The answer to 'why is my furnace smells bad' is that a smelly furnace is often caused by a buildup of dust, debris, or other contaminants inside the unit. This can lead to the growth of mold or mildew, which produces unpleasant odors. Another common cause is a problem with the furnace's air filter, which can become clogged and restrict airflow, leading to stale or musty smells.
In some cases, the smell may be due to a more serious issue, such as a gas leak or a problem with the heat exchanger. These types of problems require professional attention and should not be ignored, as they can be potentially dangerous.