Quick answer
A bathroom extractor fan needs compatible ducting and a clear route outdoors. Our UK store offers 4-inch and 6-inch matt black fans with a humidity sensor and timer, plus a 4-inch white fan with a timer only. Choose by duct size, required extraction and installation conditions—not colour or diameter alone.
4-inch or 6-inch: match the fan to the duct
In bathroom fan product listings, 4-inch generally refers to a nominal 100mm duct connection and 6-inch to a nominal 150mm connection. Confirm the actual connection and opening dimensions in the model’s installation instructions before ordering. A 6-inch fan is not a direct replacement for a 4-inch opening without assessing the whole installation.
| Model | Size and finish | Controls listed |
|---|---|---|
| 4-inch matt black extractor fan | 100mm; matt black | Humidity sensor and run-on timer |
| 4-inch white extractor fan | 4-inch; white | Timer only |
| 6-inch matt black extractor fan | 150mm; matt black | Humidity sensor and run-on timer |
The 6-inch WW-150QTH listing states a rated airflow of 255m³/h. Treat that as a product rating, not guaranteed airflow after fitting ducting. The route, bends, termination and replacement air can affect the installed result. Check current product specifications and availability on the linked pages.
Matt black can coordinate with black towel radiators, while white suits a lighter scheme. The finish helps the appearance; it does not establish ventilation performance. Browse our bathroom extractor fans and accessories to compare models.
Plan the route before buying
Ask your installer to check the duct diameter, length, bends and external outlet. A fan chosen for a short wall outlet may be unsuitable for a more demanding route. Compare the manufacturer’s permitted installation arrangements and performance information; do not assume every 100mm or 150mm fan handles the same duct run.
- Use the duct size specified for the fan. Reducing the diameter can restrict its airflow.
- Keep the route as short and direct as practical, with avoidable bends removed.
- Support flexible ducting and extend it properly so it is not crushed, sagging or sharply bent.
- Fit and seal connections as specified; check that the external grille or shutter is not obstructed.
- Discharge moist air outdoors through a suitable termination, rather than into a loft or enclosed void.
Manufacturer guidance supports checking the complete duct system. For example, EnviroVent’s SILENT MV instructions address duct sizing and extending flexible ducting. Those instructions apply to that model: use the instructions supplied with your own fan for installation.
Ducting through a cold loft
Warm, moist bathroom air travelling through a cold duct can lead to condensation. Ask your installer to assess duct insulation and any required condensate management against the fan and ducting instructions. EnviroVent’s ducting guidance discusses insulation for ducting in unheated areas. Do not add an arbitrary slope or drain without checking the system’s requirements.
The room also needs a suitable path for replacement air. If the fan cannot draw enough air into the bathroom, extraction can suffer even when the motor is running. Have the installer assess the room’s ventilation arrangements rather than treating an open window as a substitute for a correctly designed system.
When the fan runs but steam remains
| Symptom | Possible checks |
|---|---|
| Steam lingers after a shower | Blocked outlet, restricted duct, inadequate replacement air, unsuitable fan or control settings. |
| Whistling or unusually loud airflow | Obstructions, compressed flexible ducting, tight bends or installation vibration. |
| Water around the fan or duct | Condensation, insulation, condensate management and possible water ingress; arrange assessment. |
These are possible causes, not a diagnosis. Persistent damp may have other causes, including leaks. Read our guide to extractor fan run-on time after a shower and our UK bathroom mould prevention guide.
UK bathroom installation and heating
A qualified electrician should check the electrical supply, bathroom location and protection requirements for the specific fan. A splash-protection rating alone does not approve every installation position. Follow the manufacturer’s instructions and applicable local requirements; Electrical Safety First explains bathroom electrical safety.
A towel radiator can help with warmth and towel drying, but it does not replace extraction. If you are planning both, our UK electric towel radiator running cost guide explains how to estimate the heating electricity separately.
Extractor fan ducting questions
Can I connect a 6-inch fan to 4-inch ducting?
Do not assume that an adaptor makes the combination suitable. A reduction changes the duct system’s resistance. Ask the installer to check the manufacturer’s requirements and the required installed performance before changing sizes.
Is rigid or flexible ducting better?
Choose the type permitted for the installation. Smooth rigid ducting can provide a direct route; flexible ducting needs careful support and extension. Avoid using a long, compressed flexible run simply because it fits around obstacles.
Does a humidity sensor fix poor ducting?
No. A sensor can control when a fan runs, but it does not remove a blockage or make an unsuitable route work. Check the airflow path as well as the settings.
