A run-on timer keeps an intermittent fan running for a set time after its trigger switches off. A humidity-sensor fan responds to the moisture level around its sensor. The exact behaviour depends on the model and installation.
When a timer makes sense
A timer suits a bathroom with a predictable trigger, such as the light, where you want the fan to keep extracting for a set period after use. Ask how the chosen fan is triggered and how long it can run on.
When a humidistat makes sense
A humidistat can respond to moisture when the bathroom is used in daylight and the light stays off, provided the model is set up for automatic humidity control. Some models combine both features. Check what is adjustable and where the sensor will sit.
What to check on an existing installation
Take a photo of the fan label and ask a registered electrician whether your existing controls and wiring support the replacement. The two products in our range have different nominal duct sizes, so choose the duct fit separately from the controls.
Frequently asked questions
Why might a humidity-sensor fan run after I leave?
The air near its sensor may still be humid. Check the model instructions for how its threshold and any timer work.
For the full checklist, see the bathroom extractor fan buying guide.
Compare the 100 mm run-on timer fan and 150 mm humidity-sensor fan in our bathroom extractor fan range. Check the product specification with your installer before ordering.