Troubleshooting a Furnace Duct: Banging Noises

A furnace near the window in a basement.

Furnace ducts are responsible for bringing air and circulating it within the home. There are two types of main components to your ductwork. You have the supply ducts that supply the fresh air into the home and you have the return ductwork that is responsible for taking the air, sucking it out of the room and sending it back through the heating and cooling system of the home. Both components are equally important in providing a proper temperature and air flow throughout the home.

Sometimes the ductwork makes noise and bangs. Below are a few ways to troubleshoot those banging noises to bring you back peace of mind.

1 - Expansion and Contraction

One of the most common causes of banging furnace ductwork is simply the expansion and contraction of the furnace and air duct system. When the system turns on and off, the ducts may expand or contract from the change in temperature and you may hear small and large banging noises.

2 - Check the Dampers

In order to reduce or completely remove the noises that occur in your furnace ductwork, you can check the dampers. The dampers are either set in an on or off (open or closed) position. Look at those dampers that are on the supply side of your ductwork (the ducts responsible for supplying the air flow, not the returns). If you have many dampers closed on the supply side, then this could create the banging noises. Open one damper at a time and gauge the changes in the noise that is coming from your ductwork. If the noise begins to subside, then you should continue to open dampers until the noise has dissipated or disappeared completely.

3 - Add a Filter

To reduce the noise that is created you can also add a non-pleated filter inside the ductwork. Attach the filter using a couple metal braces inside the ductwork.

4 - Adjust the Actuator

Inside the duct dampers, there is an actuator. Adjust the actuator up or down in order to change the banging. The adjustments may even cause the banging noise to stop altogether. If you are unsure how to do this properly, consult an HVAC technician for advice.

5 - Clean the Ductwork

Clean your ductwork thoroughly. Sometimes items or excess dust, dirt and grime gets trapped inside the ductwork and causes the air to flow improperly through the ducts. This can promote banging and knocking noises. In order to have your ductwork cleaned properly, you should hire a professional duct cleaner or rent professional duct cleaning equipment.

6 - Clean the Fan

There is a fan that is located inside the ductwork for the air conditioning and heater. Make sure you try and clean the fan. The fan may also cause the banging and cracking of the ductwork.

7 - Add Insulation

Another trick to reducing or stopping the banging noises is in adding insulation around the ductwork. Add insulation to the entire run of ductwork that is either in your basement or your attic. Make sure you use insulation that is specially formed for insulating the ducts.

Now that you're done troubleshooting your furnace duct issues, there should be no more of those baking noises. Enjoy the peace and quiet.