Question:
The metal ductwork in the ceiling of our finished basement pops and bangs every time the furnace comes on in the winter, then makes more noise after the furnace shuts off. Is there anything we can do to stop the banging?
—William Tucek, Seattle, Wash.
Pro Tip: Richard Trethewey notes that it’s no secret that sheet-metal ducts expand when they get hot and shrink when they cool. If your ducts are making loud clicking and knocking sounds after the furnace shuts off, the thermal contraction of the metal is likely the culprit.
This Old House general contractor Tom Silva offers another approach for reducing HVAC noise: “The first and cheapest option is to have all the equipment serviced. Tell whoever is doing the work that you want to minimize noise. A little lubrication or adjustment might do a lot of good. You can also cover the ducts with sound-insulating board.” He adds that filling the joist cavities with batts of fiberglass insulation can also help dampen the sounds that travel through ductwork.
Richard Trethewey, This Old House plumbing and heating expert, replies: “Metal ducts make all sorts of noises. In some cases, excessive air flow can cause whistling. Sometimes, pressure differences between the supply and return ducts can move their walls in and out, resulting in a rumbling sound called ‘oil-canning.’ And ductwork can carry the sound of a noisy blower all over the house. But I suspect your problem is caused by hot air rushing into a cold duct, which makes the metal expand.”
Answer:
Metal ducts make all sorts of noises. In some cases, excessive air flow can cause whistling. Sometimes, pressure differences between the supply and return ducts can move their walls in and out, resulting in a rumbling sound called “oil-canning.” And ductwork can carry the sound of a noisy blower all over the house. But I suspect your problem is caused by hot air rushing into a cold duct, which makes the metal expand. You might hear some innocuous creaking, but if the duct is rubbing against wood, it might suddenly “pop” and reverberate, then pop again as the metal cools.
What can you do? Short of covering all your ducts with fiberglass insulation, or opening them up and inserting acoustic duct liners, the best you can hope for is a modest reduction in the noise. Use rubber pads to isolate ducts from wood. Make sure there’s nothing blocking the air register at the return, which can lead to pressure problems. Have an HVAC technician check the system to make sure it’s not pushing more air than it was designed for. And if your furnace doesn’t already have a flexible transition to isolate furnace noises, ask the technician to install one.
Pro Tip: Richard Trethewey advises checking for another common culprit: “Replacing your return filter is like washing your feet”—it’s basic maintenance that many homeowners overlook. A clogged return filter restricts airflow and forces air through smaller openings at higher velocity, which can cause whistling at the register.
Ross Trethewey demonstrated on Ask This Old House that closing down individual registers to reduce airflow in one room actually increases air velocity—and noise—elsewhere in the system, since the ducts are all interrelated. Instead of choking down registers at the room level, he recommends adjusting the air handler speed or using balancing dampers back near the main branch to find the sweet spot between comfort and quiet. “You got to find that balancing act between reducing that for sound but also not reducing it too much so you have a performance issue,” Ross explains.
