Water Heater Vent Pipe Leaking? CO Risk
The Short Answer: Water on a vent is condensate in B-vent or PVC/PP, rain in the termination, or a joint that is no longer a flue. It is not a “tape it” job. If the drip is at an indoor draft hood, or anyone is dizzy, shut the gas cock and leave. A licensed tech proves draft — you do not.
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Search Intent & The Real Problem
You found a wet B-vent joint in the attic, a drip at the draft hood, or a puddle under a PVC/polypropylene concentric. The internet said “condensation, probably fine.” Sometimes it is leftover flue moisture. Sometimes it is rain. Sometimes it is combustion products spilling because a joint pulled apart. You split those before you wrap the pipe.
Natural-draft tanks use Type B double-wall. Mid-efficiency power-vents use Category III stainless. Condensing tanks and tankless use PVC, CPVC, or polypropylene and make acidic condensate on purpose — that water belongs in a drain, not a joint. Related wet-flue pages: white smoke vs steam, Rinnai leaking by exhaust, Navien condensate.
What other guides get wrong
They treat every drip as harmless condensation, then recommend HVAC tape. Listed foil tape is a manufacturer joint seal on some B-vent systems, not a patch over a rusted inner wall or a 2-inch gap. They also paste tankless condensate advice onto a 40-gallon atmospheric tank. Category I B-vent is not PVC. And they skip the CO alarm.
Step-by-Step Troubleshooting
Step 1: Name the pipe and the fluid
| Vent / drip | What it usually is | First move |
|---|---|---|
| Type B (galvanized double-wall) joint wet, attic cold | Flue-gas condensate from a long lateral, oversized vent, or low fire | Shut down if indoor spill; licensed draft check |
| B-vent inner wall rusted, joint pulled, missing screws | Actual flue leak / disconnection | Gas off. Not tape. |
| Outdoor cap dripping after a storm | Rain in the termination | Cap / flashing, not a new heater |
| PVC / PP concentric, cool acidic drip at a glued joint | Condensing-unit condensate that is not in the drain | Slope, trap, condensate path |
| Stainless Category III seam wet, indoor | Power-vent condensate or a failed gasket | Licensed. Do not silicone the seam. |
| Draft hood wet, yellow flame, soot on the jacket | Combustion spilling | Leave. Utility / fire. |
- What to look for: Cool vs hot drip. Odor. Soot. Whether the wet spot is a joint, the draft hood, the roof jack, or the condensate fitting.
- Required tools: Flashlight, paper towel, CO alarm status. No wrench on the vent.
Steam at an outdoor condensing termination on a 20°F morning is often water vapor. The same cloud at an indoor draft hood is a spill. See white smoke.
Step 2: Natural-draft B-vent — condensate vs a failed flue
- What to look for: Long horizontal run (code wants rise; laterals collect condensate). Vent diameter bigger than the draft-hood outlet. Dilution air so cold the inner wall sweats. A joint that rotated and unlocked. Bird nest or rust flakes in the cap. Flame lazy and yellow, rollout at the burner door.
- Required tools: Eyes from the doorway, jacket rating plate (BTU/hr). Manometer work is licensed — typical NG manifold ~3.5 in. w.c., incoming often 5–7 in. w.c.; confirm against that plate.
Condensate in B-vent is still a venting defect. It rusts the inner wall and then the joint leaks flue gas. You do not “catch it with a bucket” for a season.
Step 3: Condensing PVC / PP — drip at the vent is a drain failure until proven otherwise
- What to look for: Drain that rises, trap dry or packed, frozen outdoor condensate, glue joint that never seated. Acidic drip (pH often ~3–5) at a coupling while the burner runs. After shutdown, leftover droplets during post-purge at the termination — same physics as a Rinnai exhaust drip.
- Required tools: Level on the vent (slope back to the appliance per the manual, commonly 1/4” per foot), wet/dry vac on the condensate drain only.
Do not fish a garden hose down the flue. That floods the fan and the flame rod. Rain after a storm on an outdoor unit is tankless after rain, not a “vent leak kit.”
Step 4: Power-vent stainless and roof jacks
- What to look for: Condensate weeping from a Category III seam because the run is long and cold. A roof jack that dumps rain into the B-vent. A disconnected elbow in a chase that only weeps when the wind is right — that is a CO leak that looks like water.
- Required tools: Phone photo of the termination and every accessible joint. No caulk.
Step 5: When the “vent leak” is the heater
- What to look for: Water in the burner tray, rust ring at the combustion chamber, wet jacket at the base — that is the tank, not the flue. Paper-towel it like a bottom leak. A condensing tankless that puts hot potable water in the exhaust is an exchanger, not dew.
- Required tools: Isolation valves on tankless. Cold inlet off on a tank if the closet is flooding — apartment shutoff.
When DIY Ends: Calling a Licensed Pro
It ended the moment the drip was indoor flue or anyone had symptoms. You may photograph joints, clear an external condensate trap, and shut the gas cock. You may not tape B-vent, silicone a concentric, or “just run it with a window open.” Combustion, vent replacement, and CO testing are licensed. A $12 roll of foil will not make a disconnected flue legal.
Frequently Asked Questions
Q: Why is my water heater vent pipe leaking? A: Condensate in a cold or poorly sloped vent, rain in the termination, or a joint that is no longer sealed. Indoor drip at a draft hood is a spill until a licensed tech proves otherwise. Do not tape it.
Q: Is condensate dripping from a B-vent dangerous? A: The water is a symptom. The danger is flue gas following the same path, and rust eating the inner wall. Shut the heater down if it is spilling indoors and hire a licensed plumber or HVAC tech.
Q: Can I seal a leaking water heater vent with foil tape or silicone? A: No. Tape is not a repair for a pulled joint, a rusted liner, or a CO leak. Silicone on a flue is how closets fill with exhaust. Replace or rejoin listed pipe.