Because our chimney flue has been eaten up by gas fumes (double-walled galv. iron) When we bought a new furnace of the 90% efficiency type, we were forced to vent out the side with two PVC pipes. One in, one out.
Our hot water heater also has a FASCO power vent attached, but although it is only about 2 years old, is too corroded by corrosive gas fumes to work any longer--and this was extended by my replacement of the pitot tube sensor several times.
I am looking for a power vent that is made from materials that will last at least as long as the water heater. These things are expensive, and making them out of sheet iron may be good 'nuf for who it's fer, as far as FASCO is concerned, but I want quality stuff.