It’s all very well having an extractor fan, but what controls it?  How does it decide when to come on?  How does it know when to turn off?  There are several options, and they all look pretty similar from the outside.

In previous articles, we’ve talked about why extractor fans are important, and how much ventilation we need. In this article, I want to look at one way of controlling the fan, and discuss exactly what a humidistat extractor fan is. This is also sometimes called an extractor fan with humidistat module.

Most fans come in several versions:

  • basic model
  • timer model
  • humidistat model
  • timer + humidistat model

The basic model is, unsurprisingly, usually the cheapest option. The top-end model with both a timer module and a humidistat module is the most expensive, and the other options fall somewhere in the middle. But what exactly is the humidistat module?

A humidistat is a switch which controls the fan depending on the amount of moisture in the air. If there’s a lot of moisture, it turns the fan on. When the humidity level is reduced, it switches the fan off.

Principles of Humidistat Sensor To Control Extractor FanUsing a fan controlled only by a humidistat is usually a bad idea. It’s true that the humidistat will turn the fan on when you have a bath or shower and the humidity is high. However, if the fan is only controlled by a humidistat, it will not turn on at any other time. Usually, a humidistat extractor fan is connected via the lighting circuit: the fan operates when the lights are switched on, and also whenever the humidity is too high. This is useful when there may be other pollutants in the bathroom, for instance odours and smells.

A humidistat extractor fan is useful if your bathroom is often humid for a long period of time. Perhaps you have a lot of condensation. Wet rooms often benefit from a humidistat as they tend to generate a lot of moisture over a wider area than a shower cubicle. It’s also useful if the house will be unoccupied for a period of time, and the humidity may increase due to variations in temperature and the weather.

You can adjust the sensitivity of the sensor, usually with a small dial or screw inside the fan. Check the manufacturer’s instructions, which should be included with the Commissioning Notice your electrician left with you when he or she installed the fan. A humidity trigger level of about 65% is a good starting point. If you find the fan is on too much, turn the trigger level up. Conversely, if the fan does not seem to be coming on even when you think the humidity is high, turn the trigger level down. Instructions for this should also be included in the Commissioning Notice. If the fan was installed in a new house (or new conversion) then you should also have received a Flow Rate Test (PEATA) Certificate from your builder – make sure you ask for this.