Furnace ducts perform the important function of carrying warm air from your furnace to the other unheated rooms. It also carries cool air from the central air conditioner to each unconditioned room. Ducts are made from a number of materials including fiberglass and sheet metal.
One of the biggest causes of inflated energy bills is the lack of a properly insulated system. Ducts that release warm air into unheated spaces will cost you dearly in terms of energy bills. It makes sense therefore to insulate ducts especially those that are in unconditioned spaces such as attics or vented crawl spaces. The type of insulation that you use also matters. Avoid using rubber-based adhesive tapes. The recommended ones are the heat-approved types. Where there is a leakage in the supply duct system, heated air is forced out into unheated areas through unsealed joints. Air that has not gone through the conditioning system can also find itself into the return ducts through the same unsealed joints. This invariably leads to higher bills in summer since the air conditioner works extended hours and likewise the same in winter when the furnace works for extended periods to keep the house warm.
How hot the furnace ducts get depends on the method of heating in use. This includes gas, electricity or fuel oils. Ideally the ducts would not very hot especially to touch. Ducts should feel warmer close to the furnace and cooler further from it. However, where there is poor insulation the furnace ducts are bound to be very hot.