Several factors can cause soot buildup in heat exchangers and vents. Incomplete combustion, where the fuel doesn't burn completely, is one common cause. This can happen due to issues such as improper airflow, dirty or clogged filters, or poor ventilation. Improper combustion can also occur if there is a cracked heat exchanger, leading to soot buildup. Additionally, rich fuel-air mixture, fixed gas pressure or lack of regular furnace cleaning can contribute to excess soot accumulation. Addressing these causes promptly is essential to ensure optimal performance and safety.