The answer to 'why is my heat exchanger leaking water' is that a leaking heat exchanger is often caused by a crack or hole in the metal heat exchanger. This allows condensation or combustion byproducts to leak out, which can lead to water puddles around the furnace. Common causes include normal wear and tear, corrosion, or physical damage to the heat exchanger.
Some other signs of a leaking heat exchanger include strange smells, soot buildup, or water stains around the furnace. This is a serious issue that requires prompt attention, as a cracked heat exchanger can allow dangerous carbon monoxide to escape into your home. If you suspect a leak, it's best to call a professional HVAC technician to diagnose and repair the problem.