The answer to 'why is my heat exchanger' is that the heat exchanger in your HVAC system may be cracked or damaged, causing dangerous carbon monoxide leaks. Common causes include normal wear and tear over time, improper installation, or a malfunctioning furnace. Symptoms include strange noises, strange smells, and a furnace that turns on and off frequently.
If you suspect an issue with your heat exchanger, it's crucial to shut off the furnace immediately and call a professional HVAC technician. Attempting to inspect or repair the heat exchanger yourself can be extremely dangerous, as a cracked heat exchanger can release deadly carbon monoxide into your home. A qualified technician will be able to properly diagnose the issue and safely replace the heat exchanger if needed.