A run-on timer or humidity sensor makes an exhaust fan far more effective — it keeps running long enough to actually clear the moisture, rather than switching off the moment you flick the light. Here’s how they work.
Runs Long EnoughClears lingering moisture
Auto OptionHumidity sensor runs as needed
Less MouldThe key to a dry room
Why a Timer or Sensor
The single biggest reason wet areas grow mould is that the exhaust fan isn’t run long enough to clear the moisture — people flick it off with the light, while the room is still full of moist air. A run-on timer or a humidity sensor solves exactly this: it keeps the fan running until the moisture is actually gone. For mould prevention, this control is often more important than the fan’s raw power, because a strong fan run for ten seconds achieves little.
How a Run-On Timer Works
A run-on timer keeps the fan running for a set time — typically 10 to 30 minutes — after you switch off the light or fan switch. So when you leave the bathroom after a shower, the fan keeps clearing the lingering moisture instead of stopping immediately. It’s a simple, effective upgrade over a plain on/off switch, and it ensures the room actually dries out. The run-on time can usually be set to suit the room.
How a Humidity Sensor Works
A humidity-sensing fan monitors the air’s humidity and switches itself on when humidity rises (as during a shower), running until the humidity falls back to normal, then switching off. It needs no thought — the room is ventilated automatically, exactly when and as long as needed. This makes it ideal for bathrooms where the fan is often forgotten, and very effective at preventing the moisture build-up that causes mould.
Which Is Right for You
A run-on timer suits households that reliably turn the fan on — it just ensures it runs long enough afterwards, at modest cost. A humidity sensor suits rooms where the fan is forgotten, or anyone wanting fully automatic, foolproof ventilation — it both starts and stops itself. For the most reliable mould prevention, the humidity sensor wins; for a simple, cost-effective improvement, the timer does the job. We can recommend based on your habits and the room.
Fitting One
When installing or replacing an exhaust fan, it’s the ideal time to include timer or humidity-sensor control — we can fit a fan with it built in, or add timer control, depending on the fan and wiring. If your current fan is just a plain switch and the room tends to stay humid or grow mould, adding this control is a worthwhile, often inexpensive upgrade. Call 0431 918 137 or request a quote. See our fan replacement guide.
Frequently Asked Questions
What is a run-on timer for an exhaust fan?
A run-on timer keeps the exhaust fan running for a set period — commonly 10–30 minutes — after you turn off the light or switch, so it clears the moisture that lingers after a shower or cooking rather than stopping immediately. Most bathroom mould comes from the fan being switched off too soon; a run-on timer solves that by ensuring the fan runs long enough to actually dry the room. It’s one of the most effective upgrades for moisture control.
What does a humidity-sensing exhaust fan do?
A humidity-sensing fan automatically switches on when it detects rising humidity (like during a shower) and runs until the humidity drops back to normal, then switches off — no need to remember to turn it on or off. It ensures the room is ventilated exactly when and for as long as needed to clear moisture. This makes it excellent for preventing mould, especially in bathrooms used by people who forget to run the fan. See our
mould guide.
Do timer or humidity exhaust fans prevent mould?
They help significantly, because the leading cause of bathroom and laundry mould is the fan not running long enough to clear the moisture. A run-on timer keeps the fan going after you leave, and a humidity sensor runs it automatically until the air is dry — both ensure the moisture is actually removed rather than left to settle and feed mould. Combined with a correctly sized fan vented outside, they’re very effective at keeping wet areas dry.
Should I get a timer or a humidity sensor?
A run-on timer is simple and effective if you reliably turn the fan on (it just keeps it running long enough afterwards). A humidity sensor is best if the fan is often forgotten, or for rooms where you want fully automatic ventilation — it both starts and stops itself based on moisture. For many households the humidity sensor offers the most foolproof mould prevention, while a timer is a cost-effective improvement over a plain switch. We can advise which suits your situation.
Can you add a timer or humidity sensor to my exhaust fan?
Often yes — depending on the fan and wiring, we can fit a fan with a built-in timer or humidity sensor, or add timer control, when installing or replacing an exhaust fan. If you’re upgrading an old fan or fitting a new one, it’s the ideal time to include timer or humidity control. Call 0431 918 137 or request a quote. See our
fan replacement guide.
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