Rinnai Leaking by Exhaust When Turned Off
The Short Answer: Water at a Rinnai exhaust after the burner shuts off is leftover condensate, rain in the termination, a blocked trap blowing droplets out the flue, or potable water from a cracked heat exchanger. Ounces of cool drip at an outdoor cap can be normal on a condensing unit. A steady stream with the unit off and isolation valves open is not. Never silicone a vent.
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Search Intent & The Real Problem
The burner went quiet. Then the flue dripped. That timing is the diagnostic. Condensing Rinnai (RU, RUR, Sensei / I-series) make acidic condensate in the secondary exchanger; after a call for heat the fan post-purges (often around a minute) and can push leftover droplets out the termination if the trap cannot take them. Non-condensing RL / V / R53 / R75 / R94 units should not be raining from the flue. Water there with the unit off and the water side still pressurized is how a cracked exchanger announces itself.
Do not confuse this with T&P discharge or a cabinet-bottom leak. Related weather failures: tankless after rain.
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 hot-water outlet; it cannot leak “by the exhaust.” Silicone on a concentric vent is how people get flue gas in a closet. A condensing unit with a clogged condensate line will throw water out the only remaining hole: the exhaust.
Step-by-Step Troubleshooting
Step 1: Identify the fluid at the flue
| Clue | Source |
|---|---|
| Cool, slightly acidic (pH ~3–5), ounces after a long fire, condensing model | Leftover condensate / post-purge droplets |
| Only after wind-driven rain, outdoor termination | Rain in the cap — not a “heater leak” |
| Cool drip that continues, trap/line uphill or frozen | Condensate backup blowing out the exhaust |
| Hot or house-temp, larger volume, unit electrically off, isolation valves still open | Potable water from a cracked heat exchanger |
| White scale crust at the flue collar or burner | Exchanger or collector leaking into combustion |
- What to look for: Temperature of the drip, pH strip if you have one, whether it is the vent cap, the flue collar at the cabinet, or the condensate fitting.
- Required tools: Paper towel, flashlight. Optional pH strip.
Step 2: 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.
- Required tools: Isolation valves. If you do not have service valves, that is an install defect; stop and hire the job.
Do not keep firing a unit that put potable water in the burner. Error history 10 (air/exhaust), 12 (flame), or 16 (overheat) next to a wet flue is the same story.
Step 3: Condensate trap, slope, and the outdoor cap
- What to look for: Trap dry or packed, 1/2” 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 “to flush it” floods the fan and the flame rod.
Step 4: Condensing vs non-condensing — different tolerance
- What to look for: Rating plate. Condensing (typically 0.90+ UEF, PVC or polypropylene vent allowed on many SKUs): some post-fire drip at a proper drain is expected; drip out the exhaust means the drain path failed. Non-condensing (Category III stainless vent, older RL/V/R53): little condensate. Water at that exhaust is rain or a water-side failure until proven otherwise.
- Required tools: The jacket sticker.
R53-class problems live here: Rinnai R53. Humming or fan noise after shutdown is a different article: Rinnai humming.
Step 5: Freeze drains are not the exhaust
- What to look for: Outdoor unit, night in the 20s °F, water from freeze-protection ports or drain-down solenoids under the cabinet — not the flue. Those ports can dump the waterways on purpose around the mid-30s °F. A stuck freeze valve looks like excessive draining. A freeze-split exchanger weeps potable water into combustion after the thaw.
- Required tools: Temperature history, eyes under the cabinet.
When DIY Ends: Calling a Licensed Pro
Stop if isolation valves do not kill the drip, the burner or PCB is wet, you have 10/12/16 after water in the flue, or anyone wants to caulk the vent. You may photograph the termination, clear an external condensate trap, and close isolation valves. Heat-exchanger diagnosis, combustion, and vent replacement are licensed. Rinnai 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 my Rinnai leaking by the exhaust when it turns off? A: Post-purge blowing leftover condensate because the trap cannot drain, rain in the termination, or potable water from a cracked heat exchanger that still has line pressure. Isolation-valve the unit; if the drip continues from the water side, shut it down and hire a licensed tech.
Q: Is a little water at a Rinnai 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 drip on a non-condensing RL/V/R53, or hot water at the flue is not.
Q: Can I seal a leaking Rinnai exhaust with high-temp caulk? A: No. Sealing a flue is a carbon monoxide failure. Fix the condensate path or replace a failed exchanger. Caulk is how closets fill with flue gas.