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How to automate exhaust fans in wet areas
Bathrooms and laundries are two of the most moisture-heavy rooms in the home.
Showers, baths, washing machines, dryers, wet towels and damp clothes can all raise humidity quickly. If that humid air is not cleared properly, the room can feel damp and stuffy. Over time, it can also contribute to condensation, mould, musty smells and general moisture problems.
Most homes already have exhaust fans, but they usually rely on someone remembering to turn them on (and off again)!
This is where smart automation makes a lot of sense.
By combining a smart humidity sensor with a smart relay, you can make your exhaust fan turn on automatically when the room becomes humid, then turn off again once the moisture has cleared. In a laundry, you can also use a smart plug with power monitoring to detect when a dryer is running and automatically turn the fan on (and off again once the cycle has finished).
It is a simple smart upgrade, but a very practical and useful one…
How to automate an exhaust fan
A smart exhaust fan setup is actually rather easy and usually relies on three main parts:
- An exhaust fan
- A smart relay to control the fan
- A sensor or trigger to tell the fan when to turn on / off
The smart relay is what turns the fan on and off. This could be a Shelly relay or another suitable smart relay installed by a licensed electrician.
The sensor is what makes the system smart. In a bathroom or laundry, the most useful sensor is usually a humidity sensor. It watches the room and detects when moisture levels rise.
For example, when someone starts a shower, the humidity climbs. The sensor detects that change, and the fan turns on automatically. Once the humidity drops back down, the fan keeps running for a short period, then switches off.
In a laundry, you can use a similar idea. If the humidity rises because wet clothes are drying, the fan turns on. If the dryer is running, a smart plug with power monitoring can detect that power usage and trigger the fan.
Bathroom exhaust fan automation
Bathrooms are one of the best places to use humidity-based fan automation.
A basic setup might turn the fan on when the humidity goes above a certain level. For example, if the bathroom reaches 70% humidity, the fan switches on.
That can work, but a better setup often looks at how quickly the humidity rises.
This is because normal humidity can change depending on the season, location and weather. In some homes, the bathroom might already sit at a higher humidity level on a rainy day. In other homes, the air might be much drier.
Instead of using only a fixed number, a smarter automation can look for a sudden increase. For example, if the humidity rises quickly after a shower starts, the fan turns on.
A good bathroom automation might work like this:
- Humidity rises quickly
- Exhaust fan turns on
- Fan keeps running while humidity is high
- Humidity drops back toward normal
- Fan runs for another 10 to 15 minutes
- Fan turns off automatically
This helps clear the room properly without relying on someone to remember the switch.
Laundry exhaust fan automation
Laundries are another great place for smart fan control.
Even if you do not have a vented dryer, laundries can still become warm and humid. Washing machines, sinks, damp clothes, indoor drying racks and dryers can all add moisture to the room.
A humidity sensor can turn the fan on when the room becomes damp, just like in a bathroom.
But laundries also have another useful option: dryer detection.
If your dryer is plugged into a suitable smart plug with power monitoring, your smart home system can detect when the dryer is running. When the dryer starts drawing power, the exhaust fan can turn on automatically.
A laundry automation could look like this:
- Dryer starts running
- Smart plug detects power usage
- Exhaust fan turns on
- Dryer finishes
- Fan keeps running for 15 to 20 minutes
- If humidity is still high, fan keeps running
- Once humidity drops, fan turns off
This is a very practical way to ventilate the laundry only when needed.
Using smart plugs to detect dryer activity
A smart plug with power monitoring does not just turn a device on and off. It can also measure how much power the device is using.
That makes it useful for detecting appliance activity.
For example, when a dryer is off, it may use little or no power. When it starts running, the power usage increases. Your smart home system can use that change as a trigger.
This can be more reliable than guessing based on time or humidity alone.
For laundry ventilation, this is especially useful because the fan can start as soon as the dryer starts. You can also make the fan keep running after the dryer finishes to clear any remaining warm, damp air.
