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Air admittance valves aren't allowed because they tend to fail over time and because they can leak sewage in a backup. Obviously such a situation can result in overflow elsewhere, but that's the rule. As you've implied, flow from upstairs creates vacuum as it rushes past such a connection. If there's an opening for air to enter, as there always is with a ...


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These are different things. The vent stack inside the house is there to prevent sewer gas from pushing back into the traps (as well as providing overall pressure relief). The vent in the yard is there to handle gas generated inside the septic tank. Whether that is required by local code, or as part of your specific design, or just because some people ...


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