The pipe is a pipe that connects the riser to the atmosphere, preventing the creation of a vacuum above the waste water falling into the riser, as a result of which water disappears from the above siphons of sinks and toilet bowls with a characteristic squelching sound, and through an empty siphon unpleasant smells from the riser . If your house has more than two floors, or you own a house with two (and more) toilet rooms, then to ensure the stability of pressure in the sewer system, you have to acquire a fan tube. It will connect the riser to the atmosphere and maintain a stable (atmospheric) pressure in the sewage system, even if there is a volumetric discharge of water from the toilet tank, causing a depression in the entire outflow line. And without a fan pipe, this rarefaction would lead to a rapid devastation of all siphons under the sinks and toilets, which would ensure direct contact of the "atmosphere" of the septic tank with the "atmosphere" of the house. Thus, the connection of the fan pipe to the sewer system not only guarantees the stability of the pressure in the discharge pipe, but also ensures the integrity of all the "water locks" in the siphons under the sewage openings that cut off the putrid air of the septic tanks from the pleasant home microclimate.