Why Is My Furnace Leaking Water?

Why is my furnace leaking water?

Your furnace leaks water because its condensate drainage path is blocked or broken, not because anything is burning wrong. A modern condensing furnace pulls so much heat out of the exhaust that the water vapor in the combustion gas cools back into liquid, roughly 3 to 5 gallons of mildly acidic condensate on a cold day. That water collects in a plastic trap and drains through a small tube to a floor drain or a condensate pump. This is normal and constant while the furnace runs. You only see a leak when the trap clogs, the tube kinks or disconnects, or the pump quits and the water backs up and spills over.

So a furnace water leak is usually a plumbing and drainage problem, and it is often cheap and sometimes a do-it-yourself fix. The important first step is figuring out where the water is really coming from, because a puddle at the base of a furnace has three very different sources: the condensing furnace's own drain, the air conditioner's evaporator coil that sits in the same cabinet, and a whole-house humidifier tied into the ductwork. Standing water near a gas furnace's burners and electrical controls is worth clearing quickly either way. The furnace maintenance checklist covers what is owner-safe to touch and what is a pro call.

Is my furnace a high-efficiency (condensing) furnace?

You can tell in about ten seconds by looking at the exhaust pipe. A high-efficiency condensing furnace (90 to 98 percent AFUE) vents through white or gray PVC plastic pipe that runs out through a side wall, and it has a visible plastic condensate drain line and trap. A standard-efficiency furnace (80 percent AFUE) vents through a metal pipe that runs up into a chimney or flue, and it has no condensate drain at all. This one detail decides whether the furnace itself can leak water in heating season, so it is the first thing to check.

If your furnace vents through PVC, it makes condensate, and the leak can be the furnace's own drain system. If it vents through metal and has no drain line, it does not produce water when it heats, so water on the floor is coming from the air conditioner, a humidifier, or a plumbing line, not the furnace. Newer installs from roughly the last 15 years are increasingly condensing units, since the higher efficiency lowers the gas bill; if you are weighing an upgrade, our note on what a good efficiency rating means covers the cooling side of the same idea.

What are the most common reasons a furnace leaks water?

The number one cause on a condensing furnace is a clogged condensate drain line or trap. The condensate is slightly acidic, and it grows a slimy mold and mineral sludge that plugs the small plastic trap and tubing; the water backs up and overflows out of the trap or the inducer housing. A clogged trap or line is behind most heating-season furnace leaks. Close behind it is a failed condensate pump, used when the furnace sits below the drain in a basement, since a stuck or burned-out pump lets the collected water pool and spill. A cracked, brittle, or disconnected drain tube does the same on older units.

After the drain system, a few less common causes are worth knowing. In cooling season the leak is almost always the air conditioner evaporator coil that sits on top of the furnace, whose own clogged drain drips down onto the furnace; that path is covered in detail in why an AC leaks water. A whole-house humidifier mounted on the ductwork can leak from a stuck solenoid or a clogged drain, which looks exactly like a furnace leak. The rare but serious cause is a cracked secondary heat exchanger, which is a gas-side failure that needs a licensed technician; never keep running a furnace you suspect has a cracked heat exchanger.

Is my furnace leaking water in summer or winter?

The season it leaks is the single best clue to the cause. Water at the furnace in summer, when the heat is off and the AC is running, is almost always air conditioner condensate, not the furnace. On most homes the AC evaporator coil sits directly on top of the furnace or inside the same air handler cabinet, so its drain pan and drain line share that space; a clogged condensate line there drips down around the furnace base and looks like the furnace is leaking. Treat a summer leak as an AC drainage problem and clear the AC condensate drain first.

A leak in winter, when the furnace is the thing running, points at the furnace's own condensate system or a humidifier. A high-efficiency condensing furnace makes water every time it fires, so a winter leak means a clogged trap, a bad drain line, or a failed condensate pump on that furnace. If your furnace is a standard 80 percent model that makes no condensate and it is still leaking in winter, the water is coming from a whole-house humidifier tied into the ducts or from a plumbing or roof leak above it, so trace it upward rather than blaming the furnace.

How do I fix a furnace leaking water myself?

The safe do-it-yourself fixes are all on the drainage side, and they clear most condensing-furnace leaks. Shut the furnace off at its power switch, mop up the standing water so it is away from the burners and wiring, then find the plastic condensate trap and drain tube and pull them to check for sludge. Flush the trap and tubing with a mix of warm water and a little distilled white vinegar to break up the mold, clear any clog, and reseat everything so it drains freely. If the furnace sits below the drain, check the condensate pump: pour water into its reservoir and confirm it kicks on and pushes the water out. A stuck float or a dead pump is a common and inexpensive part.

Keep your hands off the gas and the combustion parts. Clearing the drain trap, flushing the line, resetting or replacing a condensate pump, and clearing the AC condensate drain are owner-safe; the gas valve, the burners, the inducer motor, and the heat exchanger are not. If the water keeps coming after the trap and line are clear, or you see rust or soot streaks, or the furnace is a standard model that should not make water at all, stop and call a technician. Never attempt any repair on a heat exchanger yourself, and if you smell gas, leave and call the gas company from outside.

When should I call a pro, and what does the fix cost?

Call a pro when the leak is not a simple drain clog or when it points at the gas side. Get a technician out if the water keeps coming after you clear the trap and line, if water has reached the burners or the control board, if the furnace is a standard model leaking with no AC or humidifier running, or if you see rust, soot, or smell gas. Those last signs can mean a cracked secondary heat exchanger, which is both a carbon monoxide risk and often a reason to replace the furnace rather than repair it. When a replacement is on the table, size the new system correctly with the BTU calculator so you do not oversize it.

Most furnace water leaks are cheap to fix. Clearing a clogged condensate drain or trap runs about $75 to $200 if a pro does it, a new condensate pump is roughly $150 to $400 installed, and replacing brittle drain tubing or a trap assembly is about $100 to $300. A whole-house humidifier repair is in the same range. The expensive case is a cracked heat exchanger, which runs from several hundred dollars to well over $2,000 and pushes many homeowners toward a new furnace. For how repair prices break down across the rest of the system, see our guide on AC repair costs.

Frequently asked questions