Before you install one, it is vital to know how AAVs work. Here is a quick peek at it from our perspective.  Air admittance valves are run by gravity. When water and waste move down a drain line, it causes a vacuum air force in the pipe. This force lifts the seal in the AAV and lets air in, which permits the waste to drain away freely.  When the force ceases, the seal falls back in place to lock out sewer gas and smelly odor from the drain pipe. 

Earlier editions of this gadget were spring-operated but that is no longer a thing and such units are not reliable and do not meet code.