High efficiency furnaces were created to improve the fuel-to-heat ratio of many furnaces and burners. Some have been developed to cool gases sufficiently to prevent heat loss through water vapor.
High Efficiency and Water Vapor
It is possible that some high-efficiency furnaces can attract excess moisture. There are two reasons for this. Firstly, the high efficiency furnace causes less air movement in the house; combining this with draft prevention and the insulation of roofs, means that air is less able to circulate, leading to moisture building up inside the house. Secondly, the aim of some high efficiency furnaces is to prevent heat from being lost by water vapor becoming heated and evaporating. Vapor that does not evaporate can appear as heavy condensation on windows, or mold forming on ceilings or walls.
Chimneys can also be locations for excess moisture. High efficiency furnaces can introduce water vapor into the chimney through burning oil or gas; but it can also be caused by humid air within the house being drawn into the chimney.
Installing vents on other appliances, such as clothes dryers, will limit the amount of moisture building up. Heat recovery ventilators can be installed to improve ventilation. High efficiency furnaces often need smaller chimneys, so installing a double-walled flue can prevent excess moisture.