Flue pipes begin at the connection to the boiler or furnace where the combustion gases exit the inducer motor housing. Gas and oil water heaters have a flue pipe connected at the top. The exhaust inside a flue pipe must remain warm while traveling to the exterior of the property so that the gas does not condense into a corrosive liquid and fall back into the system.
Some systems are located close enough to the roof or the wall they penetrate that they do not require chimneys to prevent condensation of the flue gases from occurring.
However, flue pipes are often connected to heating equipment installed in the lowest areas of a house or building, such as a basement or crawlspace, but the exhaust is vented out of the top of the property, which can be several stories high.