HVAC noises

3 Smells, Sounds, and Sights That Indicate an HVAC Issue

April 26, 2016

The human body relies on the senses to make decisions and navigate the world. Certain smells, sounds, and sights immediately notify you when something is wrong. When you notice one of the following smells, sounds, or sights in your home, how will you react?


Some people might consider a bad odor to be a simple inconvenience. Sometimes a strange smell might not mean anything, especially if your heating or cooling system just barely booted up. But one of these three odors might indicate larger-and even potentially dangerous-issues.

Moldy Smell

If you smell something musty or moldy coming from your heating or air conditioning vents, you likely have a mold issue. Air conditioning units in particular end up with a lot of moisture in the system, and moisture creates an environment for mold. If you suspect mold, you should look into the problem right away, since breathing in mold might lead to a respiratory infection.

Electrical or Burning Smell

If you smell a heavy electrical odor, or if it smells like something is burning, you might have an issue with your HVAC system. Occasionally, these smells might be expected. If your heating system is starting up for the first time this season, or if you just installed a new furnace, a burning odor should cause no alarm. If this smell continues, however, you might have an electrical issue or an oil leak. Depending on the issue, an electrician or a heating specialist can fix the problem and remove the danger of a fire. While rare, HVAC-induced fires can cause serious damage when they do happen.

Gas Smell

Often described as a rotten-eggs smell, you should address a gas odor quickly. Natural gas is odorless, but most utility companies add chemicals like mercaptan to the gas so you can smell the gas if it leaks. A gas leak means combustible gas floating around your home or apartment and represents a serious explosion risk. Natural gas is generally very safe, and gas explosions are rare. Still, if you smell a pervasive gas odor you should carefully open a few windows, leave the building, and then call the gas company.


Most machines will make some type of sound. Modern HVAC appliances run much quieter than previous iterations, but you can still expect a whir or the occasional popping sound. However, obtrusive and jarring sounds usually indicate that something isn’t quite right with your furnace or air conditioning unit.

Squeals and Shrieks

A squealing sound usually indicates a bad belt. The belt is essential for your HVAC’s motor system to run properly. While a bad belt won’t cause too many problems on its own, you want to replace it before it breaks. A broken belt will shut down the motor completely.


If you hear rattling around the house, you might think you’re just hearing things. But the rattling sounds could actually be items in your ducts-especially if you have vents in the floor. If a small toy, like a Lego, falls through the vents, you can imagine where those noises come from. If the items causing the noise are still close to the vent, you might be able to grab them yourself. Otherwise, you can call a professional to clear out the noisy objects.


Scraping indicates metal rubbing against metal, and this sound usually means there’s a loose part in your unit. The harsh noise might convince you to get the problem fixed quickly, but it’s good for your HVAC system if a professional checks out scraping sounds before the issue gets worse. For example, if part of the blower system becomes completely detached, your problems will only multiply.


Your heating and cooling systems are nearly always out of sight, so you’re more likely to encounter a strange smell or sound. Still, there are a few things to look out for.


As mentioned before, air conditioning systems tend to build up moisture. That moisture should generally stay inside the system. If you notice puddles collecting near your A/C unit, you likely have a leak somewhere. The puddle is a slip hazard and increases the risk of mold, but water leaks could also be the sign of more serious internal problems in your air conditioning system.

Yellow Flame

Your gas heater should generally run with a blue flame. A yellow flame could indicate that other materials are burning alongside the methane. Depending on what those materials are, you could have a problem. Flame coloring can vary a bit, so don’t panic. Just have a professional check out your heater.

High Energy Bill

If you see a big spike in your energy bill, you might curse your luck, sign the check, and forget about the issue. But a sharp change in your bill could indicate another issue. Perhaps a part of your HVAC system isn’t functioning properly, and the system needs to work overtime to compensate. If a high energy bill or any of these other sights, sounds, or smells has you worried, seek the help of an HVAC technician. A professional opinion will give you peace of mind, and a professional’s touch can fix any issues your heating or cooling system faces.

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