Understanding Ventilation: FAQs

Airflow rate is the amount of air displaced in a given unit of time.
The flow given in the technical specifications is at free discharge, in other words, with no pressure loss.

• Airflow rates are expressed in volume of air divided by time. Most common units are m3/hour and litres/second (l/s).
• It is possible to convert m3/hour to l/s by dividing it by 3.6.

Pressure is the pushing force per surface unit that air needs to circulate and overcome the resistance of the system (i.e. pressure losses). 
Pressure is usually expressed in Pascals (Pa).
There are three types of pressure: 

1. Static Pressure (Pe): This is the force of the air in all directions on the walls of the tube, in a perpendicular direction to them.
2. Dynamic Pressure (Pd): This is the force that accelerates the air from zero to the velocity of the system. It is manifested only in the direction of the air.
3. Total Pressure: This is the sum of the static pressure and the dynamic pressure (Pt = Pe + Pd).

Noise is the audible sound that is propagated through the air and which is considered unpleasant to the receiver.
 

The Sound Pressure (Lp) level is what we really hear, and it varies according to the premises and the distance between the source of the 
noise (extractor or fan) and the receiver. Acoustic performance of extractor fans is usually Sound Pressure at a distance of 3m.
 

The Sound Power (Lw) level is the amount of sound energy of a sound source (extractor or fan) every second. It is constant, in other words, it 
does not vary according to the premises or the distance from the sound source.

Axial fans such as Xpelair Simply Silent™ C4 are most appropriate for extraction directly to the outside through a wall or window. 
If the window or wall faces a prevailing wind such as in coastal areas, exposed hills or high-rise applications, a centrifugal fan which has a 
higher pressure performance is more suited to overcome these outside conditions. Centrifugal fans are ideal for flexible ducted applications* or smooth bore ducting for longer distances. 
Xpelair Premier CF20TX with timer is ideal for internal toilets, bathrooms and washrooms with no direct access to the outside to meet Part F requirements. 

Humidistat is an electronic device which can read or calculate the moisture level in the air, and automatically signal to the fan when to turn on or off.
Automatic humidity control protects from unhealthy levels of condensation without the need to switch the fan on manually, saving energy by only switching the fan on when necessary.
All Xpelair domestic condensation control fans – such as C4HTSR, CV4SR and CF20TX – are supplied with built-in humidistats. 
If the fan cannot be located near the source of humidity, then a separate remote humidistat is available.

The extract point should be located as high as possible and provide a flow across the room with air replacement usually coming from the doorway.

If there is a negligible gap beneath the door when closed, fit a slimline door grille at low level for air replacement and effective fan operation.If there is a fuel burning appliance in the room, there needs to be adequate air replacement for both the fuel burning appliance and the fan. 

• In an upstairs bathroom, options will include a ceiling mounted fan with ductwork running to a soffit, grille or fan and light such as Xpelair Airline LED.
• In a kitchen, the right position will usually be an external wall or a window, but keep away from direct heat sources over 40ºC, such as an eye level grill.

The extract point should always be positioned to capture water vapour and odours at source. With the exception of kitchen canopies, it should be located as high as possible and provide a flow across the room with air replacement usually coming from the doorway.