Power vented water heaters use a fan to send the excess gases out through the venting system and outside. Since these systems use a fan to assist in venting, most of the fuel used goes straight to heating the water, thus making this type of water heater more efficient than its atmospheric predecessor.
Along with the efficiency advantages of installing a power vented water heater over an atmospheric one, there are also safety advantages. Due to venting relying on a fan, the gas pressure is more consistent, with less chance of gas buildup leading to potentially dangerous situations.
Yet another safety-related advantage of a power vented water heater is the elimination of backdrafting, a dangerous situation causing the exhaust gases to enter the home instead of exiting the home via the natural draft vent of an atmospheric water heater.