What is a significant risk that can lead to an explosion in the furnace of a boiler?

The significant risk that can lead to an explosion in the furnace of a boiler is the build-up of combustible gases. This situation occurs when there is insufficient ventilation or a malfunction in the fuel delivery system, causing gases that are not properly ignited to accumulate in the furnace or boiler space. These combustible gases can create a highly explosive environment, especially when they are mixed with air and then subjected to an ignition source.

Proper ventilation and regular maintenance are critical to prevent gas build-up and ensure the safe operation of the boiler. Monitoring system performance and ensuring all safety devices are functional can help mitigate this risk. In contrast, while other factors such as accumulation of water, overheating, or low water pressure can pose serious risks to boiler operation and integrity, they are less likely to directly lead to an explosive scenario compared to the specific hazard presented by the build-up of combustible gases.