have a small axial fan in the wall or ceiling, fitted because it was cheap and easy. Axial fans are fine when the air only has to pass through a short hole in an outside wall. They lose a lot of performance when they push air through long or bent ducting. The label figure is measured with free air, not with a 3m flexible duct and two bends.

That is why a new fan sometimes shows no improvement. Replacing an axial fan with another when the duct run is longer than 1.5m can leave condensation exactly where it was. Three other faults also survive a straight swap:

1. The wrong outlet. Venting into a loft or wall cavity instead of outside is a common non-compliant installation error. It trapped moisture, and it can lead to rot and mould that stay hidden until there is structural damage.
2. Controls tied to the light. A fan that stops when the light goes off removes steam only while someone is in the room. The moisture is still in the air afterwards.
3. Undersized capacity. A fan that met expectations in a small, single-user bathroom may not cope after a larger shower or an extra person in the household.

The pattern matters for anyone in Dorset, Hampshire or Wiltshire, where many period and 1930s homes have bathrooms on internal walls or in rear extensions. The route to an outside wall is often longer than people expect.

Choosing the fan type

For a bathroom more than 1.5m from an external wall, centrifugal or inline fans are consistently recommended because they keep up the pressure needed to move moist air a longer way. Inline fans also reduce noise in the room, because the motor sits away from the bathroom.

Sizing: how much airflow is enough

For a bathroom, Approved Document F gives an intermittent extract rate of 15 l/s. In existing homes, that guidance applies when ventilation is replaced or modified. The figure is a minimum for the fan in working conditions, so check the installed performance, not just the headline number on the box.

A practical approach for typical homes:

1. Note the fan’s rated flow in l/s. Some packaging states m³/h. Divide by 3.6 to convert to l/s.
2. Count the bends and measure the duct length. Each bend and each metre of duct reduces real performance.
3. If the room is larger than average, has a large bath and shower in daily use or has persistent condensation, choose a fan with a margin above 15 l/s.
4. Check the noise rating. A fan that is too loud gets switched off, which defeats the purpose.

The comparison guides from Which? and Homebuilding review individual models if you want a shortlist of products. A diagnostic survey tells you which type suits the building.

Controls that make the biggest difference

The most effective upgrade is often the control, not the motor. Three options are in common use.

• Light-switch linked. Simple, but the fan stops with the light.
• Timer overrun. The fan keeps running after the light goes off. Building Regulations typically call for a 15-minute overrun in rooms without an openable window. Set it for the full period, not the minimum the dial allows.
• Humidistat. A fan with a built-in humidistat switches on according to actual moisture levels, so the room is dried even if the light is off and nobody is in it. This suits family bathrooms and rental properties where usage is unpredictable.

In rentals, this matters. Landlord press has noted that ventilation tops tenant wish lists but often ranks low in refurbishment plans. A fan that runs automatically removes the dependence on tenant habits.

Electrical safety and the rules that apply

A bathroom fan is a mains appliance in a wet room, so the electrics matter as much as the airflow.

• Zones. Zone 1 sits directly above the bath or shower and needs a minimum IPX4 rating, with IPX5 where water jets are used. Zone 2 extends 0.6m beyond it and also needs at least IPX4.
• RCD protection. A 230V fan in Zone 1 is acceptable only with the right IP rating and RCD protection. Older homes often lack an RCD, so the circuit may need an upgrade. That is why a compliant job can cost more than a simple swap.
• Isolation. Under BS 7671, a bathroom is a special location. The installation should include a proper isolation switch for servicing, and a missing one is a frequent oversight found at inspection.
• Notification. The same source notes that bathroom electrical work is notifiable under Part P of the Building Regulations, so certification applies.

These rules are for England and Wales, and Welsh guidance is published separately in the Welsh Government’s Approved Document F. Use a qualified electrician for anything beyond cleaning. Ask for a Part P certificate or the equivalent electrical installation certificate on completion.

What a replacement costs

Budgets vary widely with access and wiring. One national cost guide updated in June 2026 gives about £75 for a bathroom fan unit, around £200 to replace an existing fan and around £300 for a first-time installation, all including VAT. These are ballpark averages, not quotes.

The usual reasons a job costs more than average:

• Ducting needs rerouting to reach an outside wall.
• The circuit needs an RCD or new cabling.
• An inline fan is being fitted in the loft, which needs access and insulated ducting.
• The extract point has to move because the old one vented into a void.

We do not publish prices for our work, because each property needs a survey first. Treat any quote that skips the duct route and the electrics with caution.

