Tips on Home Ventilation Duct Structure

Reducing Duct Volume

• One critical aspect of ventilation ductwork is the actual size of the duct should become smaller the farther from the air source (the air conditioner, furnace or heater blower). This constricting reduction in ductwork creates a forced air jet effect and results in adequate air reaching all distances from the blower. If all the ductwork was the same diameter, a large percentage of the air would exit through vents nearest the blower. If the air is forced into a smaller and smaller passage with distance, the result is higher air velocity, which forces the air through the ductwork's length.

Insulation

• When installing ductwork, the heating, ventilation and air conditioning (HVAC) technician should allow for adequate space for insulation. Insulation wrapped completely around the ductwork will keep cold air cold and warm air warm. This is necessary to reduce the amount of work the HVAC system does to lower a volume of air a given number of degrees.

  Insulation should only be outside the ducts. Insulation inside the ductwork will actually work against keeping the air a given temperature. The insulation will absorb heat from warm air and give heat to cold air. Also, insulation inside the ductwork could interfere with the free flow of air through the system.

Ductwork design

• When possible, it is best to design a duct system with few right-angle turns. Changing direction with an elbow-shaped curve improves total air flow and reduces the amount of air stalling in the ductwork when coming against a flat surface.

  Avoid installing ductwork outside the home. Even when insulated, the amount of direct heat or cold acting on the ducts will reduce the system's efficiency.

  Homes with pier foundations should have ductwork installed underneath in the crawl space with vents supplying each room. Homes built on solid concrete foundations will need to have the ductwork installed in the attic with ceiling vents. A flat-roof home on a slab with no attic will require ductwork suspended in each room. Each type of installation, however, should follow the same basic design criteria to move adequate air throughout the home at the proper temperature.