Water leaking around the base of a furnace may be caused by an internal drain system clog or an issue with the furnace's humidifier. A faulty secondary heat exchanger or furnace condensation may also be to blame, states Angie's List.
A common problem with high-efficiency furnaces that leak water around the base is sometimes a clogged drain system. This is particularly true when an air conditioning system shares its drain system with the furnace. When a clog occurs in the system, water is sent to the furnace. The humidifier inside the furnace may also be leaking into the furnace, causing the water to show up at its base.
A malfunctioning secondary heat exchanger can also cause leaking. Breaks in the line that is channeled to the drain on the floor or clogs in the line can cause condensation to pool, showing up on the floor. The same is true of an incorrectly sized flue pipe that allows hot exhaust to cool down and condense inside, draining back and leaking out on the floor.
Whatever the cause, leaks can lead to damage in the furnace's components and cause damage to the floors and walls surrounding the leak. In some instances, leaks contribute to or cause mold growth. For these reasons, contact a repairman as soon as possible, advises Angie's List.