My house has two soil stacks, the main ventilated one on the side of the house and an internal soil stack terminating in an Air Admittance Value (AAV). I was getting bad smell from the AAV and was advised to switch to an external vent by routing the soil vent pipe through the wall on the first floor and up a vent pipe to just above the eaves. The stack size is 110mm. What is the minimum size for the ventilatation pipe. Building regulations say “may be reduced in size in one and two story houses but should be not less than 75mm.” However I have seen elsewhere that since it is the secondary ventilated stack it can be 50mm. Is this correct? Thanks
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