The answer to 'why is my heat exchanger short cycling' is that there are several common causes, including a faulty thermostat, issues with the blower motor, problems with the heat exchanger itself, or an issue with the furnace's control board. Short cycling occurs when the furnace turns on and off rapidly, which can be caused by a variety of underlying problems.
Some of the key symptoms of a short cycling heat exchanger include the furnace turning on and off frequently, inconsistent heating throughout the home, and higher than normal energy bills. To diagnose and fix the issue, it's generally best to call in a professional HVAC technician, as they have the expertise and equipment to properly inspect the system and identify the root cause.