To solve a water leak from a furnace, the cause of the leak must first be identified. Common causes of water leak from a furnace include furnace condensation, a faulty secondary heater, an issue with the furnace humidifier and a clog in the internal drain system.
Furnace condensation is a common problem in high-efficiency gas furnaces. Usually, the condensation is channeled outside or to a floor drain. However, it is possible a leak is the result of the tubing used to evacuate the condensation becoming clogged. If this is the case, clearing the blockage should stop the water leak.
If condensation is not the problem, another possible source of a water leak is the secondary heat exchanger. If this is the cause of the problem, the part will need to be replaced or in some cases the whole furnace will require replacement. The furnace humidifier is also a common cause of water leaking and may require replacement depending on how much damage has been caused.
If an air conditioning unit shares an internal drain with the furnace, a likely cause of water leakage is a clogged internal drain system. If this is the case, remove the cause of the clogging and the leak should cease.