The answer to 'when to replace heat exchanger' is that a heat exchanger should typically be replaced every 10-15 years, or sooner if it shows signs of cracks, corrosion, or other damage. The heat exchanger is a critical component in a furnace or boiler that transfers heat from the combustion process to the air or water that circulates through your home. Over time, the metal of the heat exchanger can become worn down, develop cracks, or even completely break down, which can be a serious safety hazard.
It's important to have your HVAC system inspected by a professional technician annually to check the condition of the heat exchanger and identify any potential issues. If the heat exchanger is cracked or otherwise compromised, it will need to be replaced to ensure safe and efficient operation of your heating system.