Furnace Leaking Water: Three Things It Can Be

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Furnace Leaking Water: Three Things It Can Be

Short answer

Three separate things sit in or on one furnace cabinet and all three can put water on the floor: the air conditioning coil above it, the furnace's own condensate, and the humidifier bolted to the duct. Which pipes leave your furnace decides most of it. Two white plastic pipes through the wall means it makes water on purpose, and a metal pipe up a chimney means it should be completely dry.

A furnace leaking water surprises people, because a furnace burns things and nothing about that sounds wet. The answer is that three separate things live in or on that one cabinet, and all three make water.

Which of them it is comes down to two questions you can answer in a minute: what pipes leave the furnace, and what time of year it is.

Before you start

What to carry: a torch, a bucket, and old towels. Half an hour.

Gas or electric? This matters more than usual. Only a fuel burning furnace can make water of its own, and only some of those. An electric furnace burns nothing and has no flue, so it produces no condensate at all, and water around one is the air conditioning coil, the humidifier, or something that has nothing to do with the heating.

Switch it off before you put your hands in it. There is a switch on or beside the furnace that looks like a light switch. Use it, and the breaker too if the water is anywhere near the controls.

Two things worth knowing before you mop. Water and the inside of a furnace. The control board, the transformer and the blower motor are all in that cabinet. Wet electrics do not always fail immediately, which is exactly why people get a shock from something that worked fine yesterday. Power off first, every time.

A furnace with a metal flue should never produce water. If yours vents into a chimney with a metal pipe and there is water coming from inside it, that is combustion gas cooling and condensing where it should not be. It rots the heat exchanger from the inside, and it belongs with the exhaust problems rather than with a blocked drain: what an exhaust smell in the house means.

First, which furnace have you got

Two plastic pipes out through a wall, or one metal pipe up a chimney. That single difference decides whether water is normal or not.

Check 1Free, ten seconds

Look at the pipes leaving it

Two white plastic pipes going out through a side wall. A high efficiency condensing furnace. It pulls so much heat out of the exhaust that the exhaust turns back into water, and on a cold day it will make several gallons of it. Water is normal here. Water on the floor is not, and it means the drain side of it has failed.

One metal pipe going up into a chimney. A standard efficiency furnace. Its exhaust leaves hot and stays hot, so it should be dry inside, all winter, every winter.

A thick cable and no flue at all. Electric. It makes no water.

Knowing which one you have also tells you roughly what it is worth spending on it, because the two kinds are built very differently.

Three sources and one puddle

They all end up in the same place on your floor, so the puddle tells you nothing. Switching things off one at a time does.

Check 2Free, about a day

Turn things off one at a time

Is the air conditioning running? On most homes the evaporator coil sits directly on top of the furnace in its own box, complete with a drain pan. When its drain blocks, the pan fills and the water goes over the side and straight into the furnace below it. Set the thermostat to heat or off for a day. If the water stops, the furnace was never the problem: how to clear that drain.

Is the humidifier switched on? A whole house humidifier is a small box bolted to the duct beside the furnace, with a water line going into it and a drain coming out. A stuck valve leaves it running water constantly, and a blocked humidifier drain does the same thing a blocked AC drain does. It has its own switch, usually right on it. Turn it off for a day.

Is it cold out and the furnace running hard? Then on a plastic pipe furnace it is making its own condensate, and the trap or the drain is the place to look. That is check 3.

Is it none of those? Look up. A leaking pipe above, or an AC line sweating and dripping onto the cabinet, both land on the top of a furnace and run down the inside of it looking exactly like a furnace leak.

The trap and the drain

Check 3

Find the trap, then follow the hose

A condensing furnace collects its water in a plastic box, sends it through a small trap, and then out through a hose to a drain or a pump. Every one of those parts blocks.

The trap. A small clear or white plastic housing near the bottom of the furnace, often clipping off with one or two clips. It fills with a pale sludge that builds up over a season or two. Take a photo before you take it off so you know which way round it goes, rinse it out, and clip it back. This is a genuine homeowner job on most furnaces.

The hose. Follow it. It has to run downhill the whole way, and a hose that has slipped, sagged into a loop or been kinked behind the cabinet holds water and backs it up.

The pump, if there is one. Where there is no drain low enough, a small condensate pump takes it away. They have a float that sticks, and they do fail. Most have a reservoir you can lift the lid off and clean.

Outside, in a cold snap. If the drain runs out through a wall, it can freeze solid and back the water up into the house. That one fixes itself when it thaws and comes back every cold night until the pipe is insulated or rerouted.

When water is the real fault

Most of this page is a blocked drain, which is a nuisance. Two of them are not.

A metal flue furnace making water. As in the warning above. Something is making the exhaust cool down before it leaves: an oversized chimney, a liner that was never fitted, a long cold run through an unheated space, or a partly blocked flue. It is a combustion and venting job and it wants looking at properly, because the heat exchanger is corroding while it happens.

Rust or staining inside the cabinet with no obvious source. Water that has been running quietly for a while leaves a trail. Follow the stain upwards rather than guessing, and check the collector box and the inducer housing on a condensing furnace, because both crack.

If the furnace has also started behaving oddly since the water appeared, treat that as connected rather than as bad luck: the five things to check when a furnace stops.

Questions people ask

How much water should a condensing furnace make?

More than people expect. Several gallons over a cold day is ordinary, which is why the drain side of it matters so much and why a small blockage shows up quickly.

It only leaks in summer.

Then it is almost certainly the air conditioning coil above it rather than the furnace. Run the system on heat or off for a day and watch.

Is the water dangerous?

Condensate from a furnace is mildly acidic, so it is not something to have sitting on a concrete floor or in a metal pan for weeks. It is not a reason to panic, and gloves and a mop are enough.

Can I just keep emptying a bucket?

For a night, yes. Not as a plan, because the water is also going somewhere you cannot see, and a wet control board is a much bigger bill than a blocked trap.

The trap looks clean but it still backs up.

Then follow the hose past it. A sag or a loop further down the line holds water just as well as a blocked trap does.

My furnace is in the attic and the ceiling below is stained.

That is the overflow pan doing its job badly or not at all. Attic installs are supposed to have a secondary pan and a float switch that shuts the system off, and a stained ceiling means one of those is missing or has failed.

Sources: Trane "Why Is My Furnace Leaking Water?" and "Cracked Heat Exchanger", Goodman installation instructions IOG-2009E (*MSS9, *CSS9: pressure switches and blocked drains), York installation manual 561276-UIM-A-0310 (acidic condensate) and Rheem installation instructions 92-24161-124-12 (drain freezing, blocked drain shutoff)