What Are the Causes of Popping & Banging Noises in Furnace Ducts?

Improper Installation

• Furnace ductwork should be isolated from wood framing to avoid the noises that occur when the ducts expand and make physical contact with the wood. Experienced heating system installers ensure that spaces are large enough to accommodate ductwork and that the duct is free to move when heating alters the internal and external dimensions of the ducts. Allowance must also be made for lumber shrinkage over time. Openings created when the house was built may become insufficient as the home ages.

Temperature Fluctuations

• Hot air rushing suddenly into a cold metal duct may initiate a banging noise as the metal expands and a pop as the ductwork cools and contracts. Wrapping the ducts with fiberglass insulation, or installing acoustic liners may mitigate the sudden changes in temperature. Rubber pads may be placed between the metal ductwork and the wooden framing to isolate expanding metal from stationary wood framing members.

Damper and Actuator Adjustments

• Your ductwork may be making excessive noise if too many dampers on the supply side of your system have been closed. By opening one damper at a time until a discernible difference is noted, you can reduce or eliminate the banging noises. Each duct damper has an internal actuator that may be incorrectly adjusted. A certified HVAC technician can advise how to correctly set damper actuators to reduce or eliminate banging noises.

Dirt and Dust Accumulation

• Banging and knocking noises in your ductwork may be the result of restricted airflow. Ductwork should be cleaned annually by a professional duct cleaner or by renting duct cleaning equipment from a tool rental outlet and performing the job yourself. As part of this scheduled maintenance, the air-circulation fan inside your ducts should be thoroughly cleaned and any internal non-pleated air filters installed in the ducts should be replaced.