Brief Notes. Common duct mechanical extract systems, operating continuously and entirely separate from any other ventilation system, should provide for a minimum extract rate of 20 metres3/hour from a bathroom with a W.C. pan (BRE Digest 170). For preference, vertical shunts as opposed to horizontal stub branches should connect the extract grilles to the common main (or subsidiary) vertical duct. The advantages claimed for the shunt system are (i) better sound attenuation between dwellings. (ii) should a fi re occur, smoke is less likely to be carried from one dwelling to another. (iii) the system is often more compact. Integration of services is of utmost importance and with careful design, ventilation ducts can be accommodated in a single service duct with drainage, rainwater, cold and hot water, fi re mains, etc.