Which condition can increase the likelihood of a furnace explosion?

The condition that significantly increases the likelihood of a furnace explosion is the admission of fuel before an ignition source. When a furnace is filled with flammable fuel without an appropriate ignition source, the combination can create a highly explosive environment.

In typical furnace operations, proper sequencing is crucial; fuel must be strictly controlled and only introduced when there is a reliable ignition system in place. Allowing fuel to accumulate without ignition can lead to a rapid build-up of combustible gases. If these gases encounter an ignition source unexpectedly, it can result in a powerful explosion capable of causing significant damage and posing serious safety risks.

This understanding highlights the importance of maintaining strict safety protocols in handling fuel in industrial settings, ensuring that fuel is introduced only when it's safe to do so, keeping operations within regulated and safe parameters to prevent disasters.