One of the most confusing aspects of plumbing, for the average person, AND for a lot of professional plumbers, is where, how and what is a vent.  Let me start simply by telling you that the pipe or pipes penetrating your roof, is most likely a plumbing vent.  There may be a similar pipe up there that could be an exhaust for your furnace or water heater, but most likely it is a vent for your plumbing system.  Every plumbing system must be correctly vented to be safe and legal.  The main purpose of the vent or vents within your house, or any other building, is to prevent the possible siphonage of the P traps that keep the sewer gas out of the structure.  The P trap is the U shaped pipe under all of your fixtures, or built into the toilet itself.  This is designed to stay full of water, and therefor “trap” the sewer gas behind it and not allow it to enter you home. Sewer gas can be anywhere from a stinky annoyance, to as much as a toxic danger.   Without properly placed and installed vents, it is very possible that these traps can either be siphoned out by other fixtures, or by the fixture itself while draining.  Vents must be installed properly and at a precise location, or they will not do the job they are intended to.  Vents also serve other functions in the system, such as allowing fixtures to drain better, and to regulate positive and negative pressure balances, especially in larger building with very tall waste stacks.   The relatively recent introduction to AAV’s (air admittance valve) has definitely changed the way plumbing systems are installed, but not usually for the better.  AAV’s are a little device which can serve as a vent, but does not have to penetrate the roof.  These will allow air to enter when needed  (relieving negative pressure), but they will not allow the sewer gas to escape.  AAV’s have a very real drawback though, and that is that they cannot relieve positive pressure.  These devices were invented and designed with the intent of solving some very tricky plumbing venting dilemmas.  Unfortunately, since they are code approved, they are also vastly over used and abused.  A “real” vent through the roof is always the best way to install a plumbing system. 

All Area Plumbing LLC