What serious condition can arise from a blocked flue in a gas furnace?

A blocked flue in a gas furnace can lead to carbon monoxide buildup, which is a serious and potentially life-threatening condition. The flue is essential for venting combustion gases, including carbon monoxide, safely out of the home. When the flue is blocked, these gases cannot escape, leading to their accumulation in the living spaces. Carbon monoxide is odorless and colorless, making it particularly dangerous, as it can cause poisoning and serious health effects without warning.

While increased energy costs, shortened furnace lifespan, and reduced heating capacity may also result from a blocked flue or other issues with a gas furnace, they do not pose the immediate and severe health risk that carbon monoxide does. Addressing flue blockages promptly is crucial for maintaining safety and ensuring proper furnace operation.