etc.). To allow only one vent stack, and thus one roof penetration as permitted by local building code, sub-vents may be tied together inside the building and exit via a common vent stack, frequently the "main" vent. One additional requirement for a vent stack connection occurs when there are very long horizontal drain runs with very little slope to the run. Adding a vent connection within the run will aid flow, and when used with a cleanout allows for better serviceability of the long run. A blocked vent is a relatively common problem caused by anything from leaves, to dead animals, to ice dams in very cold weather, or a horizontal section of the venting system, sloped the wrong way and filled with water from rain or condensation. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage, and all the way to siphoned (empty) traps which allow sewer gases to enter the building. When a fixture trap is venting properly, a "sucking" sound can often be heard as the fixture vigorously empties out during normal operation. This phenomenon is harmless, and is different from "trap suckout" induced by pressure variations caused by wastewater movement elsewhere in the system, which is not supposed to allow interactions from one fixture to another. Toilets are a special case, since they are usually designed to self-siphon to ensure complete evacuation of their contents; they are then automatically refilled by a special valve mechanism. Internal venting Mechanical vents (also called cheater vents) come in two types: Air admittance valves and check vents, the latter being a vent with a check valve. Air admittance valves (AAVs, or commonly referred to in the UK as Durgo valves and in the US as Studor vents and Sure-Vent®) are negative-pressure-activated, one-way mechanical valves, used in a plumbing or drainage venting system to eliminate the need for conventional pipe venting and roof penetrations. A discharge of wastewater causes the AAV to open, releasing the vacuum and allowing air to enter the plumbing vent pipe for proper pressure equalization. Since AAVs will only operate under negative pressure situations, they are not suitable for all venting applications, such as venting a sump, where positive pressures are created when the sump fills. Also, where positive drainage pressures are found in larger buildings or multi-story buildings, an air admittance valve could be used in conjunction with a positive