Power Vented Furnaces
Furnaces and boilers can be "natural draft" or "power vented". In a "natural draft" furnace a chimney creates suction and pulls hot gases through the furnace's heat exchanger. However, if you don't have a chimney you need to use a fan to pull those gases through the furnace and out of the building -- such a furnace is called a "power vented" furnace.
There are two types of power-vented furnaces: induced draft and forced draft.
Power-vented furnaces can vent directly outside through a side wall -- as you might guess, if you don't have a chimney, this is a very useful feature. Power vented furnaces also tend to be a bit more efficient than natural draft furnaces, as the fan can regulate the optimum amount of gases pulled through the heat exchanger.
Power-vented furnaces tend to be a bit more expensive than natural draft models. Some homeowners have complained that side-vented furnaces can leave yellowish or gray stains on white siding.