Odors within your HVAC system can quickly spread throughout your entire home, making them quite hard to miss. However, different types of odors can point to various issues with your system. Being able to identify smells early can help make it easier for you to spot problems as they arise and before they have the chance to grow more serious and thus harder and more expensive to fix.
One of the most common smells that you will likely experience at one time or the other from your HVAC system is the smell of burning. This happens normally when you turn your furnace on for the first time after a long winter, as there will be a small layer of dust over the heating element that will be burned away in a matter of minutes. However, if you notice that the smell of burning gets progressively worse with time, comes with smoke, or is coming out of your air conditioner instead of your furnace, there may actually be an electrical issue with your HVAC system that is contributing to the risk of an electrical fire occurring. You should turn your unit off until an AC repairs professional has had the chance to take a look at the system and determine if you need to have any repairs done.
Another common smell that can come out of your HVAC system is an earthy or musty smell, sort of like wet earth or plant growth. This is a serious concern because it points to mold and mildew growth somewhere within your ductwork or your air conditioner and furnace themselves. Mold and mildew can take hold in your HVAC system fairly often throughout the summer if you live in an area with high humidity. Condensation can form in the ductwork, and the damp and dark environment is ideal for the growth to continue unchecked. Avoid running your HVAC system until a professional has had a chance to clean it out, as mold spores in the air can aggravate and cause respiratory health problems.
Finally, the last and most significant smell that can come out of your HVAC system that you must watch out for is the smell of rotten eggs. This points to a gas leak in your furnace, which is a major safety risk. At the first whiff of rotten eggs, you should turn your HVAC system off and contact a furnace professional and emergency services to report a gas leak.