Important note: not all smart plugs are suitable for dryers. Dryers can draw a lot of power, so you must choose a smart plug that is properly rated for the appliance and suitable for Australian use.
Choosing the right smart relay for the fan
The relay is the device that controls power to the fan. In Australia, it must be installed by a licensed electrician.
A Shelly relay can be a good option because it is compact, flexible and works well with smart home platforms like Home Assistant. Depending on the setup, it may be installed behind a wall switch, near the fan circuit, or in another suitable electrical location.
The right device depends on the fan and wiring.
Some fans only need simple on/off control. Others may be connected with a light, timer or multi-speed controller. Some installations may have neutral at the switch, while others may not.
This is why it is important to choose the device based on the actual wiring and fan type.
Remember, for Australian homes, anything involving 240V wiring must be handled by a licensed electrician.
Shelly 1 Gen3 Wi-Fi Relay Switch
NEW. Introducing the latest Gen3 version of Shelly's most popular Wi-Fi module. Automation of lights, garage door, irrigation solenoids, bathroom fan or any old electrical appliances… virtually anything that turns on or off! Shelly 1 Gen3 has numerous applications providing enhanced smart home control. Small enough to fit behind a wall switch or wall socket, Shelly 1 Gen3 is retrofittable and can be easily installed by a licensed electrician (or DIY for low-voltage) in less than 10 minutes. Connect Shelly 1 Gen3 directly to your Wi-Fi network (no HUB required) - or include it in your hub of choice (including Home Assistant).
Shelly 1 Gen4 Relay Switch
Shelly 1 Gen4 brings powerful upgrades to the popular Shelly 1 device. With new multi-protocol connectivity, you can choose the protocol that best suits your home network - Zigbee, Matter or Wi-Fi. An improved processor ensures faster responses and smoother performance of your automations. Shelly 1 Gen4 continues to be the perfect retrofit solution for your lights and appliances, now with enhanced compatibility and speed, all in its usual compact and easy-to-install design. Suitable for loads up to 16A.
Shelly 1PM Gen3 Wi-Fi Relay Switch
NEW. Introducing the latest Gen3 version of one of Shelly's most popular Wi-Fi modules. Automation and monitoring of power consumption for lights or electrical appliances… virtually anything that turns on or off! Small enough to fit behind a wall switch or wall socket, Shelly 1PM Gen3 is retrofittable and can be easily installed by a licensed electrician. Connect Shelly 1PM Gen3 directly to your Wi-Fi network (no HUB required) - or include it in your hub of choice (including Home Assistant).
Shelly 1PM Gen4 Relay Switch
A smart switch designed for optimal appliance control and energy savings. With new multi-protocol connectivity, you can choose the protocol that best suits your home network - Zigbee, Matter, Wi-Fi or Bluetooth. Shelly 1PM Gen4 features built-in power monitoring to track real-time energy consumption and optimise usage, helping you save energy and reduce electricity costs. Enjoy enhanced compatibility and speed, all in its usual compact and easy-to-install design. 16A max load.
Sensor placement matters
A smart automation is only as good as the sensor feeding it.
If the humidity sensor is in the wrong spot, the fan may turn on too late, too often, or not at all.
In a bathroom, avoid placing the sensor directly inside the shower area or too close to the exhaust fan. Too close to the shower, and it may react too aggressively. Too close to the fan, and it may read the air being cleared rather than the room itself.
A good location is usually somewhere that represents the general room humidity.
In a laundry, the sensor should be placed where it can detect the room’s overall moisture level. Avoid hiding it in a cupboard or placing it too close to a window, door or direct airflow path.
For dryer-based automation, the smart plug handles the dryer detection, while the humidity sensor can act as a backup or secondary condition.
