Jason Smithen, director of National Ventilation, discusses the issues around ducting, its use and the importance of correct sizing
BECAUSE it is normally hidden, a ventilation ducting system is often not considered an important part of the construction or refurbishment of a building. However, the importance of the correct ducting is slowly becoming recognised.

It is not always the fault of the fan manufacturer when the householder's dream of fresh air in the home and low noise from their fan are dashed. It can often be that the size or configuration of ducting for the fan has been incorrectly selected and causes the problem.

Air cannot be seen and because of this air movement is often seen as being more complicated than it actually is. In fact it is quite simple. Air can stop moving or get stuck too.

In ventilation installations the aim is normally to move the air from its source to the outside of the house as efficiently and quietly as possible. It is, therefore, common sense that to do this successfully, the shorter the distance the air has to travel, the quicker the aim is achieved. It is also common sense that if an obstacle is put in the way, such as a bend, the process will become more difficult as the air will lose its momentum and require more powerful assistance in order to achieve the aim.

Fans vary in extraction power, just as cars do, and the results are reflected in much the same way. A high performance/specification car should be quieter, last longer and generally provide the driver with more comfortable and satisfactory results. However, to ensure this, the car needs a straight smooth road on which to travel effectively.

The road for the air to travel when using a fan is the ducting. Again, it is common sense that a more powerful fan would need a more efficient duct - a Rolls Royce owner would not consider driving their car in off-road conditions. It is worth remembering that many fan manufacturers' guarantees will be invalidated if the correct size of ducting is not used to match the power of the fan.

Installation guidelines

· Choose the shortest and most direct route to the outside.

· Ensure that bends are kept to a minimum.

· Check the extraction capability and connection size of the fan and choose an appropriate size of ducting. Refer to the fan manufacturer for performance ratings and specifications.

· A round duct provides optimum airflow performance and should be used if installation space allows.

· Rectangular ducting is an excellent alternative when space is restricted. Its low profile enables the ducting to be easily concealed in the ceiling void or along the top of wall units.

· Rigid ducting is more efficient than flexible hose ducting and should be used whenever possible.

· If flexible hose is used, the hose should remain taut and as straight as possible.

· Provision should be made for make up air. In most dwellings this is catered for by natural leakage. However, some high performance products may require the installation of an air supply set.

· If installing a fan in a room containing an open-flued fuel-burning appliance (ie non-balanced flue), there must be sufficient replacement air to prevent to fumes being drawn back down the flue when the cooker hood is set to maximum extract.

Performance levels

Each ducting run can be planned in advance and its performance levels evaluated so it can be ascertained if the desired installation will meet the required level.

Technical data is available for the installer or system designer in order to achieve the optimum overall extraction rate. This data highlights how much pressure (measured in Pascals) is generated in the ducting run and will show if the required airflow rate can be achieved.

Flow rate data and charts should be available from the fan manufacturer. The data will detail the maximum duct system resistance when using the particular fan. Therefore, by reducing the resistance of the duct system, the extract performance of the fan is increased. A duct system with a low resistance is a duct system with a good performance.

The total resistance of the duct system can be calculated by simply adding up the resistance of each component used. Each component has a measured resistance value for the three UK Building Regulation flow rates and should be available from a reputable ducting manufacturer.

National Ventilation encourages good practice and recommends that only the correct choice of ducting be selected for the application in question otherwise those dreams of fresh homes become nightmares!


National Ventilation T: 01823 690 290