Power-vent heaters have a fan that blows exhaust gases out the vent pipe. They are used when the heater location is far inside a building and long, possibly horizontal vent runs are needed. They also have electronic controls that sometimes are finicky and they're more expensive than simpler water heaters.

Commonly they have a bulky motor housing on top of the tank. Anybody thinking about using a powered anode should look long and hard to make sure that the anode socket that sticks up a couple of inches above the heater won't run afoul of this housing.