Why Your Bathroom Extractor Fan Stops When the Light Turns Off (And Why It's Causing Mould)

After installing probably 500+ bathroom fans over twenty years, here's what actually works:

Basic Extractor Fan (The One That Causes Problems)

How it works: Wired directly to your light switch

What happens: Runs when light is on, stops when light is off

Cost: £15-35

Installation: Simpler (one wire to light switch)

The problem: Stops before moisture is removed

I installed dozens of these when I was starting out. I thought I was doing good work. Every single one ended up with mould complaints within a year. Every. Single. One.

Example of what NOT to buy:

• Manrose CF100T Basic Fan (£22 on Amazon)

  • Extract rate: 16 litres/second

  • Runs with light only

  • No timer, no humidity sensor

  • Will cause mould in most bathrooms

The only time a basic fan works is if you have a massive openable window that you actually open after every shower. And let's be honest—in February in London, nobody's doing that.

Timer Extractor Fan (The One That Actually Works)

How it works: Runs for 15-30 minutes after you turn the light off

What happens: Actually removes moisture after you've left

Cost: £35-85

Installation: Slightly more complex (needs permanent live wire)

The reality: No mould, no problems, no call-backs

What I actually install now:

Budget Option:

• Manrose MF100T Timer Fan (£42 on Amazon)

  • Extract rate: 17 litres/second

  • Adjustable timer (15-30 minutes)

  • Backdraft shutter included

  • Quiet operation (26dB - whisper quiet)

  • Easy timer adjustment with screwdriver

  • This is what I fit in 60% of bathrooms

Premium Option:

• Vent-Axia Silhouette 100XT (£85 on Amazon)

  • Extract rate: 20 litres/second

  • Built-in humidity sensor (turns on automatically when humid)

  • Adjustable timer (15-30 minutes)

  • Ultra-quiet (23dB - silent)

  • 5-year warranty

  • This is what I fit when customers want the best

High-End Option:

• Vent-Axia Lo-Carbon Sentinel Kinetic (£165 on Amazon)

  • Extract rate: 25 litres/second

  • Humidity sensor + presence sensor

  • Intelligent control (learns your patterns)

  • Energy efficient motor

  • App control available

  • This is what I fit in new builds or high-end renovations

The difference between the £42 fan and the £165 fan?

The cheap one does the job perfectly well. The expensive one does it with more features and lasts longer. Both prevent mould.

Both are better than any basic fan.

This is where it gets interesting. Most people don't know the law requires proper ventilation.

Building Regulations Part F (Ventilation) states:

For bathrooms WITH an openable window:

• Minimum 15 litres/second extract rate

• OR window that opens to at least 1/20th of floor area

For bathrooms WITHOUT a window (internal bathroom):

• Minimum 15 litres/second continuous extract

• OR 60 litres/second intermittent with 15-minute overrun timer

That "15-minute overrun" means the fan must keep running after you leave. It's not optional for windowless bathrooms. It's the law.

I see this mistake constantly. DIY person or cheap electrician installs basic fan, wires it to light switch, jobs done.

Except it's not compliant. And more importantly, it doesn't work.

One of my early mentors used to tell me:

"Always assess the space by your view and experience, and act on base of that. Because if things go wrong, this is your fault—not the person who just told you what to do."

He meant: if a customer asks for a basic fan in a windowless bathroom, you explain why that's not going to work.

Don't just do what they ask because it's easier. Do what's right.

I've learned to have that conversation upfront now. "You can have the £20 fan that'll need replacing when mould appears in six months, or the £45 fan that'll work properly. Your choice."

Nobody chooses the cheap one when you explain it like that.

I've fixed enough of these to write a book.

Here are the exact mistakes I see repeatedly:

Mistake 1: Fan connected directly to light switch

What happens: Fan only runs when light is on

Who does this: 70% of DIY installations, 40% of "electricians"
The result: Mould within 3-6 months

The fix: £120-150 to install timer fan properly

Story: A landlord in Dulwich called me about mould in three rental properties. All three had basic fans wired to lights. I replaced all three with timer fans in one day. Cost him £450 total. No more mould complaints in 18 months since.

He could've saved £450 if the original electrician had spent £40 more per fan in the first place.

