Which of the following can lead to a furnace backdraft?

A furnace backdraft occurs when exhaust gases from the furnace are not properly vented outside, causing them to flow back into the living space instead. This can lead to hazardous conditions, as carbon monoxide and other harmful gases can enter the home. Blocked flues or improper ventilation directly obstruct the path that exhaust gases need to take to escape, creating a buildup of pressure within the system that can force these gases backward.

In contrast, while excessive gas pressure can contribute to other safety hazards, it does not specifically lead to backdrafting. Inadequate insulation in the home primarily affects heating efficiency and comfort rather than the operation of exhaust systems. Faulty thermostat settings can certainly affect furnace operation, but they are not directly connected to causing backdrafts. Therefore, the presence of a blocked flue or improper ventilation is the key factor that can lead to a backdraft situation in a furnace.