<P> As with all drains, ventilation must be provided to allow the flowing waste water to displace the sewer gas in the drain, and then to allow air (or some other fluid) to fill the vacuum which would otherwise form as the water flows down the pipe . </P> <P> An island fixture vent provides an elegant solution for this necessity: when the drain is opened, water displaces the sewer gas up to the sanitary tee, the water flows downward while sewer gas is displaced upward and toward the vent . The vent can also provide air to fill any vacuum created . </P> <P> The key to a functional island fixture vent is that the top elbow must be at least as high as the "flood level" (the peak possible drain water level in the sink). This ensures that the vent never becomes waterlogged . </P> <P> The cost of installation is high because of the number of elbows and small pieces of pipe required . The largest cost outlay with modern plastic drain pipes is labor . Use of street elbows is helpful . </P>

Where is the vent pipe in my house