How Much Does It Cost to Fix a Furnace Leaking Water?

Furnace water leak repairs range from $100 to $1,500 depending on the cause. A simple condensate drain clog costs $100–$250, while a cracked heat exchanger can run $500–$3,500. Learn what's causing the leak and what the fix will cost.

Most homeowners pay

$100avg $375$1,500

Based on national averages. Condensate drain repairs average $100–$250; heat exchanger issues average $500–$3,500.

Furnace Water Leak Repair Costs by Cause

Item	Low	High	Note
Condensate Drain Line Clog (cleaning)	$100	$250	Most common cause
Condensate Drain Line Replacement	$150	$450	If line is cracked or corroded
Condensate Pump Replacement	$150	$450	If pump fails on basement units
Condensate Trap Replacement	$100	$300	Blocked or cracked trap
Humidifier Leak Repair	$100	$400	Feed tube, solenoid, or pad
Humidifier Replacement	$300	$800	If unit is beyond repair
Flue Pipe / Vent Pipe Repair	$150	$500	Improper sizing causes condensation
Secondary Heat Exchanger Repair	$300	$1,200	Condensing furnace specific
Primary Heat Exchanger Replacement	$500	$3,500	Most serious cause
Diagnostic / Service Call Fee	$75	$200	Usually credited toward repair

Prices include parts and labor at standard rates. Emergency service adds 50–200% to labor costs.

Why Is Your Furnace Leaking Water?

Water around or under your furnace is never normal, but it's not always an emergency either. The cause — and the repair cost — depends largely on what type of furnace you have.

High-Efficiency Condensing Furnaces (90%+ AFUE)

If your furnace has a PVC (white plastic) exhaust vent rather than a metal flue pipe, you have a high-efficiency condensing furnace. These units extract so much heat from combustion gases that the gases cool enough to condense into water. This is completely normal — the furnace is designed to produce water.

The condensation is supposed to flow through a drain system: condensate trap, drain line, and sometimes a condensate pump. When any part of this system clogs, cracks, or fails, water backs up and leaks around the furnace.

Common condensate system problems:

Clogged drain line ($100–$250): Algae, mold, or sediment blocks the line. A technician clears it with compressed air or a wet/dry vacuum. This is the most common cause of furnace water leaks and the cheapest to fix.
Cracked or disconnected drain line ($150–$450): PVC drain lines can crack from age or freeze in unheated spaces. Replacement is straightforward.
Failed condensate pump ($150–$450): If your furnace is in a basement below the drain level, a pump pushes condensate up to the drain. Pumps last 3–5 years on average.
Blocked condensate trap ($100–$300): The trap prevents exhaust gases from escaping through the drain. When it clogs, water backs up into the furnace.

Standard-Efficiency Furnaces (80% AFUE)

Standard-efficiency furnaces vent exhaust through a metal flue pipe and should not produce condensation. If water is pooling around a standard-efficiency furnace, the cause is typically external to the furnace itself:

Oversized or improperly pitched flue pipe ($150–$500): If the flue pipe is too large or doesn't slope upward correctly, exhaust gases cool inside the pipe and condensation drips back down to the furnace.
Whole-house humidifier leak ($100–$800): Many furnaces have a humidifier mounted on the plenum or ductwork. A leaking solenoid valve, cracked water panel, or disconnected feed tube can drip water onto or around the furnace.
AC evaporator coil drain issue ($150–$400): If your AC system shares ductwork with the furnace, a clogged AC condensate drain can cause water to drip onto the furnace during cooling season.

Heat Exchanger Cracks — Diagnosis and Cost

A cracked heat exchanger can cause water to leak from the furnace because combustion gases escaping through the crack cool rapidly and condense. This is particularly common on condensing furnaces where the secondary heat exchanger develops pinhole leaks from the acidic condensate it handles.

Signs of a Heat Exchanger Crack

Water pooling that doesn't correspond to a visible drain line issue
Yellow, flickering, or irregular burner flames (healthy flames are blue and steady)
Soot or carbon buildup inside the furnace cabinet
A strong, unusual smell when the furnace runs
Headaches, nausea, or flu-like symptoms that improve when you leave the house
Carbon monoxide detector alarms

Repair Cost: $500–$3,500

Heat exchanger replacement is technically possible but extremely labor-intensive (6–10 hours). The part itself costs $300–$2,000 depending on the furnace model, with total installed cost of $500–$3,500.

In most cases, heat exchanger replacement is not recommended. The repair cost is so high that it often approaches the cost of a new furnace ($3,000–$8,000), especially when you factor in that:

The furnace is usually old enough that other components will fail soon
A new furnace will be significantly more energy efficient
The new furnace comes with a full warranty (typically 5–10 years parts, lifetime heat exchanger)

For a deeper analysis, see our Furnace Repair Cost Guide and the $5,000 rule for repair vs. replacement decisions.