Mistake 2: Timer set too short

What happens: Fan runs for 5 minutes when it needs 20

Who does this: Installers who set it and forget it

The result: Some mould, just slower

The fix: 2-minute adjustment with a screwdriver (free if you know how)

Most timer fans have a small dial or switch inside. You remove the cover, adjust the dial, job done. Takes 2 minutes if you know where to look.

But most people don't know it's adjustable. They assume 5 minutes is correct. It's not. For most bathrooms, 20 minutes is minimum.

Mistake 3: Wrong size fan for room size

What happens: Small fan in large bathroom can't move enough air

Who does this: People buying "whatever's cheapest"

The result: Inadequate ventilation, mould in corners

The fix: £350-£450 including correct fan

Rule of thumb:

• Bathroom up to 4m²: 100mm fan (15 l/s minimum)

• Bathroom 4-6m²: 100mm fan (17 l/s minimum)

• Bathroom 6-10m²: 120mm fan (20+ l/s minimum)

• Bathroom over 10m²: Consider two fans or 150mm fan

I saw a massive 19m² bathroom with ensuite in Norwood.

They'd installed one basic £20 fan. Mould everywhere bathroom and room. I installed one Turbo tube TT100PROT (300m3/h)

Problem solved.

Cost to do it wrong: £20

Cost to fix it: £440

Cost to do it right first time: £200

Mistake 4: Ducting done wrong

What happens: Duct kinked, too long, or venting into loft instead of outside

Who does this: DIY installations where cutting through wall looks hard

The result: No airflow, moisture in loft, or cold air flooding back

The fix: £200-400 depending on access

This was my mistake when I was 20"s. First solo bathroom job. I didn't want to core-drill through the brick wall, so I vented the fan into the loft. Thought I was being clever.

Three months later, customer calls. Loft insulation soaking wet. Mould growing on rafters. I had to come back, cut through the wall properly, replace £300 of insulation, and didn't charge the customer a penny extra.

That mistake cost me £600 and time and taught me a lesson I never forgot: do it properly or don't do it at all.

Mistake 5: No backdraft prevention

What happens: Cold air floods back through fan when it's not running

Who does this: Cheap fans without built-in shutters

The result: Cold bathroom, higher heating bills, draft

The fix: £80-120 to replace with proper fan

Good fans have a backdraft shutter—plastic flaps that close when fan isn't running. Cheap fans don't. You get a cold draft coming in.

Customer in Crystal Palace complained her bathroom was freezing. I looked at her fan. £15 basic model from eBay, no shutter. Wind was literally blowing straight through it.

Replaced with proper fan with shutter. Problem solved. She said heating bill dropped noticeably that winter.

Mrs. Patel in Sydenham.

This is the woman I mentioned at the start.

Black mould covering 60% of her bathroom ceiling. She'd paid two different companies £400 each to treat it. It came back both times within six weeks.

When I looked, I could see everything wrong:

2. 1. Basic £18 fan from Screwfix wired to light switch

3. 2. Fan undersized (4m² bathroom, fan rated for 2m²)

4. 3. Duct was kinked behind the fan (no airflow)

5. 4. External grille had no cowl (wind pushing moisture back)

6. 5. No timer, so ran for maybe 30 seconds total

I explained: "Your fan is doing almost nothing. Every shower creates 1.5 litres of moisture. Your fan runs for 30 seconds while you turn the light off. All that steam stays in here and condenses on your ceiling every single night."

She understood immediately. "Why didn't they just tell me that?"

Good question. Maybe they didn't know. Maybe they just wanted to sell mould treatment.

What I did:

• Removed basic fan

• Installed Manrose MF100T timer fan (£42)

• Straightened the kinked duct

• Added external cowl with backdraft shutter (£8)

• Set timer to 20 minutes

• Tested airflow (strong pull now)

Total cost: £180 including materials and labour

Time: 90 minutes

Six months later, she sent me photos. Ceiling completely clear. Not a spot of mould anywhere.

She'd spent £800 on treatments that didn't work. £180 to fix the actual problem.

That's why I tell people: treat the cause, not the symptom.

