Answer first, context after
Why is my furnace leaking water?
If you have a high-efficiency furnace, water is normal and a leak usually means the condensate drain is clogged. High-efficiency units pull so much heat out of the exhaust that the moisture in it condenses, and that water has to drain somewhere. On an older standard-efficiency furnace, water is not normal and often points at the vent or at the AC coil sitting above it. Either way, shut it off if water is near the electrical parts.
Water under a furnace looks like a plumbing problem, but it usually is not. Which explanation fits depends on what kind of furnace you have.
High-efficiency furnaces make water on purpose. A condensing furnace, the kind rated around 90% or higher, extracts so much heat from the exhaust that the water vapor in it turns back into liquid. A healthy one produces several gallons on a cold day and pipes it to a drain. So the question is never “why is there water,” it is “why is the water not going where it belongs.” Usually one of these:
- A clogged condensate drain or trap. The most common cause by a wide margin. Kern County dust, plus the gunk that grows in a wet line, and it backs up.
- A cracked or disconnected drain line, often where it was bumped during another repair.
- A failed condensate pump, if your furnace has to pump the water up to a drain rather than let gravity carry it.
- A plugged inducer drain or pressure switch hose, which can also cause the furnace to shut down entirely.
On an older standard-efficiency furnace, water is a warning. Those are not supposed to condense. Water usually means the exhaust is cooling before it leaves, which points at a vent that is the wrong size, blocked, or badly sloped, and that deserves attention because it is the same system that carries combustion gases out of your house.
It might not even be the furnace. In a lot of Bakersfield homes, the AC evaporator coil sits in the same cabinet, directly above the furnace. A coil draining badly in summer drips straight down onto it and looks exactly like a furnace leak. That is a different fix: why is my AC leaking water.
Heat pumps and mini-splits are a separate case. A heat pump outdoor unit dripping or steaming in winter is usually the defrost cycle doing its job, which we cover in why does my heat pump blow lukewarm air. Water running down an indoor wall from a mini-split head is not normal and usually means a blocked condensate line.
What to do right now. If water is pooling near wiring, the control board, or the blower, shut the system off at the switch and let it stop making more. Mop up what you can see so you can tell whether it comes back. Then call. Most of these are a drain cleared and a part replaced, quoted flat from the published furnace repair menu, with the $89 diagnostic waived when you book the repair.
Find out which furnace you have first.
On a high-efficiency furnace water is normal and the drain is the suspect. On an older furnace water usually means something else, and that difference changes the fix.