Furnace leaking water: is it dangerous?

High-efficiency condensing furnaces pull so much heat out of the exhaust that water vapor in it turns to liquid. That water, called condensate, is supposed to run through a trap and a drain line to a floor drain or a small pump. When any part of that path clogs, cracks or comes loose, the water ends up on the floor or inside the cabinet.

Standard-efficiency furnaces with a metal flue do not make condensate in normal operation. Water at one of those comes from something attached to it, or from a flue problem.

• Clogged condensate drain or trap. Slime, dust and debris build up and the water backs up into the furnace.
• Failed condensate pump. The pump tank fills and overflows.
• Loose, cracked or badly sloped drain tubing.
• AC coil drain. In cooling season, the coil above the furnace makes water. A blocked AC drain overflows onto the furnace.
• Humidifier leak. A stuck water valve, a clogged pad or a blocked humidifier drain.
• Leak inside the furnace. A cracked collector box, inducer housing or internal drain hose, or a failing secondary heat exchanger.
• Flue condensation on a standard furnace. A flue that is too large, too long or blocked lets exhaust cool and condense. This needs a professional look at the venting.

The water itself is mildly acidic and not hazardous to touch, though it is hard on metal and concrete over time. The risks are indirect. Water on the control board or wiring can short out the furnace. A blocked drain trips a safety switch and leaves you without heat. Long-term leaks rust the cabinet and the heat exchanger, and persistent dampness grows mold.

The one situation to treat as urgent is water at a standard-efficiency furnace with signs of a venting problem, such as rust streaks on the flue, soot or a carbon monoxide alarm. Leave the furnace off and call.

The technician clears and flushes the trap and drain line, tests or replaces the condensate pump, and checks the slope and connections of the tubing. If the leak is inside the cabinet, the inducer housing, collector box, pressure switch hoses and secondary heat exchanger get inspected. Drain repairs are quick. Internal leaks take longer to pinpoint.

Ask the technician to show you where the water was coming from and what was done to stop it. If the drain clogged with slime, ask whether the trap can be cleaned at each tune-up, and whether a safety switch can be added to shut the furnace off the next time the drain backs up.

A drain or pump problem is a routine repair. A leaking secondary heat exchanger, or a primary heat exchanger rusted through from years of water exposure, is a different matter. Read cracked heat exchanger: repair or replace before approving that work, and see the furnace repair guide for how to compare quotes.