How to Plan Duct Placement

Plan on positioning hot air ducts along the bases of exterior walls for a separate central heating system. For forced air heating, the best placement for ducts is under windows or along the bottom of exterior walls. When the heated air rises and meets with opposing cold air near the exterior walls and windows, convection currents are formed. These currents circulate toward the centers of the rooms, providing a larger mass of circulating heated air.

Position cold air ducts along ceilings or the tops of exterior walls for a separate central cooling system. Cold air colliding with the warmed air in the room below creates a convection current. The current naturally moves the cold air into the center of the room, filling the room and improving the efficiency of the space conditioning system. Situated at the top of exterior walls, the cooled air circulates down and around the room much easier and the conditioning unit performs its duties more efficiently.

Install return air ducts equal in size to the supply ducts for each room. For maximum efficiency, comfort and, in keeping with building codes in some municipalities, every room with a supply duct must also have a return duct of equal or greater cubic space. For example, a supply duct that measures 8 inches by 4 inches by 10 feet long provides 320 cubic feet of conditioned space; in such a case, plan on installing a return duct in the room that supplies at least 320 cubic feet of return air ducting.

Install ducts within the conditioned space, if possible. Ducts placed in the rooms they serve prevent energy waste and offer nearly 100 percent efficiency. Ducts installed inside unconditioned wall cavities and attic spaces lose much of their efficiency, as much as 25 to 40 percent of the heating or cooling energy if the ducts are leaky. Seal duct joints well with mastic, a special duct adhesive, to prevent leaks.

Create paneled boxes to cover exposed ducts in living spaces. Besides improving the appearance of the room, covered ducts are protected from damage and prevented from spewing dust or absorbing house odors or polluted air from one room to the rest of the building.