Although it may be a disgusting thought, bear in mind that many of the odors coming up through your registers will likely be animal related. These may be due to “animal smells” getting lodged in the filters, thereby sending the odor throughout the home through the ductwork. It could also be due to a mouse or rat that has died either in the ductwork itself or inside a building cavity. Thus, if replacing the filter doesn’t work, you may opt to call a professional who can locate the source of the stench and remove it.
Combustion Flue Gases
Combustion flue gases are dangerous and are loaded with carbon monoxide, so if you smell things that seem out of place – i.e. a “burning” scent, LP gas or oil, or anything else that you shouldn’t notice – it is crucial to contact someone right away, particularly if you do not have carbon monoxide detectors in your home. The release of flue gases into the ductwork can be caused by any number of things, but it is most commonly caused by a hole in the heat exchanger that allows the gases to seep into the ducts.
Mold odors are also incredibly common in ductwork, but they can also be dangerous. Mold can have a severe impact on your overall health and cause persistent allergy symptoms, infections, and even permanent damage to your body. If you notice a mildew smell emanating from your vents, then it is best to contact a professional who can not only locate the source of the mold, but figure out how it got there in the first place. It is best to have it removed as soon as possible.
Septic or Sewer Odors
Another common odor that homeowners report, primarily in the winter season on days or nights when the furnace is used, is a sewer smell. There are a number of causes for this, but it might be that a sewer line has cracked or is leaking onto ductwork. Thus, when the furnace is running and air is coming from registers, it is bringing that rather strong sewer smell with it. It may also occur when the condensate drain is connected to the sewer without a trap. In this case, sewer gas may leak up through the condensate drain and into the furnace. A professional plumber can install a trap in the condensate line to alleviate this problem.
Remember that if you are smelling mold, flue gases (or the odor of combustion byproducts), or sewer odors in your home, it is important to have these issues addressed immediately as they present serious hazards to your health and perhaps even the risk of explosion in the case of methane (sewer) gas in high concentrations.
Strange and unpleasant odors in your ductwork can be frustrating or even embarrassing when you have friends and family in your home. If you’ve noticed some unusual smells emanating from your registers and you’d like to take care of them in time for the holidays, you’ve come to the right place. Though some of the odors might be considered benign and simply annoying, others have the potential to be dangerous. You may need to clean, repair, or replace parts of your ductwork to solve the problem.