What causes a furnace to explode?

A furnace can explode due to several factors, primarily involving the accumulation of gas and improper ventilation. If there is a gas leak or if the burners are malfunctioning, unburned gas can build up in the combustion chamber. When this accumulated gas ignites suddenly, it can cause an explosion. Additionally, blockages in the ventilation system can lead to pressure buildup, further increasing the risk of an explosion.