Look for leaks: water can leak down into your furnace several ways, ultimately causing failure. If you discover these leaks early it is an easy fix for your furnace technician.
• Check for water leaks from the air conditioning evaporator, located in the supply duct on top of the furnace.
• Humidifiers can also leak and damage the furnace.
• High-efficiency furnaces create a steady stream of condensation (water). If the drain tubes get disconnected, water leaks in and around the furnace. Since this condensate is also slightly acidic, it will corrode steel very fast.