Water Dripping From Water Heater Exhaust

The Short Answer: Water at a water heater exhaust is leftover condensate, rain in the termination, a blocked condensate path blowing out the flue, or potable water from a cracked heat exchanger. Ounces of cool drip at a condensing vent can be normal. A stream with the unit off is not. Never silicone a flue. Rinnai-specific shutdown drip is Rinnai leaking by exhaust.

Search Intent & The Real Problem

The flue is wet. Atmospheric B-vent, power-vent PVC, or a tankless concentric — they all get searched as “exhaust dripping.” Condensing appliances make acidic condensate on purpose; it should leave through a drain, not the cap. Non-condensing tanks should not rain from the draft hood unless the chimney is dumping weather or the collector is leaking. A puddle at the cabinet with a dry flue is inside drip or Navien bottom leak, not this page.

What other guides get wrong

They tell you to caulk the exhaust, replace the T&P because the flue is wet, or treat every drip as condensation you can ignore. The T&P is on the water side; it cannot leak “by the exhaust” unless someone tied the discharge into the vent — a code failure. Silicone on a concentric or B-vent is how closets fill with flue gas. A condensing unit with a clogged condensate line will throw water out the only remaining hole: the exhaust. Brand-specific condensate plumbing: Navien condensate drain.

Step-by-Step Troubleshooting

Step 1: Identify the appliance and the fluid at the flue

• What to look for: Temperature of the drip, whether it is the cap, the collar at the cabinet, or the condensate fitting. Rating plate: condensing (often 0.90+ UEF, PVC/polypro vent) vs non-condensing (B-vent or Category III stainless).
• Required tools: Paper towel, flashlight. Optional pH strip.

Step 2: Tankless isolation-valve test — this is the exchanger question

• What to look for: Close cold and hot isolation valves. If the exhaust drip dies after residual waterways empty, that water was potable under line pressure — exchanger or internal waterway, not dew. If a small cool drip finishes and stops with valves still open, leftover condensate is plausible. No service valves is an install defect; stop and hire it.
• Required tools: Isolation valves.

Do not keep firing a unit that put potable water in the burner. Error history for air/exhaust, flame failure, or overheat next to a wet flue is the same story. Related weather: tankless after rain.

Step 3: Condensate trap, slope, and the outdoor cap

• What to look for: Trap dry or packed, 1/2-inch drain that rises, frozen outdoor condensate, pump dead. On condensing indoor units a blocked collector backs into the exchanger and the fan shoves it out the vent during post-purge — then it still drips after the fan stops. Outdoor concentric: missing or cracked rain cap, termination too close to a soffit that dumps roof water into the intake.
• Required tools: Eyes on the drain path, wet/dry vac on the condensate drain only.

Clearing an external trap is fair DIY. Fishing a hose down the flue floods the fan and the flame rod.

Step 4: Atmospheric B-vent and power-vent tanks — rain vs a dying collector

• What to look for: Missing chimney cap, a lateral that holds water, a power-vent PVC run with a belly. Cool rain after a storm at a draft hood can be weather. Water that continues in dry weather, or hot water at the hood, is a tank or collector failure — treat like water inside and do not fire a wet burner tray. Tissue at the draft hood should pull in with the burner on; if it spills, shut it down. House static over 80 psi and a T&P dumping 150 psi / 210°F steam near the vent is not an exhaust leak — never cap that valve.
• Required tools: Flashlight on the vent path, CO alarm status. No combustion-chamber teardown.

A 4500 W electric tank has no exhaust. If you searched this on an electric, you have a steam leak or a wet element — different cavity.

Step 5: Freeze, silicone, and the “caulk it” dead end

• What to look for: Outdoor tankless in the 20s °F dumping freeze-protection ports under the cabinet — not the flue. A freeze-split exchanger weeps potable water into combustion after the thaw. Any joint someone sealed with high-temp caulk. That caulk comes off and the vent gets repaired or replaced.
• Required tools: Temperature history, eyes under the cabinet. No caulk tube.

White vapor at an outdoor condensing termination on a 20°F morning is usually steam — white smoke. A cloud indoors at a draft hood is a spill.

When DIY Ends: Calling a Licensed Pro

Stop if isolation valves do not kill the drip, the burner or PCB is wet, the draft hood spills, you smell exhaust, or anyone wants to caulk the vent. You may photograph the termination, clear an external condensate trap, and close isolation valves. Heat-exchanger diagnosis, B-vent/power-vent repair, combustion, and Category IV vent replacement are licensed. Manufacturers will not smile on an exchanger that ran with a blocked condensate line or a flue full of silicone.

Frequently Asked Questions

Q: Why is water dripping from my water heater exhaust? A: Condensate the drain cannot take, rain in the cap or chimney, or potable water from a cracked heat exchanger or tank collector. Isolation-valve a tankless; if the drip continues from the water side, shut it down and hire a licensed tech.

Q: Is a little water at a water heater vent normal? A: On a condensing unit, ounces of cool condensate at a proper drain after a long fire can be normal. A stream from the exhaust, any hot drip, a wet atmospheric draft hood in dry weather, or indoor flue-gas smell is not.

Q: Can I seal a leaking water heater exhaust with high-temp caulk? A: No. Sealing a flue is a carbon monoxide failure. Fix the condensate path, the rain cap, or replace a failed exchanger or tank. Caulk is how closets fill with flue gas.