what cause furnace blow back?

Furnace blowback typically occurs due to a blockage in the exhaust vent, which prevents combustion gases from properly exiting the system. This can be caused by debris, a bird's nest, or a buildup of soot. Additionally, improper air pressure or a malfunctioning blower fan can contribute to blowback, leading to gas accumulation and potentially dangerous situations. Regular maintenance and inspection of the furnace and venting system can help prevent this issue.