When Should You Replace the Ducting in HVAC Systems?

Upgrading Your System

• When upgrading HVAC equipment, your contractor will utilize Manual J, the industry standard software to calculate the precise heating and cooling load of your home. With that data, he can recommend a furnace or air conditioner with the required capacity to do the job in the most energy efficient way. However, if old, existing ductwork is too small or large to properly accommodate the capacity specified by the Manual J calculation, new ducts should be installed to facilitate the performance and efficiency of the new system. This frequently is the case when upgrading to a heat pump from a conventional fossil fuel system. Heat pumps require higher system air volume to properly heat and cool the home, and existing ducts may not accommodate that specification.

Too Much Ductwork Is Outside the Home’s Thermal Envelope

• Ductwork routed through unconditioned zones such as the attic, an unconditioned basement or the crawlspace under the house poses several drawbacks. First, thermal loss from the ducts because of acute temperatures in these zones wastes energy and may not be adequately offset by insulating the ductwork. Leakage of conditioned air into unconditioned zones means the heating or cooling effect is completely lost, while air leaks from ductwork contained inside the thermal envelope still benefits the home. In addition, leaks in return ducts frequently draw air into the system versus spilling air out. Return ducts in unconditioned zones such as crawlspaces may pull in unhealthy air contaminated with mold, bacteria or other pollutants, and disperse them into living spaces. Replacing ductwork routed through unconditioned areas with ducts inside the conditioned envelope alleviates these negative consequences.

Heating and Cooling Expenses Soar

• If you’ve noticed steadily increasing utility costs and have eliminated other possible causes, your ductwork may be deteriorated and silently leaking. A study prepared by the Brookhaven National Laboratory for the U.S. Department of Energy reports that typical residential duct systems lose 25 to 40 percent of heating and cooling energy through a combination of duct leakage and conduction. While a certain number of pinhole leaks and leakage at elbow joints can be effectively sealed, extensive rust and corrosion, widespread seam deterioration, and collapsed ducts may require ductwork replacement. An HVAC contractor can pressurize the ducts and perform a leak test to determine the percentage of leakage in relation to the total system airflow. This figure will help determine whether replacement is necessary.

Inadequate Returns

• Airflow in an HVAC system enters rooms through supply ducts, then flows into return ducts to be conveyed back to the air handler. Instead of dedicated return ducts in individual rooms, however, many homes were built with a single return register and duct located in a common area such as a hallway. The air path to a single return may be obstructed when doors to rooms are closed, or the return may not be centrally located to efficiently convey air back to the air handler. This results in a chronically air-starved system and disruption to air balance in rooms. Replacing the single common return system with dedicated return ducts in each room improves performance and efficiency.