Why a Furnace Leaks Water
A high-efficiency furnace makes water on purpose. A puddle means that water is not leaving through the drain, or the leak is something else entirely.
Water at a furnace is not one fault. The age and type of furnace decide what the water is. A high-efficiency furnace, the kind with PVC exhaust pipe, is supposed to make water. It condenses moisture out of the flue gas. That water is acidic and has to go out a drain. A puddle means the path out failed. An older furnace with metal flue pipe is not supposed to make condensate. Water under that kind of unit has a different source.
The high-efficiency drain
Condensate leaves the collector box through a hose, often a trap, then a floor drain or a condensate pump. Algae and sludge plug the trap. A pump full of slime stops lifting water, and a float switch may shut the furnace off so it does not overflow. That shutdown looks like no heat. The water on the floor is the clue that the drain quit before the burners did.
- You can look at a hose that has popped off a fitting and, if it is obviously unseated and there is no gas odor, push it back onto the barb. Do not take the collector box apart.
- Do not pour bleach through the furnace. It attacks metal and does not rebuild a cracked hose.
- If the furnace shares a pump with the air conditioner, a summer plug can show up as a winter leak the first time the furnace runs.
When the furnace should not be wet
A standard-efficiency furnace that is wet may be collecting water from a humidifier, from the air conditioner coil above it, or from a vent pipe that is not drafting. A rust streak at the burners is not a DIY wipe-down. Combustion problems, including a cracked heat exchanger, are the reason we do not tell anyone to pull the blower door and look with a flashlight as the diagnosis. If you smell gas, leave and call the gas utility from outside. If a carbon monoxide alarm sounds, get everyone out and call emergency services.
Water plus no heat is a heating repair visit. The drain, the inducer, and the safeties get checked before anyone talks about a new furnace.
Common questions
Is some water inside a furnace normal?
Inside the condensate path of a high-efficiency furnace, yes. On the floor, or in a furnace that does not have a PVC flue, no.
The furnace will not start and the floor is wet.
A float switch on the drain or the pump may have shut it down on purpose. Clearing the overflow without finding the plug only resets the same shutdown.
Can a leaking furnace mean carbon monoxide?
A drain leak and a combustion problem are different. Any CO alarm, or symptoms that feel like one, means leave the house and call emergency services. Do not stay and troubleshoot the hose.
