What type of maintenance does spalling indicate is needed in a furnace?

Spalling is a condition that refers to the breaking off or chipping away of material, often occurring in furnace linings or refractory materials. When you observe spalling, it indicates that the insulation or refractory lining is deteriorating, which can compromise the furnace's efficiency and safety.

In this context, the correct answer highlights the necessity for insulation replacement, as replacing the spalled insulation will restore the furnace's thermal efficiency and structural integrity. Addressing spalling promptly by replacing the damaged insulation prevents further heat loss, protects other components of the system, and promotes safe operation.

Other choices, while relevant to furnace maintenance, do not directly relate to the issue presented by spalling. Regular cleaning would help maintain performance but wouldn't specifically tackle the damage shown by spalling. Pressure testing assesses the integrity of the pressure vessel, which is not directly related to spalling effects. Temperature monitoring is essential for operational efficiency but does not address the material degradation indicated by the presence of spalling.