As of 2015, some types of gas furnaces do not need a chimney, others work with unlined chimneys and yet more require a lined chimney. Greater efficiencies of modern furnaces complicate the problem.
In some cases, gas furnaces need a smaller chimney to function correctly. If the furnace is not generating enough water vapor and other gases, a large chimney might not provide a sufficient updraft, causing the water vapor to condense. In addition, many furnaces also come with water-heating features, which further complicate the issue depending on how the system works. Before installing a new gas furnace, homeowners must determine whether it needs a chimney and if their current chimney needs modification.