The Basics of How Furnace Ducts Work

If you have a forced air furnace, you’ll inevitably have furnace ducts as well. Furnace ducts are usually made of sheet metal, especially in older houses, and carry the heat from the furnace to different rooms in the house. If you have a heat pump, the ductwork will be used for both heating and cooling.


Furnace ducts are generally made of out sheet metal that’s formed into tubes. These are in sections and are screwed together with sheet metal screws or welded into a tubular shape. Elbows join the sections, and these are especially useful for taking the furnace ducts around corners.

There are basically two kinds of ducts; those that distribute the warm air from the furnace and the larger, rectangular duct that runs from the plenum to the furnace drawing in cooler air. The plenum, or cold air return, is usually a large grid that sits on the ground floor of a house.

Newer constructions use plastic flex duct. This is also fitted in sections and is easier to use that sheet metal as well as being less dangerous. The furnace ducts can either be insulated or uninsulated (the latter is usually the case). If the furnace ducts aren’t insulated, they’ll lose air along their length which can often be emitted into places where you don’t require heating.


Cool air comes in through the ductwork from the plenum into the furnace via an air filter. It’s important to make sure this isn’t obscured or covered by anything to keep enough cool air coming into the furnace. Once the air reaches the furnace, it’s heated to a high temperature and a fan, run by electricity, pushes the warm air into the heat exchanger and through the ductwork. The fan also overcomes any resistance in the ductwork.

The further away a room vent is from the furnace, the lower the pressure of heat coming into the room since the fan can only exert so much pressure. The air will also be cooler because it has a longer distance to travel. This can be especially true in the upper stories of a house.


Furnace ducts need to be cleaned periodically as dust will accumulate easily. Cleaning will remove mold and give a better flow of warm air in the furnace ducts which, in turn, will reduce your heating bills.

Insulating your ductwork with a sleeve will also improve the levels of heat running through the furnace ducts. You will be able to insulate the portion of your ductwork that’s visible in the basement. If you have kids, they will inevitably have dropped small items through the room vents into the furnace ducts at some point. It’s worth unscrewing the vents and removing them, using the nozzle of a vacuum cleaner to remove the object and any accumulated dust.


The furnace ducts in the house are usually installed when the house is built so they’re actually part of the fabric of the home. Under most circumstances, they will never need to be replaced or moved to another location unless you make room additions to your house.