Choosing the right bathroom fan

Bathroom Extract Fans

Inadequate bathroom ventilation is one of the most common causes of condensation and poor air quality in homes.  In this short guide we try to help you choose the best solution for your bath or shower room.

How much airflow do I need?

Current UK and Northern Ireland building regulations state minimum intermittent extract rates of 15 litres per second (l/s) for bathrooms and 6 l/s for sanitary accommodation, i.e. a room with just a toilet and wash basin.  Unsurprisingly , most conventional 100mm dia (4”) fans sold in the UK can achieve 15l/sec and therefore comply with the regulations.

However in practice the minimum 15 l/s is rarely sufficient to cope with the large amounts of moisture generated from showering and bathing, and we would generally recommend between 25 and 50 l/s for a domestic bathroom or ensuite shower room.  The specific requirement will depend on several factors such as how often the bathroom or shower room is used, the size of the room itself, and the type of shower or bath installed.

Where should the fan be fitted?

Ideally the fan needs to extract from ceiling level, directly above the largest source of moisture in the room.  This is generally the shower (if one is present), although Jacuzzi type baths can give off even more water vapour.  If fitting a surface mounted (wall) fan directly above a shower, it may need to be a Safety Extra Low Voltage (SELV) model to comply with electrical safety regulations.

Wall fan or inline fan? 

Wall fans are unsurprisingly designed to be mounted on a wall, with the fan discharging straight through the wall to outside.  Wall fans with ball bearing motors such as the S&P SILENT-100-CRZ can also be mounted in the ceiling, with the discharge air ducted through the ceiling void to the wall and outside, however the additional resistance caused by this ducting can significantly reduce the airflow, especially when flexible ducting is used.

Inline fans such as the S&P TD MIXVENTare designed for fitting into a ducted system, and as such can generally cope with much higher resistances and hence longer duct runs than a wall fan.  They are therefore ideal for applications where the shower or bath is not sited near an outside wall and the ducting must travel through a ceiling void or roofspace.  Inline fans generally deliver much higher airflows than equivalently sized wall fans, so are well suited to larger bathrooms or showers that are heavily used.  A single inline fan can be connected to multiple extract grilles, serving two or more distinct areas where moisture is generated.  The downside of an inline fan is that a separate ceiling grille and grille box is required.

How will the fan be controlled?

There are various ways to control your bathroom extract fan:

Pull-cord.  Some wall mounted bathroom fans have a short pull cord hanging from the bottom of the casing, providing a simple means of switching the fan on and off.  Whilst still common in other countries, this type of control is sometimes considered to be relatively out-moded in the UK.

Lightswitch control.  This is the most common way of switching a bathroom extract fan.  The fan runs when the light is on, and stops when the light is turned off.  The disadvantage of this method is that if your bathroom has a window you have to turn the light on even during the daytime if you want the fan to run.

Lightswitch control with run on.  This is a mandatory requirement for bathrooms or WCs with no window.  The fan comes on when the light is turned on, but when the light is turned off the fan continues to run for a pre-set time (minimum 15 minutes for an internal room).  This option requires a permanent live supply to the fan in addition to the switched live.

PIR control.  This option uses a Passive Infra Red (PIR) detector to sense when a person enters the room and switch the fan on.  Most PIR switches also contain a built in run on timer to keep the fan running on for a period of time after the person leaves.

Humidity control.  This option uses a humidity sensor (either built into the fan or a remote unit) to detect increases in moisture levels in the air.  When the humidity level exceeds a pre-set (usually adjustable) level, the fan comes on and stays on until the humidity has reduced again.

Some models combine two or more of the above features.  For example the SILENT-100CHZ fan by S&P has a built in humidity sensor but this can also be overridden by a lightswitch input so that the fan comes on with the light even if the humidity level is not high.