How to Diagnose the Source of the Leak

Before calling a technician, you can narrow down the cause with a few simple checks:

Step 1: Identify Your Furnace Type

Check the exhaust vent. PVC (white plastic) pipe = high-efficiency condensing furnace. Metal flue pipe = standard-efficiency furnace. This immediately narrows the likely causes.

Step 2: Check the Condensate System (High-Efficiency Units)

Follow the drain line from the furnace. Look for visible water pooling at connection points, cracks in the PVC pipe, or a condensate pump that isn't running. If the pump's reservoir is overflowing, the pump has likely failed.

Step 3: Inspect the Humidifier

If your furnace has a whole-house humidifier mounted on the supply or return plenum, check for dripping water around it. Turn off the humidifier's water supply and see if the leak stops.

Step 4: Check the AC Drain (Summer Leaks)

If the leak only occurs during cooling season, the issue is likely the AC evaporator coil's condensate drain, not the furnace itself.

How to Save on Furnace Water Leak Repairs

Prevent clogs with maintenance. Adding a cup of white vinegar or bleach to the condensate drain line every 3–6 months prevents algae and mold buildup — the most common cause of clogs.

Replace the condensate pump proactively. If your furnace uses a condensate pump and it's over 3 years old, replacing it during a routine maintenance visit ($150–$300) is cheaper than an emergency call when it fails mid-winter ($250–$500+).

Get the diagnostic fee credited. Most HVAC companies charge $75–$200 for a diagnostic visit but credit that amount toward the repair. Always confirm this before scheduling.

Don't assume the worst. While a heat exchanger crack is the most expensive and dangerous cause, it's also the least common. The vast majority of furnace water leaks are condensate drain issues that cost $100–$250 to fix.

Check your warranty. Heat exchangers on many premium furnaces carry 20-year or lifetime warranties. If your heat exchanger is cracked and still under warranty, you'll only pay for labor ($300–$800), not the part.

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Furnace Water Leak Repair FAQ

Why is my furnace leaking water?

The most common cause is a clogged condensate drain line, especially on high-efficiency condensing furnaces (90%+ AFUE). Other causes include a failed condensate pump, leaking humidifier, improperly sized flue pipe, or — in the most serious cases — a cracked heat exchanger.

Is a furnace leaking water dangerous?

Usually not. Most furnace water leaks are caused by condensate drain issues, which are inconvenient but not hazardous. However, if the leak is caused by a cracked heat exchanger, carbon monoxide can enter your home — this is a serious safety hazard. If you smell unusual odors, see yellow burner flames, or your CO detector goes off, shut down the furnace and call for service immediately.

How much does it cost to unclog a furnace condensate drain?

Clearing a clogged condensate drain line typically costs $100–$250, including the service call fee. This is one of the cheapest and most common furnace repairs. If the drain line needs to be replaced entirely due to cracks or corrosion, expect $150–$450.

Can I fix a furnace water leak myself?

Some condensate drain clogs can be cleared with a wet/dry vacuum applied to the drain line exit. You can also check for disconnected or cracked drain lines and inspect the condensate pump. However, any repair involving the heat exchanger, gas connections, or internal furnace components should be handled by a licensed HVAC technician.

Should my furnace be producing water at all?

If you have a high-efficiency condensing furnace (90%+ AFUE with PVC exhaust venting), yes — condensation is a normal byproduct of operation. The furnace should produce 0.5 to 1 gallon of condensate per hour of operation. Standard-efficiency furnaces (80% AFUE with metal flue pipe) should not produce water.

How often should I clean my furnace condensate drain?

Pour a cup of white vinegar or diluted bleach into the condensate drain opening every 3–6 months to prevent algae and mold buildup. Have the entire condensate system inspected during your annual furnace maintenance visit.

My furnace leaks water only in summer. Why?

If your furnace only leaks water during cooling season, the problem is almost certainly the AC evaporator coil's condensate drain, not the furnace itself. The evaporator coil sits above or next to the furnace, and its condensate can drip onto the furnace when the drain is clogged. This is an AC repair, not a furnace repair, typically costing $150–$400.

Will a furnace water leak cause damage to my home?

Yes, if left unaddressed. Standing water can damage flooring, drywall, and the furnace itself. It also creates conditions for mold growth. Even a small, steady drip can cause significant water damage over weeks or months. Address furnace water leaks promptly — most are inexpensive to fix.

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Last updated March 13, 2026