Our perspective: ventilation is a moisture problem, not a gadget problem

We survey damp and mould in homes across Dorset, Hampshire and Wiltshire, and bathroom fans are a regular part of the picture. The usual story is a homeowner who has already replaced the fan once and still has black mould on the ceiling.

Our approach is to find the root cause before recommending a treatment. For a bathroom, that means asking where the moisture is produced, where it goes and whether the fan can move it there. Sometimes the answer is a better fan. Sometimes it is a shorter duct, a new outlet through the wall, or a humidistat control. Sometimes the bathroom is only one source and the moisture is also coming from elsewhere, such as a leak, penetrating damp or poor ventilation across the whole property. Our guide on telling rising damp from condensation explains how to separate those.

In-house specialists carry out our work, not subcontractors, and treatment work carries a 25-year insurance-backed guarantee. For condensation, we look at the whole building: condensation and moisture control, and where a continuous, quiet background system suits the property, PIV and air quality solutions. A fan that clears the shower steam but leaves a damp, cold house will not solve the mould.

— Quay

A stopgap while you plan

While you plan the work, open the window after bathing, keep the door closed during showers so steam does not spread to colder rooms, and wipe down wet surfaces. A portable unit can reduce moisture in the short term, and dehumidifier and air dryer deals are a way to compare options. It is a stopgap, not a fix for a fan that cannot move air.

When the fan is not the whole answer

Watch for signs that a new fan will not be enough:

• Mould returns within weeks of a fan upgrade.
• Damp patches or a musty smell appear on walls outside the bathroom.
• The ceiling below or beside the bathroom is stained.
• Timber around the bathroom floor, joists or window is soft or discoloured.
• Mould is present in other rooms and the property is cold.

These suggest a leak, penetrating damp, a ventilation shortfall elsewhere or timber decay, and each needs a different treatment. Our guide to fixing damp that keeps coming back covers why repeat failures happen. If you find black mould, see our page on black mould removal and prevention. If timber is affected, our timber treatment service covers rot and woodworm.

Property owners with several units, such as landlords and managers, can also look at energy efficiency upgrades. Insulation and ventilation work together, because warmer surfaces collect less condensation.

Upgrade checklist

1. Clean the existing fan and test the airflow.
2. Measure the duct run and count the bends.
3. Confirm the outlet discharges outside, not into a loft or cavity.
4. Choose axial, centrifugal or inline according to the duct length.
5. Select a fan above 15 l/s with a realistic noise rating and a suitable IP rating for its zone.
6. Add a timer overrun or humidistat.
7. Have a qualified electrician check the circuit, RCD protection and isolation switch.
8. Keep the certificate with your property records.

Book a survey if the problem keeps returning

If you have replaced a fan and the bathroom is still damp, the next step is a diagnosis. Our complimentary property survey identifies where the moisture comes from and gives you a detailed written report. If treatment work is needed, it is carried out by our own specialists. Our wider damp, mould and timber services cover the Dorset, Hampshire and Wiltshire area, and the guide to what a damp survey involves explains what to expect.

FAQ

How long should a bathroom fan run after a shower?

Building Regulations typically require a 15-minute overrun for bathrooms without an openable window. In practice, run the fan until the mirror and walls are dry. A humidistat model adjusts this automatically.

Is a more powerful fan always better?

Not necessarily. Airflow in real conditions depends on the duct length, bends and outlet, and an oversized fan can be noisy enough that people switch it off. Choose the right fan type for the duct first, then check the rated flow is above 15 l/s.

Can I replace a bathroom fan myself?

If it is a like-for-like swap with no new wiring and you can isolate the circuit safely, some people do. Anything involving new wiring, a fan in Zone 1 or 2, or an older circuit without an RCD should go to a qualified electrician, because bathrooms are special locations under BS 7671.

Why is my new fan not clearing steam?

The usual causes are a long or bent duct, an axial fan used where a centrifugal one is needed, a blocked or wrongly placed outlet, or controls that stop the fan too soon. If the fan is fine and mould continues, the moisture may come from another source, which a survey can identify.

Sources

• Approved Document F: Ventilation Requirements, squote
• Bathroom extractor fan buying guide, Checkatrade
• Extractor fan installation cost breakdown 2026, Checkatrade
• Bathroom electrical regulations UK, Bright Sparks of York
• UK Bathroom Regulations 2026, Bowmans
• Where bathroom money goes furthest in a rental property, Landlord Today
• Approved Document F Volume 1, Welsh Government
• How to buy the best bathroom extractor fan, Which?
• 6 best bathroom extractor fans, Homebuilding

Recommended

• PIV & Air Quality Solutions Dorset
• Mould & Condensation Solution Bournemouth