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Home > Help > Glossary of Ventilation Terms

Glossary of Ventilation Terms


From one man’s expertise in the areas of fan design and air flow measurement, in 1955 the Airflow Development group of businesses was founded. Initially based in one factory in High Wycombe, Airflow has since blossomed into a flourishing multi-million pound international manufacturing and research group headquartered in High Wycombe, England. Airflow’s ongoing expansion was underpinned by opening subsidiaries in Germany and the Czech Republic, sustained by a world-wide sales distribution network. With a wealth of knowledge gained from over 56 years experience, Airflow is widely considered as a world leader in air movement and ventilation innovation.

Airflow has acquired its status as a world leader by constantly supplying innovative, quality products, endorsed by in-built reliability and developed by experts in air movement, and has been manufacturing to the stringent requirements of ISO9001 since 1994. Airflow’s extensively equipped laboratories and state of the art computer aided design section permits Airflow’s research & development department to constantly seek new ways of moving air, by integrating the latest technology and expertise combined with environmentally friendly energy usage. All of these rudimentary constituents come together to provide energy efficient and reliable products backed by Airflow’s commitment to quality, value and customer service which will extend for many years to come. See the full range of Airflow Fans.
Axial Fans
An axial fan is one in which the extracted air is forced to move parallel to the shaft about which the blades rotate. Most domestic wall fans, including the fans on this website are axial. Axial fans are great for small duct runs of less than 5m. They usually come in different versions, being operated, for example by a timer, humidistat, pull cord or PIR and may be switched on by the lights. Some axial fans are available in both 240v and SELV 12v for when installing in Zones 1 and 2 of the bathroom.

Axial fans are not good for long duct runs. Anything over 5m will impair their ability to extract air because of the larger air pressures present in the duct. When installing an axial fan one should also avoid bending the duct if possible as this will also add to the pressure in the duct. In these situations one should use a Centrifugal Fan.

Back Draught Shutter
A back draught shutter can take a couple of forms but performs the same function: They prevent air travelling back up the duct, through the impellars of the fan and into the room - particularly important in winter! Back draught shutters may be integral to the fan, in which case they may be as simple as a one way hinged membrane (they blow backwards when the fan is on, but cannot bend forward when it's off), or may be thermo-electrically controlled. However, back draught shutters are also available as stand alone units which may be fitted any where in the duct run. These are more commonly used when one is fitting a centrifugal fan.
Bathroom Fan
The term 'bathroom fan' is in some ways a misnoma because no one thing actually is a bathroom fan. This is principally due to the UK electrical regulations which divide the bathroom into 4 separate zones. Extractor fans permitted to be situated in zone 3 of the bathroom, for example, may be 240v (mains voltage) whilst fans installed in zone 1 and 2 must be SELV (safety extra low voltage 12v) or IP*5 waterproof rated. In other words, a 'bathroom fan' can be two very different things depending on where it is installed. now you're confused!... but there's no need to be. If you're in the process of choosing a bathroom simply order one which is suitable for zone 1 and it then doesn't matter where you install it. Our advice, in summary, is to consider the whole bathroom as zone 1.
Boost Speed
This term is used specifically in relation to MEV and dMEV continuous ventilation units which offer a low continuous extraction rate, (referred to as Trickle) and higher 'Boost' extraction rate for times when increased airflow is required - for example when the shower is in use. 