Based on what I charge in South London and what I see other qualified tradespeople charging:

DIY Fix (If You're Competent with Electrics)

Materials needed:

• Timer fan (£35-60)

• External cowl if needed (£8-15)

• Duct sealant (£5)

• Silicone sealant (£6)

• Total materials: £54-86

Your time: 2-4 hours depending on experience

Skill level needed: Basic electrical, understanding Part P

Risk: If you get wiring wrong, you're liable

When DIY makes sense:

• You're confident with electrics

• It's like-for-like replacement

• Wiring is accessible and sound

• You have the right tools

Professional Installation (What I Charge)

Supply and fit timer fan: £150-180 Includes:

• New timer fan (good quality)

• All materials and fixings

• Testing and adjustment

• 12-month guarantee on workmanship

Time: 90-120 minutes

What you get: Compliant installation, properly tested, Full Replacement (When Ducting Needs Work)

New fan + re-route ducting: £250-400

Includes:

• New timer fan

• New ducting (rigid where possible)

• External vent and cowl

• Core drill through wall if needed

• Making good (filling, painting touch-up)

Time: 3-5 hours depending on access

Why it costs more: Cutting through walls, running new duct, making good

Worst Case (Mould treatment + New ventilation)

Professional mould treatment + ventilation fix: £500-800

Includes:

• Professional mould kill treatment

• Surface cleaning and treatment

• New ventilation system

• Anti-mould paint

Time: 1 full day

Reality: Much cheaper than ignoring it for another year

If you're doing this yourself and you're competent:

Essential Tools:

Wera Kraftform Screwdriver Set (from £35 on Amazon)

1. • You'll need this for fan removal and terminal connections

   • Proper electrician's screwdrivers, not cheap ones

     Fluke T90 Voltage Tester (from £45 on Amazon)

2. • Safety first—test everything is dead before touching

   • Worth every penny

     100mm Hole Saw Kit (£25 on Amazon)

3. • If you need to cut new ducting holes

   • Get a good quality one

     CT1 Silicone Sealant (£12 on Amazon)

4. • Best sealant I've used in 20 years

   • Sticks to anything, stays sealed

     Materials You Need:

     Manrose MF100T Timer Fan (£42 on Amazon)

5. • My go-to for 80% of installations

   • Reliable, adjustable, quiet

     100mm Flexible Ducting (£8/metre on Amazon)

6. • Get quality ducting, not the cheap thin stuff

   • Rigid is better where possible

     External Vent Cowl - 100mm (£8 on Amazon)

7. • Must have backdraft shutter

   • White or brown to match exterior

     Aluminium Duct Tape (£6 on Amazon)

8. • Not fabric tape—aluminium foil type

   • Seals duct joints properly

     Don't Cheap Out On:

• The fan itself (timer is essential)

• The ducting quality (cheap ducting splits)

• The external vent (needs backdraft shutter)

• Your tools (cheap screwdrivers round off screws)

  Where You Can Save Money:

• Use existing wiring if it's sound

• Use existing duct route if it's good

• Shop around for delivery (often free over £20)

• Do the painting touch-up yourself

I need to be very clear because I've seen what happens:

Timeline of bathroom mould:

Weeks 1-2: Tiny black spots appear in ceiling corners

Weeks 3-4: Spots spread across ceiling in patches
Month 2: Black patches cover 30-40% of ceiling, smell starts

Month 3: Mould spreads to walls, grout lines, silicone sealant

Month 4-6: Paint starts peeling, structural damage begins

Month 6-12: Health effects start (breathing, allergies)

Year 2+: Ceiling replacement needed (£800-1,200)

Real example: Customer in Penge ignored bathroom mould for 18 months. By the time she called me, the ceiling plasterboard was saturated and soft. We had to replace the entire ceiling, treat all the joists with fungicide, plaster, and paint.

Total cost: £1,200

Original fan fix cost: £180

She paid 8 times more by waiting.

Worse?

Her two kids had developed breathing problems. Doctor asked about damp at home.

That's when she finally called me.

My old mentor used to say:

"Share what you know while you can. One day, you’ll understand that teaching others is how we leave something behind."

I'm sharing this so you don't make the same expensive mistake she did.

