An Introductory Guide to Ventilation

Rigid vs Semi-rigid vs Flexible

Rigid ducting consists of a solid plastic or metal pipe with smooth interior walls. These achieve by far the best air movement figures. They are also recommended on longer runs.

Semi-rigid ducting usually consists of a metal pipe which is flexible. They are suited for use over shorter runs where flexibility is the key requirement. The internal walls of the duct are not smooth however which impedes airflow somewhat.

Flexible ducting is the poorest choice, though due to cost, it is particularly common. It consists of a thin plastic which contains a thin wire coil. The pipes are prone to collapsing or being crushed, have poor air movement values, and often block up with lint when used with tumble dryers. They are not suited to runs over two or three metres.

Round vs Flat Channel

Round ducting is the more common variety of ducting. It offers a larger diameter pipe, and is able to extract more air than an equivalent flat channel of the same size.

Flat channel ducting is useful in locations with limitations on the height of the ducting, due to the height usually being around half that of the round variety.

With regard to most extraction fans, round ducting is able to achieve higher air movement values than flat channel due to the round nature of the duct offering less resistance. However, flat channel is preferable in use cases where a cross-flow fan type is being used due to the output on the fan usually being rectangular in shape.


Round ducting is commonly sized at 100mm (4 inch), 125mm (5 inch) and 150mm (6 inch).

Flat channel ducting is commonly sized at 110mm x 54mm and 204mm x 60mm though other sizes are also available.


Reducers allow for changes in the size of ducting used. Although they are named reducers, they can also be used in reverse to move from a smaller duct to a larger duct if required.

Whenever possible, the duct should be designed to use the best combination of size and requirement throughout the entire run. A reducer however is particularly useful when multiple runs are to be combined for output through a single larger vent or grille.


An adaptor is used to convert from one type of ducting to another. This can include round to flat channel or rigid to flexible.

Although adaptors are useful, it is recommended to avoid the use of them where possible due to the resultant impact on airflow.

Where possible, the design should incorporate thought on adaptors which may need to be used. Some are designed better than others, and their use case is also important. For example, many round to flat-channel duct adaptors feature flat edges which can build up dust or grease.


If a flexible duct is to be connected to a rigid type, it is highly recommended to use a screw connector for attaching the flexible duct. This provides a screw-like thread on which the flexible duct is affixed, and ensures it will stay firmly attached while not putting strain on the duct material.

Vents and Grilles

There are a variety of vents and grilles which have different uses, and pluses or minuses depending on the application.

If the outlet is to be installed on a vertical wall, a gravity flap vent or cowled vent can be used. These feature a flap which opens when the air extraction is turned on, but close via gravity when the extraction is off.

The gravity flap vent is suitable for areas with low to moderate wind, and areas without significant wet. This differs to the cowled vent variant which is better suited to areas with strong winds and driving rain.

In situations where the vent will be installed on a non-vertical surface, a louvred grille can be used. These are commonly installed on soffits or walls which have a off-vertical standing. They are also perfectly usable on vertical walls however are recommended in locations where there is little wind or potential for water ingress. They should not however be used for tumble dryer extraction due to the likelihood of lint building up quickly.

Backdraught Shutters

Although gravity flap vents and cowled grilles offer some resistance to wind, it may be beneficial to also install a backdraught shutter in locations where wind may enter the ducting the run. These are designed to prevent wind blowing back through the duct and out into the space being vented.

Generally, there are two types of shutter. The first is a gravity based one for horizontal runs, while a spring-loaded variety is available for runs which are vertical or non-level.

The suitability of installing them however requires the extraction fan being powerful enough. They should also not be installed on duct runs used to extract tumble dryer air due to the potential for blockages. It is also recommended to install the backdraught shutter as close to the outlet as possible.


Condensation is an important consideration when running ducting. If the extractor is to move warmer air through a colder space, condensation will need to be thought about. It is also a greater concern with metal ducting over the plastic alternatives.

A common issue is to extract air from a bathroom through a loft space, which results in condensation forming on the duct walls. This often collects in dips, and can even run down into a fan causing failure.

Ducting should be insulated when running through colder spaces such as lofts, to reduce the risk of condensation development within the duct. This can either be done using loft-type insulation, or by purchasing insulated ducting.

Where possible, it is recommended to ensure the duct run has a gradient which allows water to escape such as through a grille or vent. An alternative is to install a condensation trap which can then pipe the excess water away separately. It is also especially important to note situations where condensation could run into fan electronics.


In some installations, care will be required in the design to ensure the integrity of any fire-rated ceilings. This can be remedied by an appropriate firecuff which is fitted onto the duct, and is designed to expand inwards during a fire, cutting off the duct and preventing the spread of fire or smoke.

Key Points


Is it ever worth using a reducer to go from a smaller duct to a larger duct?

In most circumstances, it is not. Once the size of the duct has been reduced, the air movement value has dropped, and increasing the duct size does not aid extraction rates after a decrease.

Using an example, a duct run which goes from 150mm round duct to 100mm round to pass through a restriction is recommended to continue at that size to the end of the run, rather than return to 150mm.

The key situation where a reducer can be used to move from a smaller duct to a larger duct is for future proofing, when a larger section may be used as a common for multiple smaller ducts.

Can multiple fans connect to a single outlet?

Yes, but there are some considerations to make. The first is the amount of air to be shifted will need to determine the size of the common portion of the duct.

For example, in a ventilation system where multiple fans could be running at the same time, the common portion will probably need to be larger than the individual portions of the ductwork. However, in a home which is ventilation bathrooms, a single size could be used due to the seldom use of two or more fans at once.

Another consideration is that backdraught shutters will also need to be installed on the individual legs of the ductwork to ensure air does not flow back through the system.