What situation has the greatest chance of igniting?

The scenario involving wood stored against a heating duct presents a significant ignition risk due to the heat transfer from the duct to the wood. Heating ducts can reach high temperatures, especially if they are handling hot air from a furnace. If wood is in close proximity to these ducts, it can easily absorb the heat and eventually reach a temperature that causes combustion.

Additionally, this situation is exacerbated by the fact that wood is a combustible material. The proximity of a heat source, particularly one that is frequently used or can generate significant warmth, creates a direct and potentially dangerous path for ignition.

In contrast, while electrical wires near water sources can present electrical hazards, the direct ignition potential is lower unless there’s a short circuit or other specific circumstances that lead to sparking. Paper products in direct sunlight may degrade or become brittle over time due to UV exposure, but sunlight alone typically does not provide sufficient energy to ignite paper without an additional heat source. Heating appliances placed on wooden surfaces can result in combustion; however, if properly rated appliances are used and maintained, they typically have safeguards to prevent overheating, making them less likely to cause ignition compared to the direct and constant heat exposure associated with wood against a heating duct.