Vents....vent -- poisonous exhaust gases, that is, and through the roof or wall to the outside. While that might seem a no-brainer, it is important that it be done in a certain way. If the proper water heater draft diverter isn't used, or the vent diameter is wrong, it can cause the water heater to soot up, or to backdraft, so that the fumes stay within the water heater closet instead of going out.

Here, we borrowed a photo from the Closet of Horrors, since the yellow arrows just happen to delineate the vent.