The answer to 'how to maintain heat exchanger' is to regularly inspect, clean, and service the heat exchanger to ensure it is operating safely and efficiently. This includes visually inspecting the heat exchanger for cracks, corrosion, or other damage, cleaning any buildup of soot or debris, and testing the heat exchanger for proper operation.
It's recommended to have a professional HVAC technician service the heat exchanger annually as part of a routine maintenance check-up. They have the specialized tools and expertise to properly evaluate the heat exchanger and make any necessary repairs or adjustments. Attempting to service the heat exchanger yourself can be dangerous if you are not properly trained, so it's best to leave this task to the professionals.