A bathroom extractor fan is one of the hardest-working appliances in your home. It quietly removes steam, odours and excess moisture, protecting your décor, your health and your heating bills. Yet many UK homeowners either don’t have one, run it for too short a time, or neglect it until it rattles. Getting the fan right – and using it properly – is one of the simplest energy-efficient home improvements you can make.

Why good bathroom ventilation matters

Every shower releases litres of water into the air. Without effective extraction, that moisture condenses on cold surfaces: mirrors, tiles, windows and walls. Over time, this leads to mould growth, peeling paint, damp patches and even timber decay. It also makes your bathroom feel colder, because damp air takes more energy to heat than dry air. A suitable fan removes the moisture at source, so your heating system doesn’t have to work as hard.

Building Regulations Approved Document F sets minimum extract rates for bathrooms. For intermittent fans, the usual figure is 15 litres per second (about 54 cubic metres per hour). For a typical 4–6 square metre bathroom, that’s a sensible starting point. Larger bathrooms, or those with a power shower, may need more.

Sizing and choosing the right fan

Start by measuring your bathroom’s volume: length × width × height in metres. Multiply by the number of air changes per hour you need – typically 6 to 10 for a bathroom. Then convert to litres per second. For example, a 2.4m × 2m × 2.4m room is 11.5 cubic metres. At 8 air changes per hour, that's 92 cubic metres per hour, or about 25 litres per second. That’s higher than the minimum, so choose a fan rated at 25 l/s or more.

Fan types matter too:

• Axial fans – simple, cheap, best for short duct runs (under 1.5 metres) through a wall.
• Centrifugal fans – more powerful, handle longer duct runs and higher pressure. Ideal for ceiling or loft installations.
• Inline fans – mounted in the ducting, often in a loft. Quiet and effective for long runs.

Always match the ducting size to the fan. A 100mm fan with 100mm ducting is standard, but 125mm ducting reduces resistance and noise. Keep duct runs as short and straight as possible, and use rigid ducting rather than flexible hose where you can.

Controls: timers and humidity sensors

Manual on/off switches are easily forgotten. Better options include:

• Overrun timer – the fan continues for a set period after you turn off the light, typically 15 minutes. This clears residual moisture.
• Humidity sensor – detects rising moisture and runs automatically until humidity drops to a set level. Excellent for showers and drying laundry.
• PIR or pull-cord – simple activation, but less precise.

Humidity sensors are the most energy-efficient choice because the fan only runs when needed. Look for adjustable sensitivity and a manual boost override.

Running times and everyday habits

For a timer fan, set the overrun to at least 15 minutes after a shower. For a humidity sensor, let it do its job – it will typically run for 10–20 minutes after the moisture peak. Avoid leaving the fan running for hours; it wastes heat and electricity. On the other hand, don’t switch it off immediately. A common mistake is to turn the fan off as soon as you leave the room, leaving steam behind.

Also, keep the bathroom door closed while showering and for a few minutes afterwards, so moist air is drawn out rather than spreading through the house. If you have a window, opening it briefly can help, but in winter a fan is far more effective and cheaper than losing heat.

Retrofitting a fan in an existing bathroom

If you don’t have a fan, or yours is old and noisy, retrofitting is straightforward. A qualified electrician can core-drill a wall and install a duct. Key points:

• Choose a wall with a short, straight path to outside. Avoid long, convoluted runs.
• Ensure the duct slopes slightly downwards to the outside, so condensation drains out rather than back into the fan.
• Insulate ducting that passes through cold lofts to prevent condensation inside the pipe.
• Fit an external gravity shutter or cowl to stop draughts and rain.
• Consider a fan with a DC motor – they use 5–10 watts instead of 20–30 watts, and run quieter.

Cleaning, maintenance and energy efficiency

Dust and lint build up on fan blades and grilles, reducing airflow and making the motor work harder. Clean the fan every six months:

• Switch off the power at the isolator or fuse box.
• Remove the grille or cover (usually a twist or clip fitting).
• Wash the grille in warm soapy water and dry thoroughly.
• Vacuum dust from the blades and housing with a soft brush attachment.
• Check the ducting for blockages or kinks, and make sure the external shutter opens freely.

For the best energy efficiency, pair a humidity-sensing fan with a short duct run and a DC motor. If you’re undertaking a larger retrofit, mechanical ventilation with heat recovery (MVHR) can recover up to 70% of the heat from extracted air, but it’s a bigger investment. For most UK bathrooms, a well-chosen, regularly cleaned extractor fan remains the simplest, most cost-effective way to keep moisture under control – and keep your heating bills down.