The answer to 'how much to fix heat exchanger' is that the cost can vary significantly depending on several factors. Generally, the repair of a heat exchanger can range from a few hundred dollars to over a thousand dollars, depending on the extent of the damage, the age and model of the HVAC system, and the labor costs in your local area.
Some key factors that influence the cost include the type of HVAC system, the accessibility of the heat exchanger, the complexity of the repair, and whether any additional components need to be replaced. It's important to have a qualified HVAC technician inspect the system and provide a detailed estimate before proceeding with any repairs.