Technical Guides

Bathroom fan run-on timers – a short guide


Lots of bathroom and toilet fans come with a built-in run on timer, but do you know if this is what you need?  Here we look at what this feature is and when you need it.

Fans with run on timers are primarily designed for light switch operation, i.e. when someone comes into the room and turns the lights on, the fan comes on.  Other types of fan control are available such as PIR occupancy sensing or humidity sensing, but for the purposes of this article we are considering only those controlled by an externally switched mains feed.

What does the run-on timer do?

Simply put, it keeps the fan running for a pre-set (usually adjustable) time after the lights are turned off.

Why is it needed?

Running the fan for a period after the room has been vacated helps to disperse moisture and odours after the room is no longer occupied.    It is a building regulations requirement for toilets and bathrooms without openable windows, but is used in the majority of basic domestic fan installations, and makes good sense if you are using a light-switch controlled fan.

How does it work?

The fan has two live electrical connections; a permanent live and a switched live.  The permanent live is present continously, and provides the power to run the fan motor and control circuits.  The switched live comes from the switched side of the light switch, and is just a signal to the fan control circuit to let it know when to come on.  When the switched live from the light switch is turned off, the timer starts running and keeps the fan motor on for the pre-set time.

Most run-on timers allow the length of run-on to be adjusted via a simple potentiometer inside the casing.  The range of adjustment is typically from 30 seconds up to 15 minutes, but manufacturers vary in what they offer.

Do I need a fan with a built-in timer?

For replacement installations, if the original fan had a timer, then you should replace it with a timer fan.  For new installations, building regulations stipulate that if there is no openable window the extract fan should have a run on timer set to at least 15 minutes, unless humidity control is used.