What happens during the pre-purge phase of a forced air furnace?

During the pre-purge phase of a forced air furnace, the primary objective is to ensure safety and proper operation by clearing out any potential gas vapors that may have accumulated in the combustion chamber or duct work before ignition occurs. By circulating air through the system, this phase helps to expel these gases, which can be crucial for preventing hazards such as explosions or harmful emissions when the furnace ignites.

This process typically involves the blower motor operating to move air through the system and can last for a specific duration set in the furnace's control system. This is essential for ensuring that when ignition does occur, it does so in a safe environment, free from combustible gases that could otherwise pose a risk to the system and the occupants of the building.

The other processes mentioned, such as sealing off air ducts, bypassing the thermostat, or igniting the furnace itself, do not take place during the pre-purge phase and occur in different stages of the furnace's operation.