To prevent the danger of a furnace explosion during start-up, what must the programmer allow?

The correct answer is allowing a pre-purge period to prevent the danger of a furnace explosion during start-up. This process involves clearing any unburned gas from the combustion chamber before ignition occurs. By evacuating potentially explosive gas, the pre-purge period helps ensure that the environment within the furnace is safe for ignition.

Pre-purging is a critical safety measure that prevents the accumulation of gas that might otherwise ignite all at once, causing an explosion. This procedure typically occurs right after the furnace has been turned off and before the start-up sequence is initiated. A thorough understanding of gas behavior and the necessary precautions during the furnace operation is essential for safe and efficient performance.

While options like a post-purge period, cool-down period, and safety shutoff period can play roles in furnace operation, they do not directly address the immediate risk of an explosion during the critical start-up phase, which is specifically mitigated by implementing a pre-purge period.