Centrifugal Fan
Centrifugal fans blow air at right angles to the intake of the fan, and spin the air outwards to the outlet (by deflection and centrifugal force). The impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the scroll-shaped fan casing. A centrifugal fan produces more pressure for a given air volume, and is used when long duct runs are necessary. Some centrifugal fans, such as the Vent Axia Solo on this site can be used when the duct run is up to 50m! Centrifugal fans come in two main types : As an Inline duct fan or as a wall mounted fan.
Continuous Ventilation
Increasingly popular, this term is applied to ventilation devices which run 24 hours a day at a low 'Trickle Rate'. See also dMEV, MEV and System 3.

dMEV - Decentralised Mechanical Extract Ventilation
dMev is the term given to a single room MEV unit. The distinguishing feature between a dMEV unit and normal extractor fan is the fact that a dMEV unit is continuous running. They constantly extract air from a room at what is known as a 'Trickle Rate'. Most dMEV units also feature a 'Boost' flow rate which is comparable to the extraction offered by a standard extractor fan for times of increased demand - such as shower time when more steam is present. We do not know why the 'd' of dMEV is lower case ..but it always is!
Duct or ducting is the pipe which takes the extracted air away from the extraction zone. It comes in various sizes to match the spigot sizes of extractor fans. The most common domestic sizes are 4 and 6 inch, but commercial fans will use 9 and 12 inch ducting. The most common types of ducting are solid, pvc, aluminium and insulated.

Extraction Rate
Extraction rate is the term given to the amount of air extracted over a given period. This figure is commonly represented in one of two ways: Either as metres cubed per hour (m3/hr) or litres per second (L/s). A half decent extractor fan should be somewhere around 70m3/hr or 20L/s. To convert m3/hr into L/s simply multiply the L/s figure by 3.6 so 25L/s = 90m3/hr.

The Grille finishes off the installation. Internal and external grilles prevent objects entering the duct run. Gravity grilles, fitted on the outside of the house, are used to stop backdraughts, having several hinged flaps which hang down when the fan is not in operation. Upon being used the extracted air pushes the hinged flaps up so that they open allowing the extracted air to pass.

Heat Exchange Unit
A device manufactured from thermo-conductive materials used in heat recovery systems such as MVHR units.
Heat Recovery
In ventilation this term specifically refers to the practice of taking the heat from old / stale / moist air which has been extracted from a room or building and transferring it to fresh incoming air going back in to the building via a heat exchanger. Increasingly popular, Heat Recovery lowers heating bills whilst improving air quality.
A humidistat measures the relative humidity of the air. If the RH reaches a pre-set level the humidistat will turn the fan on. Humidistats may be integral to the fan, (in which case the fan code usually includes an 'H' ..such as QT100HT or VA100LH) or may be separate switch units. In the latter case, remote humidistats are used to switch inline duct fans, which of course do not sit in the room from which they are extracting air. 

Icon Fan
The Icon extractor fan is a well known and very popular domestic 100mm extractor fan from Airflow, notable for its 'opening eye' shutter.
The impeller in a fan is the blade assembly. Impellers come in two main types: axial or centrifugal. The more common for domestic fans is axial as these are cheaper to produce and require smaller housing. Impellers may be backward or forward curved and may have any number of blades. Axial impellers commonly have between 8 and 11 blades.
Inline Fan
Inline extractor fans, as the name suggests are situated in the length of ducting. They therefore look completely different from wall fans but most importantly are not installed in the room from which they are extracting air. Ducting will run from one end of the fan to a ceiling grille in the extraction zone and from the exhaust end of the fan to the outside grille. Inline fans are excellent for use above bathrooms or kitchens where extra extraction power is required. Inline fans almost always use a centrifugal spigot and as such can usually extract over very long duct runs.
'Kinetic' is the name of an MVHR range from Vent Axia.
One of the leading manufacturers of Extractor Fans, celebrating its 23rd birthday this year, Manrose Manufacturing continues to go from strength to strength. Already Britain's fastest growing manufacturer of domestic ventilation fans and equipment with over 600 items in the product range, Manrose continues to lead the way in practical, innovative and cost-effective ventilation solutions. Since the launch of the company's first product, a 4" bathroom fan, the Manrose research and development team has continued to listen to what customers say and to develop new and innovative products to address those requirements.
MEV - Mechanical Extract Ventilation
MEV is the term given to a special type of ventilation unit which extracts air from the whole house. Situated centrally in the loft or sometimes cupboard, MEV units has multiple duct connections which are receive stale air from multiple rooms. It then expels the waste air through one duct run to the outside world.
Mixed Flow Fans
Mixed flow fans combine the characteristics of both axial fans and centrifugal fans. The air moves in both an axial and radial direction relative to the shaft. Mixed flow fans, like centrifugal fans, develop higher pressures than axial fans. Since this technology is only ever used in Inline Fans, the terms are interchangable.
MVHR - Mechanical Ventilation Heat Recovery
MVHR is the specific term given to a special type of ventilation system that not only extracts stale air form a room or whole house, but also returns fresh air from outside that has been warmed by the extracted air through a heat exchanger. MVHR systems are becoming a very popular method of ventilation as it both keeps air fresh and saves heating costs.
Nine Inch Extractor Fan
A nine inch extractor fan is a very powerful fan usually used in commercial installations including larger kitchens, restaurants, club rooms, pubs, shops and offices.
Over Run Timer
A device usually integral to the fan which permits operation after the fan has been manually turned off. Usually adjustable from around 1 - 30 minutes the over run period is determined during installation. This functionality ensures all steam is extracted from the bathroom.

