The answer to 'furnace smells bad' is that there are several common causes for a foul odor coming from your furnace, including dirty filters, buildup of dust and debris, and issues with the heat exchanger or combustion system. Symptoms may include a musty, burning, or rotten egg-like smell, and in some cases, the odor can be a sign of a serious problem that requires immediate attention.
If the smell is mild and the furnace is still functioning properly, you may be able to address the issue with some basic DIY maintenance. Change the air filter, vacuum out any visible dust or debris, and check for any blockages or obstructions in the ductwork. However, if the smell is strong, persistent, or accompanied by other issues like strange noises or reduced airflow, it's best to call in a professional HVAC technician to diagnose and resolve the problem.