Book - School of Science and Technology

358 VentilationTo fans at roof levelServiceshaftShunt ductsMain ductFigure 13.11 Extract duct from toilets in multi-storey flats (fire barriers not shown)WC. In order to overcome stack effect and other chance pressure differences due to opendoors, etc., a duct system design as Figure 13.11 was recommended; this has the addedadvantage of providing an extended travel for voice transfer between dwellings. In thecase of office blocks and other large buildings, an extract rate of 10±12 air changes perhour is desirable.In this context, most authorities require that the exhaust fan must be duplicated, witharrangements for automatic change-over in the event of failure. Conventional fans may befitted in parallel but it is more convenient to fit a purpose-made twin fan unit of the typeshown in Figure 13.12.As in other instances, provision must be made for a supply of inlet air from outside and,in a block of flats where toilets and bathrooms are likely to be arranged one above theTwin centrifugal fansGlass fibrecasing (topremoved)LinkedchangeoverdampersOutlet oneither sideInlet duct tosuction boxFigure 13.12 Twin fan unit for toilet extract