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Duct noises can be alarming. If the noise just suddenly starts happening, your first thought may be that something’s wrong with the HVAC system and that an expensive repair is on the horizon. Fortunately, the cause of most duct noises is often not a reason for alarm.

What’s That Noise?

Chances are, the main noises you’ll hear from ductwork are booms, bangs and pops. Generally, the noises occur after the HVAC system comes on and the conditioned air begins rushing through the ductwork on its way to being delivered in your home. The noises are usually caused when the thin sheet metal of the ductwork expands to accommodate the sudden change in pressure caused by the airflow.

Quite often, these sorts of pops and bangs occur right after a duct cleaning. By removing dust and debris from dirty ductwork, the air can flow unimpeded through the ducts and may well cause the metal sides to expand in a way that didn’t occur prior to cleaning. Air pressure is measured in cubic feet per minute, or cfms, and the increased cfms that occur after a good cleaning will likely cause some popping. That said, bangs and pops can also occur in dirty ductwork.

Size and Thickness of Ducts Count

Some ductwork may be more prone to popping or banging than others. Ductwork is found in three shapes: round, square and rectangular. These three shapes don’t handle air pressure the same way. Round or circular ducts can handle more air pressure before popping, followed by square and then rectangular. The rectangle has two short sides and two long sides, and it’s the long sides that pop.

Most residential ductwork is made from thin sheet metal, in contrast to the thick sheet metal used in commercial ductwork. That’s another reason for residential ductwork being prone to popping or other sounds.

Other Causes of Ductwork Popping

There may be several other causes of ductwork popping beyond cleaning.

  • Delayed ignition in a furnace can cause ductwork to pop. Furnaces have five or six burners that, over time, may become blocked with dirt. When the thermostat calls for more heat, gas will be emitted, but if the burners are blocked, they can’t ignite. What usually happens is that the ignition is delayed by, in some cases, as much as a couple of minutes while the gas builds up. When ignition is finally achieved, there may be a pop.Two people can determine if this is happening by having one initiate the call for heat, while the other either watches the burners or stands by the furnace to see how long it takes for ignition. If delayed ignition is occurring, it needs to be fixed or a cracked heat exchanger could be the result. Make sure burners are cleaned yearly.
  • Undersized ductwork can cause popping. Ducts on the supply side expand with the increase in pressure, creating a slight vacuum on the return side. Ducts on both sides will expand and contract, and then when the blower comes on, the ducts will begin to emit a banging sound. Noises may also be caused by the ductwork hangers making contact with the ductwork when it expands. Insulating the ducts may be beneficial.
  • Sometimes duct noises originate from the duct joint between the trunk and plenum. This joint is under considerable fluctuation in temperature because it is located near the furnace. An expansion collar may help.

Solutions

If you determine that your home’s ductwork is making popping or banging noises because of the expansion of the metal when the conditioned air turns on, and you determine that the sounds aren’t because of a malfunction, then you may decide that you can just live with the banging, annoying as it may be. On the other hand, if you want to remedy it, you may have to look into changing the ductwork.

Two options to consider are replacing ducts with those made of thicker gauge metal, or else changing the shape.

Buildup of dirt and debris can also cause a variety of noises, so try having the ducts cleaned if they haven’t been in some time.

For more information on how to identify the cause of and how to reduce duct noises, contact us at Aqua Plumbing and Air. We serve Sarasota, Bradenton and the surrounding area.