Is a furnace leaking water dangerous?

The water itself is usually harmless condensate, but what it damages is not — a puddle under a furnace sitting in a garage or crawlspace can reach the control board, the blower motor, or the floor structure. Standing water near electrical components is a real risk, and a leak that trips the safety switch will shut the furnace down mid-winter. Mop it up, shut the system off at the thermostat if water is pooling under the cabinet, and get it diagnosed.

Can I keep running my furnace if it is leaking water?

For a day or two in mild weather, usually yes, as long as the water is not reaching wiring, the control board, or the burner compartment. If water is dripping onto electrical parts, if you smell gas, or if the furnace is cycling on and off repeatedly, shut it off at the thermostat and call. A condensate overflow switch is designed to stop the furnace before damage happens, so a system that will not restart after a leak is doing its job.

How much does it cost to fix a furnace leaking water?

Furnace repairs in this region run $150 to $1,500 in standard markets, $175 to $1,800 in affluent markets, and $200 to $2,000 in luxury markets like Bellevue and Gig Harbor. Clearing a clogged condensate drain or replacing a cracked hose sits at the bottom of that range. Condensate pumps, inducer assemblies, and drain pan work sit in the middle; a failed secondary heat exchanger is the one repair that usually turns into a replacement conversation.

Why is my furnace leaking water in the summer when the heat is off?

Because it is almost certainly not the furnace. On most Puget Sound homes with central air, the AC evaporator coil sits in the cabinet directly above the furnace and drains through the same line. In cooling season that coil produces far more condensate than a furnace ever does, so a clogged drain, a rusted drain pan, or a frozen coil shows up as water under the furnace.

Does a leaking furnace mean I need a new one?

Usually not. The large majority of furnace water leaks are drainage problems, and drainage parts are inexpensive. Replacement enters the conversation when the leak traces back to a corroded secondary heat exchanger or a cracked primary heat exchanger, or when the furnace is already 15 or more years old and the repair costs more than about half of a new system — gas furnace replacement runs $4,000 to $8,000 installed in standard markets.

Furnace Leaking Water? Causes and What the Repair Costs

Water under a furnace looks worse than it usually is. A furnace burns gas and moves hot air — water is not supposed to be part of the story, so the first assumption is that something expensive broke. Nine times out of ten, it did not. It is condensate going somewhere it should not, and the fix is measured in dollars, not thousands of dollars.

Here is how we work through a leaking furnace on a service call in Federal Way, Kent, Tacoma or Olympia, what each cause actually costs, and the one scenario where the leak is telling you the furnace is done.

First question: what kind of furnace do you have?

Everything about diagnosing a water leak depends on this, and you can usually tell from the exhaust pipe.

High-efficiency condensing furnaces (90% AFUE and up)

If a white PVC pipe runs out through the wall or rim joist, you have a condensing furnace. These are supposed to make water. They pull so much heat out of the exhaust that the water vapor in it condenses back into liquid — several gallons a day in cold weather on a hard-running system. That water is mildly acidic and has to drain away through a trap, a hose, and either a gravity drain or a condensate pump.

That drainage path is the single most common failure point on a modern furnace. Every piece of it is plastic, every piece of it stays wet, and it collects sludge.

Standard-efficiency furnaces (80% AFUE)

If the exhaust is metal pipe going up a chimney or B-vent, your furnace is not designed to condense at all. It should produce no water. So a puddle under an 80% furnace means something else is wrong — and in most Puget Sound homes with central air, that something else is sitting right on top of the furnace.

The causes we actually find

1. Clogged condensate drain line or trap

The number one cause, by a wide margin. The trap and tubing fill with a gray biofilm that eventually plugs the line. Water backs up into the collector box, overflows the cabinet, and lands on your floor. Many furnaces also have an overflow safety switch, which is why some homeowners discover a leak only because the heat quit working.

2. Failed or clogged condensate pump

If the furnace sits below the nearest drain — common in Puget Sound basements and crawlspace-adjacent utility closets — a small pump lifts the condensate out. Pumps are consumables. The float sticks, the motor fails, or the discharge tubing plugs, and the reservoir simply overflows.

3. Cracked, kinked, or disconnected drain hose

The soft tubing between the furnace and the drain gets brittle, gets bumped during a filter change, or was never properly secured. This is the cheapest repair on the list and one of the most common, especially in garage installs where things get moved around.

4. Plugged inducer drain or collector box

Upstream of the trap, the inducer housing and collector box have their own small drain ports. When those plug, water pools inside the furnace itself and finds the seams. You may also hear a gurgling or sloshing sound when the inducer runs.

5. The air conditioning coil above the furnace

In cooling season, the evaporator coil sitting in the cabinet above your furnace produces far more condensate than the furnace ever will. A plugged coil drain, a rusted-through drain pan, or a frozen coil thawing out all dump water straight down onto the furnace. If your leak shows up in July and not January, start here, not with the heating side. A furnace that is also running your central air shares one drain for both jobs.

