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A GUIDE TO VENTILATIONUNDERSTANDING DUCTINGPERFORMANCE FIGURESAs has already been explained, theperformance of the fan is dependent on theperformance capability of the fan itself, thesize of the duct, the ducting configurationand the type of wall terminal installed.Each ducting run can be planned in advanceand its performance levels evaluated so itcan be ascertained if the desired installationwill meet the required level.Technical data is available for the extractorfan/cooker hood installer or system designerin order to achieve the optimum overallextraction rate. This data highlights howmuch pressure (measured in Pascals) isgenerated in the ducting run and will show ifthe required airflow rate can be achieved.Pressure(Pa)200150100500High PressureLow PressureLow Flowrate15 30 60 80Flowrate (l/s)High FlowrateThe diagram above shows a typical pressureflowratecurve for an extractor fan or cookerhood (flow rate information should beavailable from the fan/cooker hoodmanufacturer). The information it contains isobtained by a series of laboratorymeasurements starting with free flow at zeropressure, then, by steadily increasing thepressure that the fan has to operate against,the volume flow and pressure at each pointis derived in order to plot the curve.As detailed in the diagram, in order toachieve the current UK Building Regulationflow rate requirement for a kitchen of 60 l/s(or 216 m 3 /hr, 30 l/s or 108 m 3 /hr, for a hoodwithin 300mm of hob centreline), themaximum duct system resistance when usingthis particular cooker hood can be no greaterthan 150 Pascals (Pa). Therefore, it can bereadily seen that by reducing the resistanceof the duct system, the extract performanceof the fan or cooker hood is increased.Furthermore, it can be stated that a ductingsystem with a low resistance is a ductsystem with a good performance.The performance of a ducting system isgreatly affected by both the overall length ofthe ducting, the number of bends containedwithin it and the type of outlet. The totalresistance of the duct system can becalculated by simply adding up theresistance of each component. Each productwithin the Domus range has a measuredresistance value for the three UK BuildingRegulation flow rates. These figures aredetailed within the ducting system sectionsof this catalogue, against each component.Note: For every extractor fan or cooker hoodinstallation, provision should be made formake-up or return air. In most dwellings thisis catered for by natural leakage. However,some high performance products mayrequire the installation of an air supply set(see page 37-38).The following is a calculation example usingthe typical ducting layout shown in thediagram.Therefore the airflow performance of thecooker hood connected should exceed39 Pa @ 30 l/s.System Configuration (as kit 227)ResistanceProduct Description (Pa) @ 30 l/s2 x 510 1m Flat Channel (2 off) 3.01 x 520 Flat Channel Connector 0.41 x 541 Elbow Bend with 125mm Spigot 11.11 x 561 125mm Flexible Hose, 1m 0.7*1 x 501 Airbrick with Damper 23.8Total 39.0* Based on a 1m length of 125mm pipe, presuming that the hose is pulled taut.Within the Ducting section, in order toencourage good practice (for guidanceonly) each system is supported with anicon depicting the application for which itis suited.TECHNICAL ADVICEDomus offers expert technical advice on allventilation installations and has individualdimensioned drawings available on request.www.domusventilation.com7