The answer to 'why is my heat exchanger not working' is that the heat exchanger is likely cracked or damaged, preventing it from properly heating your home. This is a serious issue that requires immediate attention, as a faulty heat exchanger can release dangerous carbon monoxide into your home.
Some common causes of a malfunctioning heat exchanger include normal wear and tear over time, corrosion, cracks or holes from physical damage, or improper installation. Symptoms may include reduced heating output, strange noises coming from the furnace, or the furnace cycling on and off frequently. In some cases, a cracked heat exchanger may also trigger the furnace's safety sensors, causing it to shut off entirely.