The answer to 'heat exchanger blowing cold air' is that there are several common causes, including a cracked or damaged heat exchanger, a problem with the blower fan, or issues with the thermostat or control board. This can result in the heat exchanger blowing cold air instead of warm air.
Some key symptoms to look for include cold air coming from the vents, the furnace not turning on, or the blower fan running continuously. Potential solutions may involve repairing or replacing the heat exchanger, troubleshooting the blower fan, or addressing issues with the thermostat or control board. In many cases, it's best to call a professional HVAC technician to diagnose and fix the problem, as working with a heat exchanger can be dangerous for homeowners.