There are various different options when it comes to extractor fans. One of the more popular options for bathroom installations is a fan with a humidistat function. This involves a fan unit with a built-in sensor that can register the humidity levels in the air, triggering the fan into operation when this level of humidity rises above a set trigger point.
So - how does this work? The air around us holds moisture. The level of moisture in the air is dependent on temperature, location, the weather and other contributing factors. Looking at Bathrooms in particular, the main factor affecting humidity is your shower. Because shower heads deliver multiple streams of hot water that fall through the air for a good 2 metres, this gives a large continuous surface area for water moisture to evaporate from into the cooler air of the bathroom.
Fans with a humidity sensor will sense this moisture increase in the air which will trigger the fan. The sensors can be set, usually via a dial or sometimes through set increments via dip switches, to trigger when the humidity reaches a certain percentage in the room - these settings are usually selectable between 60% and 90% humidity.
So, what setting should you choose when setting up your fan at the initial installation? Well, the first thing to note is that fans with built-in humidity sensors often can run continuously for a couple of days when first installed while the sensor calibrates. With this in mind, it is preferable to set the humidity setting to the least sensitive setting. This can be confusing as fans that have a dial to set the sensitivity will need to be turned all the way clockwise to make the sensor less sensitive and all the way anti-clockwise to make it more sensitive to humidity which feels counter-intuitive! If in doubt, the user manual for the unit will state the correct way to set this level.
Once the humidity setting is set, we would suggest setting any over-run function to the minimum setting. Now wait for the fan to run the calibration process - this will be done when the fan turns off.
To set the fan so that it is triggered by the high humidity caused by a shower or hot bath, run the water in the room to create a raised humidity atmosphere and then slowly dial back the humidity dial anti-clockwise a little at a time until the fan triggers. If your fan’s humidity trigger point is set via dip switches, change the switches to the next lower humidity setting and wait a few minutes before trying the next lower setting to allow time for the sensor to adjust until the fan kicks in. This should now mean that the fan will only be triggered by the high humidity level caused by a hot bath or shower and should stop any nuisance triggering due to changes in the ambient humidity levels caused by changes in the weather and temperature. Of course, you can now also set your over-run timer as applicable.