A common reason for water under a furnace is a clogged condensation drain. Condensate is the name given to the moisture that has been condensed out of the air in your home when the air conditioner is running. This can be the result of clogged condensation tubing, breaks in the condensation line or other possible issues with the pump. Humidifier issues are also a possible cause of furnace leaks. A clogged filter or drain in the humidifier can easily cause any furnace to produce unwanted moisture. High-efficiency furnaces create condensation through the heat exchange process. When operating normally, this condensation is drained safely out and away from your furnace through the condensate line and drain trap. If there's a leak or a clog in the condensate lines, you'll quickly find a puddle around your furnace.