The answer to 'why is my heat exchanger smells bad' is that a bad smell coming from your heat exchanger is typically a sign of a serious issue that requires professional repair. The heat exchanger is a vital component of your furnace that transfers heat from the combustion process to the air circulated through your home. If it's producing a foul odor, it could indicate a crack or leak that is allowing dangerous combustion gases to escape.
Some common causes of a smelly heat exchanger include a buildup of dust, debris, or soot, a cracked heat exchanger, or a problem with the furnace's combustion process. This can allow carbon monoxide and other hazardous gases to leak into your home, posing serious health and safety risks. It's important to address this issue promptly and have a qualified HVAC technician inspect and repair the problem.