The answer to 'heat exchanger blowing cold air' is that the heat exchanger in your HVAC system is likely not functioning properly, causing cold air to blow instead of warm air. This can be caused by a cracked or damaged heat exchanger, a problem with the gas supply, or issues with the blower motor or other components.
Some common symptoms of a faulty heat exchanger include cold air blowing from the vents, inconsistent or unreliable heating, and unusual noises or smells coming from the HVAC unit. If you notice these issues, it's important to have a professional HVAC technician inspect the system as soon as possible, as a malfunctioning heat exchanger can be a serious safety hazard.