Long duct runs also have more potential for leaks, because there are more sections of ducks that can fail or become loose. Leaky ducts, in fact, are a primary cause of heat loss in homes, which affects the heating and cooling bills. And longer runs are more fertile ground for mold and other indoor pollutants.
Duct material is typically made from thin sheet metal that is folded in welded into rectangular or around sections. These sections are connected into runs with screws and duct tape, the latter sealing the joints between sections. Sheet metal is not only durable, but acts as a thermal conductor to help maintain temperature is the heated or cooled air travels through the ducts.
In some cases and especially in warmer regions of the country, duct runs are constructed of sections of coiled metal wrapped in a thin, insulated sheathing. A bright, reflective layer on the sheathing acts as a thermal barrier to block heated or cooled air from escaping.
The flexibility of these ducts, while more difficult to connect and seal and sheet metal, makes them better able to accommodate high framing conditions to reach certain areas of the house. On occasion, flexible ducts serve as branch sections from the main sheet metal duct.
Both sheet metal in flexible duct runs require support for longer length. Sagging duct runs not only lead to failures at the joints, but reduce the performance of the forced air system. And, like plumbing pipes, ducts are generally kept out of the exterior walls to mitigate heat loss through the walls to the outside. Attic insulation can often help with such issues.
As with the plumbing rough, the homes frame is often designed to accommodate duct runs in and around the structure. In a basement or insulated crawlspace, deduct runs extends from the furnace directly to the registers cut into the floor above and around the air conditioner.
An improperly sized HVAC unit or system, whether providing too much or too little capacity for the space and conditions, wastes energy and reduces the effective life of the equipment. An undersized furnace, for instance, must work harder and suck off more gas or electricity to heat the house, straining its inner workings in the process.