Heater vents can emit odors for a variety of reasons, but some common culprits are animal excrement or urine, dead animals in the duct work and mold. Gas, oil and burning smells indicate a potentially imminent danger.
Animals can track dust, dirt, excrement and other materials into a home. These contaminates end up being circulated through the heater vents. In addition, animals can sometimes enter ductwork and become trapped. A sweet, sickly smell or a nauseous stink indicates a dead animal. A musty smell, especially after rain, indicates the presence of mold. The smell of burning plastic or rotten eggs indicates an electrical malfunction and natural gas leakage, respectively.