Part F - Building Regulations
In England and Wales, requirements for ventilation are contained in Approved Document Part F, Ventilation and can be downloaded from The latest issue of Part F was in 2010. Like all Buiulding Regulations, Part F must be adhered to in order for any works to be certified.
A pullcord is a cord  attached to an extractor fan, which turns the fan on and off. Not so common nowadays on Bathroom Fans, but still popular on Kitchen Fans.
A long standing name in the ventilation industry, Silavent have been specifying products in housing schemes for over 50 years. With a reputation for reliable, robust products some of their fans have not changed design in 30 years! See the full Silavent range.
System 3
Under Part F of the Building Regulations, System 3 covers continuous mechanical extract ventilation (MEV). This can be either a whole house centralised MEV system, or localised decentralised MEV fan.
System 4
see MVHR
Trickle Speed
see 'Boost Speed'.
Vent Axia
One of the best known names in UK ventilation products, Vent Axia have been at the very forefront of design and innovation for over 60 years. Their range includes pretty much everything in the air movement sector from domestic bathroom fans to large commercial roof fans as seen on hotels and whole house ventilation systems. See the full Vent Axia range.
Volumetric Flow Rate
The volumetric flow rate of a system is defined as a measure of the volume of fluid passing a point in the system per unit time. The volumetric flow rate, Q, can be calculated as the product of the cross-sectional area, A, and the average flow velocity, v. The volume of fluid that flows past a given cross sectional area per second is Q = A.v. Strictly v is a vector, but in internal flows through pipes, for example, there is only one possible flow direction. The SI units for volumetric flow rate are cubic metres per second, although many other units are commonly used depending on the industry, such cubic feet per second or litres per second.
In fluid dynamics and hygrometry, the volumetric flow rate is the volume of fluid which passes through a given surface per unit time. As defined by Darcy's law, volumetric flow rate should not be confused with volumetric flux. Volumetric flux is represented by the symbol q and its SI units are m3/(m2.s) or m/s. The volumetric flow rate is given by the integration of a flux over an area. When the fluid flows in an angle θ, the volumetric flow rate Q can be calculated as:
Q = A ʋ cos θ where A is the area of the pipe and ʋ is the uniform velocity of the fluid with an angle θ. When the fluid flow is perpendicular to the area A, the angle θ = 0. The volumetric flow rate is
Q = A ʋ cos θ
The above equation is for one-dimensional incompressible flows and it is commonly referred to as the continuity equation. If the velocity of the fluid through the area is non-planar then the volumetric flow rate must be calculated by using a surface integral.
Xpelair extractor fans are one of the best known ventilation brands in the UK. Indeed their reputation now places them among Vent Axia, Manrose and Airflow.

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