The answer to 'why is my furnace leaking water' is that there are a few common causes, including a clogged condensate drain line, a cracked heat exchanger, or issues with the furnace's condensate pump. Leaking water from a furnace is usually a sign of a problem that requires professional diagnosis and repair.
The most common cause of a furnace leaking water is a clogged or blocked condensate drain line. As the furnace operates, it produces condensation that needs to be properly drained away. If the drain line becomes clogged with dirt, debris, or algae growth, the water has nowhere to go and will start leaking out of the furnace. Other potential causes include a cracked heat exchanger, which can allow water to leak, or issues with the condensate pump if the furnace has one installed.