I'm going to be straight with you:

Call a professional if:

• You're not confident with electrical work

• The fan is ceiling-mounted (not wall)

• You need to run new ducting through walls

• You need Part P certification for selling/renting

• The existing wiring looks dodgy or old

• You've already got mould (needs treating first)

• You're not sure what you're doing

You can probably DIY if:

• It's a simple like-for-like wall fan replacement

• Wiring is accessible and in good condition

• You understand basic electrical work

• You're competent with tools

• You're willing to take responsibility

I'm not trying to talk you out of DIY. I learned by doing it myself. But I also learned by fixing my own mistakes, and some of those mistakes were expensive.

If you're unsure, spend £60 on a professional assessment before you spend £200 on materials and tools you might not need.

Step 1: Check your current fan (2 minutes)

Turn your bathroom light off. Does the fan keep running?

• If NO → You have a problem

• If YES → Time how long it runs (should be 15-20 minutes minimum)

Step 2: Calculate your bathroom size (3 minutes)

Measure length × width in metres. For example: 2m × 2.5m = 5m²

You need minimum 15 litres/second extraction for that 5m² bathroom

.

Step 3: Check your ducting (5 minutes if accessible)

Can you see where your duct goes? Check:

• Is it kinked or bent sharply? (Bad)

• Does it go outside through a wall? (Good)

• Does it go into your loft? (Bad)

• Is it long and flexible? (Less effective)

• Is it short and rigid? (More effective)

Step 4: Decide your next move

Option A: Book a property Health check from (£80)

• I'll assess your ventilation properly

• Check if it's compliant with Building Regs

• Identify all issues

• Give you written report

• Quote for fix if needed

Option B: DIY replacement

• Buy proper timer fan (£42+)

• Follow manufacturer instructions carefully

• Test thoroughly

• Accept responsibility

Option C: Professional Installation (£150-180)

• I supply and fit proper fan

• Test

• 12-month guarantee

• Part P certificate provided

My honest recommendation?

If you've got mould already, call someone.

Don't mess about—mould gets worse fast.

If your fan just needs upgrading and you're handy, DIY might work. But get it certified afterwards if you're renting or selling.

If you're unsure, spend £60 - £80 on an assessment. I'll tell you exactly what needs doing and what it'll cost.

No surprises.

Here's what I've learned after 20 years: small problems become big problems if you ignore them.

That's why I started offering Property Health Checks. For £60, I spend 45 minutes going through your property looking for issues before they become expensive.

(This is a visual maintenance inspection only and does not replace legally required compliance certificates.)

What I check:

• Ventilation (bathrooms, kitchen, whole house)

• Visible damp or condensation

• Electrical safety concerns

• Plumbing leaks or problems

• Door and window operation

• Anything else you're concerned about

What you get:

• Written report of findings

• Prioritized list (urgent vs. can wait)

• Cost estimates for any work needed

• Advice on DIY vs. professional

• Photos of any issues found

from £120 inspection
If work proceeds within 14 days → credit £40 only.

Landlords love this. I check their properties annually. Catches small issues before tenants complain. Saves thousands.

Homeowners use it before selling. Fix issues before they show up on buyer's survey. Avoids price negotiations.

Investors use it before buying. I've saved people from buying properties with £10,000+ of hidden problems.

One customer in Norwood nearly bought a "perfect" renovation. I spotted that all the electrics were DIY, ventilation was non-compliant, and there were three hidden leaks. He walked away. Saved himself at least £15,000 in fixes.

🚫 What It Does NOT Include:

This inspection:

• Is visual only

• Does not include electrical certification

• Does not include gas safety checks

• Does not replace EICR or Gas Safety Certificate

• Does not include invasive investigation

Book a Property Health Check:

• Email: lovelofters@gmail.com

• Website: Lovelofters/property-health-check

Your bathroom fan stopping with the light isn't just annoying. It's creating a mould problem that'll cost you hundreds or thousands to fix if you ignore it.

I've fixed this exact problem hundreds of times. The solution is simple: install a timer fan properly. Cost: £180 professionally installed, or £50 in materials if you DIY.

Don't wait until you've got black mould everywhere. Fix it now while it's cheap and easy.

And if you're not sure what to do, book a Property Health Check. I'll assess everything properly and tell you exactly what needs doing.

One of my mentors once told me:

"Quality takes a little extra time, but it leaves a lasting impression. The care you put into your work today shapes how people see you tomorrow.”

I prefer to be known for doing things properly. That's why I'm taking the time to explain this to you.

Do it properly, do it once, and never worry about bathroom mould again.