Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 23·2.9
Non-proprietary ventilation assembly requirements
When multiple components form a ventilation assembly, it should be kept simple and smooth, with the equivalent area determined by the component with the smallest equivalent area.
Where two or more components are to be used to provide a non-proprietary ventilation assembly, the assembly should be kept as simple and smooth as possible. The assembly should be taken to have an equivalent area equal to that of the component with the smallest equivalent area in the assembly.
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