How to Muffle Noisy Furnace Ducts
We can't sleep! What's the solution?
Our oil-burning furnace heats up the house nicely, but after it shuts off, the ducts in the basement make such loud clicking and knocking sounds that we can’t sleep. How do we put an end to the noise? —Simon Rosenfeld, Laytonsville, MD
Richard Trethewey replies: It’s no secret that sheet-metal ducts expand when they get hot and shrink when they cool. There’s nothing to stop that from happening. But the noises you describe are another story. They’re probably being caused by the metal rubbing against the wood framing or otherwise being restricted from moving at various points. Those choke points can be padded and muffled to quiet your unwelcome symphony.
Go to the basement where the ducts are exposed, look for any spots where they’re touching wood, and listen for squeaks. When you pinpoint them, insert rubber pads to keep the two materials separate. That alone should tamp down a lot of the cacophony you’re hearing.
Next, wrap the ducts in R-6 foil-faced fiberglass—sold as duct wrap (shown)—to help keep them from heating up the neighboring pipes and to muffle reverberations in areas you can’t reach. When you’re done, you might find your house warms more quickly, as less heat will be lost through the duct walls. As a side note, the same strategy of isolating and insulating works for noisy hot-water supply pipes.
Finally, have a licensed HVAC specialist check where the main supply duct connects to the furnace. There should be a flexible expansion-joint collar in that location to isolate the duct from noise generated by the furnace blower and to absorb duct movement. If there is no collar, have one installed. Also, ask the specialist to check that the blower is sized correctly. You don’t want one that pushes more air than your ducts are designed to handle.
It’s a good thing that you aren’t losing sleep due to noises from the ducts in your walls. Fixing them would require tearing back the drywall and installing spacers and insulation where necessary. Wearing earplugs might be an easier solution!