The answer to 'when to replace heat exchanger' is that a heat exchanger typically needs to be replaced every 10-15 years, or sooner if it shows signs of significant wear or damage. The heat exchanger is a critical component of your furnace that transfers heat from the combustion process to the air that circulates through your home. Over time, the metal in the heat exchanger can become cracked or corroded, reducing its efficiency and potentially allowing dangerous gases like carbon monoxide to leak into your home.
It's important to have your furnace and heat exchanger inspected annually by a professional HVAC technician. They can assess the condition of the heat exchanger and let you know if replacement is recommended. Signs that your heat exchanger may need to be replaced include cracks, holes, excessive rust or corrosion, or poor heat transfer performance.