Vent stacks that protrude out of a roof are used to protect the water that is inside the ‘P’ trap in a DWV system. It allows air into the system to balance the pressure inside the system and prevents the water in a ‘P’ trap from being siphoned off. The ‘P’ trap in all drains is used to prevent sewer gasses from backing up into an occupied space and causing obnoxious and unhealthy fumes. This is a simple and reliable method that has been used for many years and up until now was the only method available. Air Admittance Valves (AAV’s) are now a viable alternative. AAV’s are a pressure-activated, one-way mechanical vent. AAV’s allow air into a DWV system just like a vent stack to protect the ‘P’ trap and do not allow air to escape out. When water flows into the drain system, the valve opens to allow air into the system. Once the pressure in the system is equalized, the valve closes thus allowing water to remain in the ‘P’ trap and prevent sewer gasses from escaping into the occupied space. Otherwise the AAV remains closed preventing the escape of sewer gas through the vent. AAV’s allow for easier installation, more design flexibility and uses less piping material than traditional vented systems. They eliminate the use of unsightly air vent stacks through the roof and lessen the risk of roof leaks. AAV’s are code approved and are an excellent alternative to vent stacks especially during remodeling and in new construction.