6. Whole-home humidifier

If a humidifier is strapped to the supply plenum or return, it has its own water supply line, solenoid valve, and drain. Any of the three can leak, and the water lands in the same place.

7. Secondary heat exchanger failure

The uncommon one, and the serious one. On a condensing furnace, the secondary heat exchanger lives in the acidic condensate stream for its entire life. When it corrodes through, it leaks water — and it also leaks combustion gases. This is not a routine repair. On a furnace past the 12-to-15-year mark, it is a replacement conversation.

What to check before you call

Free, five minutes, no tools beyond a flashlight:

Find the source. Is the water coming from the furnace cabinet itself, from the drain tubing, from the pump, or from the pipe joint at the wall?
Check the filter. A clogged filter freezes an AC coil, and a thawing coil floods. If the filter looks like a wool blanket, that is a lead.
Look at the condensate pump. Lift the lid. If the reservoir is full and the pump is silent, the pump is the problem.
Confirm the drain pipe is not blocked outside. Condensate terminations get buried under bark, blocked by a shrub, or iced over during a cold snap.
Mop the water up and watch it. Knowing whether it refills in ten minutes or ten hours tells a technician a lot.

What not to do: do not run the furnace with water reaching the control board or burner compartment, and do not bypass an overflow safety switch to get heat back. That switch is protecting the equipment and your floor.

Rule of thumb: if the water is clean and the leak only appears while the furnace or AC is running, it is condensate. If the water appears when nothing is running, look for a humidifier line or a plumbing issue near the furnace.

What the repair costs in Puget Sound

Repair	Typical cost, standard markets
RepairClear a clogged condensate drain or trap	Typical cost, standard marketsLow end of $150 – $1,500
RepairReplace cracked drain hose or fittings	Typical cost, standard marketsLow end of $150 – $1,500
RepairReplace a condensate pump	Typical cost, standard marketsMid range of $150 – $1,500
RepairClear inducer or collector box drain	Typical cost, standard marketsMid range of $150 – $1,500
RepairReplace a rusted AC drain pan	Typical cost, standard marketsUpper range of $150 – $1,500
RepairSecondary heat exchanger failure	Typical cost, standard marketsUsually replacement: $4,000 – $8,000

Affluent markets run $175 to $1,800 on repairs, luxury markets $200 to $2,000. You get a price before any work starts, not after. Most leaking-furnace calls are among the least expensive visits we run all winter, which is why it is worth getting an actual diagnosis rather than guessing.

If you want a ballpark on replacement before anyone comes out, the instant estimate tool gives Good, Better and Best starting prices in about 60 seconds.

When the leak means the furnace is done

Three situations flip this from repair to replace:

A corroded secondary heat exchanger. The part cost plus labor on an out-of-warranty furnace rarely makes sense against a new system.
A furnace 15 or more years old facing a four-figure repair. Our standing rule: under 15 years and under half the cost of replacement, repair it. Past that, replace.
Repeat leaks from rusted internal components. Water inside a furnace cabinet rusts the blower housing, the burner assembly and the sheet metal. Once corrosion is general rather than local, you are patching.

Gas furnace replacement runs $4,000 to $8,000 installed in standard Puget Sound markets and $5,000 to $10,000 in affluent markets, with most swaps done in one day. If you are replacing anyway, it is worth running the numbers on a heat pump instead — it covers heating and cooling in one system, and local utility rebates may apply to heat pumps, where they generally do not apply to a furnace swap. The amount depends on what is being replaced and the equipment installed; programs change, so we confirm what your utility actually pays before quoting and file the paperwork. Our heat pump versus gas furnace comparison lays out that math honestly, including when staying with gas is the better call.

Why Puget Sound furnaces leak more than you would expect

Two regional factors. First, a lot of furnaces here live in unconditioned garages and crawlspace-adjacent closets, where cold surfaces and damp air encourage condensation outside the drain path and where drain lines are more likely to freeze at the termination during a cold snap. Second, our long shoulder seasons mean condensing furnaces run in short, frequent cycles for months — plenty of condensate, never enough sustained flow to keep the drain scoured.

Both are preventable with a yearly service that includes flushing the condensate trap and line. It is the single cheapest thing on a maintenance checklist and it heads off the most common leak there is.

What a Varsity visit looks like

We find the source, show you where the water is coming from, and give you a price before any work starts. If it is a $200 drain clog, we tell you it is a $200 drain clog — we are not going to open with a replacement proposal on a furnace with good years left in it. If the heat exchanger is the problem, we will show you that too, and then price repair and replacement side by side.

Varsity is owner-operated out of Federal Way, licensed in Washington (VARSIHC766RJ), and every installation we do carries a 10-year parts and labor warranty. See the full scope of furnace repair and replacement work, or check that we cover your town on the service areas page.

Get it looked at

Service hours are Monday through Friday, 8 to 6, with same-week scheduling for most of the service area. Call (253) 785-4622 or request an estimate online and we will get a technician out to find the leak before it finds your subfloor.