Lennox Furnace Leaking Water: Causes, Fixes & Costs

Last updated Sep 29, 2026 · By fixme.vip Editorial

Lennox furnace leaking water: what it means

“Leaking water” is not a specific Lennox blink or E-code — it is a physical symptom you find as a puddle under or around the furnace, or water dripping from the cabinet. On most modern Lennox systems, water at the furnace comes from the condensate system of a high-efficiency (condensing) furnace, from the air-conditioning coil sitting on top of the furnace, or from a flue/venting problem.

High-efficiency Lennox furnaces (90%+ AFUE, such as the SLP99V, EL296V and similar) are designed to produce condensate — acidic water wrung out of the flue gases. That water is supposed to flow through a collector box, condensate trap, and drain line. Any blockage or crack in that path shows up as a leak.

Because a leak can also indicate a cracked secondary heat exchanger — a genuine safety and health concern — this is rated pro. There is no fault code to reset; instead, the safest response is to shut the furnace down and have the source identified.

Lennox furnace leaking water: common causes, ranked by probability

1. Clogged condensate drain line or trap — Algae, dust and debris block the drain, so condensate backs up and overflows. The single most common cause on condensing furnaces.
2. Full or overflowing condensate trap — The trap collects debris or dries out/cracks and leaks at its seams or fittings.
3. Condensate pump failure (if your furnace uses one) — The pump reservoir overflows when the pump fails, floats stick, or its discharge line clogs.
4. A/C evaporator coil drain issues — In summer, the coil above the furnace produces condensate; a clogged coil drain pan or line drips onto the furnace and looks like a furnace leak.
5. Clogged or improperly sloped flue/vent (PVC) — Condensate pools in the venting instead of draining back to the trap, then leaks at joints.
6. Cracked collector box or hoses — Internal plastic condensate components crack with age and leak inside the cabinet.
7. Cracked secondary heat exchanger — Less common but serious; produces condensate leaks and requires professional inspection.

A Lennox-specific wrinkle

On many Lennox condensing furnaces the condensate path — collector box, drain hoses and trap — sits behind the blower access panel, not out in the open on the side of the cabinet. That matters for two reasons: a leak from those components can run down inside the cabinet toward the blower and control board before you ever see a drop on the floor, and there is no way for a homeowner to inspect that assembly without opening the cabinet, which is technician work.

Lennox installs also commonly use an inline drain arrangement where the installer selects a left- or right-side drain-hose routing in the field to suit the closet, attic or basement layout. If a furnace was moved, re-piped, or the drain was re-routed after the original install, the hose configuration and the slope of the drain line are worth mentioning to the technician — a leak that started right after other work is often a routing or slope issue rather than a failed part.

How to fix Lennox furnace leaking water: safe checks first

These are the only checks a homeowner should perform, in this order:

• Turn the furnace off at the thermostat (set to Off).
• Cut power at the breaker or the furnace service switch before you go near standing water. Do this first if there is any water at the furnace base, near wiring, or near the control board area.
• With the power off, sponge or wet-vac the standing water away from the furnace base and electrical parts. Never wet-vac around a furnace that still has power to it.
• Check your air filter. A severely clogged filter can cause coil freeze/thaw that drips water in cooling season; replace it if dirty.
• Look at the condensate drain line’s exit. If it terminates at a floor drain, sink or outside, make sure that endpoint isn’t clogged or frozen and the line isn’t kinked.
• Check the condensate pump (if you have one) — confirm it has power and its reservoir isn’t overflowing. Do not open or disassemble it.
• Look for A/C-related dripping — if it’s summer and the leak is coming from the coil box on top, note that for the technician.

Do not open the furnace cabinet, remove hoses, poke at the collector box, or run the furnace to “dry it out.” Anything inside the cabinet is technician work.

If a CO alarm sounds, anyone in the home feels dizzy, nauseated or headachy, or you smell gas or burning: leave the house immediately and call 911 or your gas utility from outside. Do not stop to troubleshoot, and call the HVAC company only after the home has been cleared.

How a technician will diagnose it

• Confirms whether the water is combustion condensate, A/C condensate, or flue condensation by tracing where it originates.
• Inspects and clears the condensate drain line and trap, often flushing or vacuuming the blockage.
• Tests the condensate pump, floats and discharge line.
• Checks flue/vent slope and joints for pooling condensate and improper pitch.
• Inspects the collector box and internal condensate hoses for cracks, including the assembly behind the blower access panel.
• Verifies the drain-hose routing and slope are correct for how the unit is installed.
• Performs a heat exchanger inspection (visual, camera, and sometimes combustion analysis) to rule out a cracked secondary heat exchanger.
• Checks the evaporator coil drain pan and line if the leak is seasonal/cooling-related.

A good quote should name the specific source — vague “it’s leaking” without identifying the component is a red flag.

Symptom, cause and what to do

Lennox furnace leaking water repair cost

• Drain line/trap clearing: $0 DIY (endpoint only) – $150–$250 professional cleaning.
• Condensate pump replacement: $200–$600 installed.
• Condensate trap or hose replacement: $150–$350.
• Flue/vent re-sloping or resealing: $150–$450 depending on access.
• Collector box replacement: $300–$700 (part plus labor).
• A/C coil drain pan/line service: $150–$400.
• Cracked secondary heat exchanger: $1,200–$3,000+ installed, even when the part itself is still covered by a parts warranty, because the labor to pull and rebuild the furnace is substantial. On older units, replacing the furnace is often more economical.

Prices vary by region, accessibility and whether parts are under Lennox warranty. Labor is typically covered only if you have a labor warranty or service plan; the Lennox parts warranty may cover the heat exchanger itself but usually not the labor to install it.

Related pages

Before you match a leak to a “code,” check how your furnace actually reports faults. Lennox fault display varies significantly by control board: newer units such as the SLP99V and EL296V use a 7-segment unit controller that shows alphanumeric alert codes, while older ML- and SL-series boards use LED flash/blink codes and have no E-codes at all. Read the code chart on the label inside your furnace’s access panel rather than assuming one universal Lennox code set — the same symptom can carry a different code, or no code, depending on the board in your cabinet.

• Lennox Furnace Pressure Switch Problems — a clogged condensate drain can also trip the pressure switch, so leaks and pressure-switch faults often appear together. How that fault is displayed depends on your control board.
• Lennox Furnace Keeps Shutting Off: Causes, Fixes & Costs — condensate backups can cause safety-trip shutdowns.
• Lennox Furnace Short Cycling: Causes, Fixes & Costs — drainage faults can contribute to short cycling.