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Aqara Zigbee Temperature & Humidity Sensor T1
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Aeotec Z-Wave aërQ Temp and Humidity Sensor
Know when environmental factors within a room become too uncomfortable with the Aeotec aërQ Temperature & Humidity Sensor. This 3-in-1 environmental sensor features aërQ technology used to monitor indoor temperature, humidity, and air quality within a single room and sends data to your Z-Wave automation hub. The aërQ Temperature & Humidity Sensor is compatible with all major Z-Wave hubs and controllers, allowing it to provide real-time updates and trigger automation scenes within your network. It also includes Z-Wave S2 security, which will enhance your device's performance with faster automation, advanced signal encryption, and better battery life.
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Aeotec Z-Wave MultiSensor 7
The Aeotec Z-Wave MultiSensor 7 can measure motion, humidity, light levels, temperature, vibration and UV all from one very small (and very clever) device. Featuring the latest Z-Wave Gen 7 (700 Series) chipset and firmware – this is the cutting edge of Z-Wave sensor technology. USB cable and 2 x CR123A batteries included.
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Shelly HT Gen3 Wi-Fi Humidity & Temperature Sensor
Shelly H&T Gen3 combines advanced technology with a modern, minimalist design that fits perfectly into any home or office. Featuring an e-ink display with low power consumption and keeps you informed about the temperature and humidity in your spaces. One year battery life with USB C connection for optional USB power. Wi-Fi connectivity (no hub required). Works with the Shelly App, Home Assistant, Alexa and Google Home. Available in Black or White.
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Good automation rules to use
A good exhaust fan automation should feel invisible. It should turn on when needed, run long enough to be useful, and then turn off by itself.
Some useful automation ideas include:
- Turn the bathroom fan on when humidity rises quickly
- Keep the fan running until humidity drops close to normal
- Add a 10 to 20 minute run-on timer after humidity drops
- Turn the laundry fan on when the dryer starts
- Keep the laundry fan running after the dryer finishes
- Turn the fan on if humidity stays high for too long
- Allow manual control from the wall switch
- Automatically turn the fan off if someone forgets
The run-on timer is especially important. Moisture often remains in the room even after the shower or dryer has finished. Keeping the fan running a little longer helps clear the space properly.
Common mistakes to avoid
One common mistake is using only a fixed humidity number.
For example, setting the fan to turn on at 70% humidity may work in one home but not another. Humidity changes with weather, seasons and location. A smarter option is often to detect a humidity increase compared to the room’s normal level.
Another mistake is turning the fan off too soon.
If the fan stops the moment humidity starts dropping, the room may still be damp. A short delay helps clear the remaining moisture.
It is also easy to overcomplicate the automation. Most bathrooms and laundries do not need complex rules. A humidity trigger, a run-on timer and manual override are usually enough.
Finally, make sure the hardware is suitable. Fans, relays and smart plugs all need to be correctly rated and installed safely.
A practical smart home upgrade
Automating exhaust fans in bathrooms and laundries is one of those smart home upgrades that is genuinely useful.
It is not just about controlling a fan from your phone. It is about making the home respond properly to moisture, steam and appliance use.
In a bathroom, the fan can turn on automatically during a shower and keep running until the room clears.
In a laundry, the fan can respond to rising humidity or start automatically when the dryer is running.
With the right combination of smart humidity sensors, relays, smart plugs and automation logic, wet area ventilation becomes much easier to manage.
The result is a home that feels fresher, smarter and more comfortable, without relying on people to remember the fan switch.
Need help choosing the right smart hardware?
If you’re planning a smart laundry ventilation setup and want to automate a motorised damper properly, SmartHome can help you choose the right Shelly hardware for the job. Whether you’re building a straightforward relay-based system or tying everything into Home Assistant, getting the right gear from the start makes the whole project much smoother.
Feel free to contact us, we are always happy to help.
I’m curious about adding an ‘odour’ or air quality sensor into the automation – such as the ones built into air purifiers. So that the exhaust fan can vent things other than humid air…