Simple Explanations for Odd Duct and Furnace Noises

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iStock_000012354020Medium compressedYour home’s HVAC system is supposed to be a silent partner when it comes to heating and cooling your home. When noises develop within the ductwork, your first inclination may be to figure out what's going wrong. You can use this guide to diagnose the various duct and furnace noises that might be keeping you up at night or annoyed during the daytime.

Under Pressure

As the furnace provides conditioned air, the metal ducts undergo pressure changes depending on the temperature and volume of air that passes through. As the pressure builds up, it causes the metal ductwork to expand and contract as the pressure decreases. This series of expansion and contraction can create popping noises. It’s harmless, but it can drive you up the wall if it becomes loud and frequent enough.

The amount of popping you might hear comes down to the ductwork’s shape and thickness:

  • Most residential homes use rectangular shaped ductwork with 24 to 26-gauge sheet metal. (The higher the gauge, the thinner the metal.) Although it's affordable, it handles the least amount of pressure before it begins to expand and pop.
  • Some homes use square ducts, which offer increased strength over rectangular ducts and, therefore, less popping.
  • Round ducts feature a reduced surface area with no sharp corners, making them stronger and less subject to major pressure drops. These ducts are commonly found in commercial buildings.

You could solve the popping dilemma by swapping out part of the current duct system with new ducts that feature thicker sheet metal or a different shape, but it may be an expensive and lengthy job if you’re not already renovating your home.

If you’re using rectangular ducts, your HVAC specialist may add installation or deaden the sound by placing a crease or bend on the wider sides of ducts. Adding creases to this section helps stiffen the surface, preventing pressure changes within the ducts from causing a racket.

Popping Noises After Cleaning

In many cases, you may start hearing duct and furnace noises right after your home’s HVAC system has received a good cleaning. Fortunately, there's a simple explanation for this problem as well.

Popping noises are actually quite common with brand-new ductwork. As time goes on, the dirt and dust that naturally accumulates on the blower fan and other parts of the HVAC system help reduce the amount of pressure placed on the ductwork, which eventually brings an end to the popping noises.

Cleaning every inch of the HVAC system of dust and other debris restores lost pressure and causes the sheet metal to pop once more. In addition, it can also help improve your home’s indoor air quality.

Things That Go Boom in the Night

Among other duct and furnace noises, you may hear a "boom" when you turn the furnace on. In most cases, the cause can be traced back to a faulty burner assembly. Over time, rust or debris can block one or more burners, causing a momentary failure to ignite when it’s supposed to. It could take a large quantity of gas passing through the furnace to ignite the affected burner, resulting in the characteristic "boom."

You can easily fix this problem by having your technician thoroughly clean the affected burner or replace it all together. This should also be accompanied by a thorough furnace cleaning, as large accumulations of dust and debris can create a serious fire hazard.

Whistling Issues

If you’re hearing whistling noises as the HVAC system operates, especially after it’s been cleaned, it may be due to increased air flow passing through the heat vent louvers. You could try solving the problem with the following steps:

  • Make sure you have the right vent covers installed. Many homeowners accidentally use covers that are too large or too small for the ducts.
  • Use a lower fan speed to reduce the volume of air and hopefully eliminate the whistling noises. Unfortunately, this may not be a feasible plan during the summer, when the HVAC system needs all of the airflow it can get to prevent evaporator icing and other problems.

The next time you want to take care of those duct and furnace noises and need professional expertise, contact McWilliams & Son Heating & Air. Our team of qualified HVAC technicians proudly serve the heating and cooling needs of Lufkin and Nacogdoches area residents.