• Checking the vent system for clogs, blockages, holes, or leaks
• Checking the heat exchanger for rust, dents, and other signs of damage.
• Checking the burners to ensure proper operation in accordance with the furnace specifications.
• Check wiring/switches for proper function, signs of damage, etc.
• Filters need to be checked/replaced - Note that it is wise to check your furnace filter quarterly to ensure that it is not clogged. A clogged filter greatly reduces the efficiency of the furnace in addition to no longer filtering particulates from the air supply.