Where branch pipes are long or steep, negative pressures can develop in the system which can cause loss of
seal in water seal traps by self siphonage. EN 12056-2, Table 6 and Approved Document H1, Diagram 3 give
guidance on when provision of branch pipe ventilation is required. This is traditionally by provision of a water
seal trap incorporating an AAV. Where HepVO
is used in place of a water seal trap there is no further need for a branch air admittance valve for
venting protection at that appliance.
® will only allow air to enter the systems and will prevent backflow of air from the stack.