Rinnai Water Heater Leaking

Common Symptoms

• Water dripping from the bottom of the Rinnai unit
• Water pooling under the unit on the floor
• T&P relief valve dripping from the discharge pipe
• Water at the union fittings on the cold inlet or hot outlet connections
• Code 03 displayed — freeze protection activated
• Condensate drain line producing more water than usual
• Water seeping from the heat exchanger body (perforation from scale or freeze)

Most Likely Causes

1. 1

   Condensate Drain Line Leak (Normal Flow vs. Heat Exchanger Perforation)

   Rinnai RUR and RUS condensing models produce acidic condensate (pH 3–5) during normal operation — a small, steady flow from the 1/2-inch flexible condensate drain tube is expected and not a leak. An abnormal increase in condensate volume, condensate dripping from the heat exchanger body rather than the drain tube, or water with a bluish or rust-colored tint from the drain indicates a more serious problem. Code 14 (thermal fuse trip from overheating) can precede a heat exchanger perforation on heavily scaled condensing units — scale creates localized hot spots that eventually erode the heat exchanger wall. A perforated heat exchanger allows water to contact the exhaust gas side, producing an acidic mist that drips from unexpected locations. Test: check if the 'condensate' is actually soapy or has a metallic smell — these indicate it is potable water from a perforation, not normal condensate. If perforation is confirmed, the heat exchanger must be replaced.

2. 2

   T&P Pressure Relief Valve Drip (Expansion Tank Required on Closed Systems)

   The T&P (temperature and pressure) relief valve on a Rinnai tankless unit opens at 150 PSI or 210°F to prevent catastrophic overpressure. Intermittent dripping from the T&P discharge pipe in a system that has a check valve, pressure-reducing valve (PRV), or backflow preventer installed on the cold supply is almost always caused by thermal expansion pressure — not a failed valve. When water heats up and expands, the expansion has nowhere to go in a closed system, so pressure rises until the T&P opens. Fix: install a thermal expansion tank on the cold supply between the shutoff valve and the unit. After installing the expansion tank, if the T&P continues to drip, test the pressure with a gauge; above 80 PSI, adjust the PRV. Replace the T&P valve only if it drips at normal operating pressure after an expansion tank is installed.

3. 3

   Inlet/Outlet Union Fitting Leak (Teflon Tape + Pipe Dope Required)

   Rinnai tankless water heaters use threaded union fittings on the cold water inlet and hot water outlet connections. These unions can weep at the threads if they were not sealed with both PTFE (Teflon) tape and pipe thread sealant (pipe dope) during installation, or if the unions were tightened and loosened during service and not re-sealed. A union leak is typically a slow drip that is worse when the unit has been running. Fix: shut off cold water supply, open a hot tap to depressurize, unscrew the leaking union, clean the male threads with a wire brush, wrap with 3–4 turns of PTFE tape clockwise on the threads, apply a thin coat of pipe dope over the tape, and re-tighten (hand-tight plus 1–2 turns with a wrench — do not over-torque the plastic union nut).

4. 4

   Flow Sensor O-Ring Failure

   The cold water flow sensor inside the Rinnai unit contains an O-ring seal between the sensor body and the water manifold. Over time, the O-ring can degrade and allow water to seep past the sensor housing, producing a drip at the bottom of the unit that appears to come from the heat exchanger area. The leak is usually a slow weep rather than a stream, and may worsen as water pressure increases. Diagnosis: with the front panel removed, run the unit and watch the flow sensor body — a weep visible at the sensor housing confirms an O-ring failure. Fix: shut off water supply, remove the flow sensor (model-specific procedure — consult Rinnai service manual), replace the O-ring with an OEM-spec Rinnai part, lubricate with silicone grease, and reinstall.

5. 5

   Freeze Damage — Code 03 and Post-Freeze Inspection

   Rinnai tankless units (all V, RU, RUR, RUS series) have built-in freeze protection circuits that circulate water through the heat exchanger using small electric heaters when ambient temperature drops near freezing. Code 03 indicates the freeze protection circuit is active or was activated — it is informational and clears when the unit warms up. However, the built-in freeze protection ONLY works when the unit has AC power. If power was lost (storm, tripped breaker) during a hard freeze, the heat exchanger may have cracked. Post-power-outage freeze inspection procedure: close the cold water isolation valve; open a hot tap to relieve any remaining pressure; slowly open the cold valve just enough to pressurize the unit; inspect all connections and the heat exchanger body for dripping or weeping. If the heat exchanger shows cracks or weeping after freeze exposure, the exchanger must be replaced — do not operate the unit.

6. 6

   Scale-Perforated Heat Exchanger on RUR/RUS (Descale vs. Replace Decision)

   On Rinnai RUR and RUS condensing models with years of scale accumulation, the heat exchanger can develop pinholes or micro-fractures where scale-induced hot spots thinned the tube walls. This produces a characteristic leak: water appears inside the combustion/exhaust chamber rather than at plumbing fittings, and the condensate drain may carry potable-water volume far exceeding normal condensate production. Descaling at this stage rarely seals existing perforations — descale first to confirm the heat exchanger is not simply coated (a successfully descaled exchanger with no leaks visible is reusable), but if leak points are confirmed after descaling, the heat exchanger assembly requires replacement. Rinnai heat exchanger replacement is a significant repair ($400–$800 parts + labor) — compare against the unit's age and the 12-year residential warranty before proceeding.

Quick DIY Checks

1. 1Step 1 — Identify the leak source precisely: remove the front cover and use a flashlight and dry paper towel to find exactly where water originates. Wipe all visible components dry, then run a hot water draw and watch under flow. Key locations: union fittings on cold inlet and hot outlet (threaded connections at the top of the unit), the T&P valve discharge pipe (usually a 3/4-inch pipe running to a drain), the flow sensor body on the cold side, the condensate drain port at the bottom of RUR/RUS units, and the heat exchanger body itself. Accurately locating the source before starting any repair prevents unnecessary disassembly.
2. 2Step 2 — Repair a union fitting leak (Teflon tape + pipe dope): close the cold water supply valve and open a hot tap downstream to release pressure. Remove the leaking union fitting (cold inlet or hot outlet). Clean male threads with a wire brush to remove old sealant residue. Wrap 3–4 turns of PTFE (Teflon) tape clockwise around the threads, starting at the first thread. Apply a thin coat of pipe thread sealant (pipe dope) over the tape. Re-thread the union and tighten by hand, then one to two turns with a wrench. Do not over-torque plastic union nuts — hand-tight plus one full turn is sufficient. Restore water supply and check for drips under pressure for 5 minutes.
3. 3Step 3 — Address T&P valve dripping (expansion tank): if the T&P valve drips intermittently — especially after a hot water draw — and the system has a check valve or PRV on the cold supply, the fix is an expansion tank. Install a potable-water thermal expansion tank on the cold supply between the shutoff valve and the unit (typically 2-gallon for tankless; a plumber should verify sizing based on supply pressure). Pre-charge the tank's air bladder to match cold supply pressure (typically 50–80 PSI) before installing. After installing the expansion tank, monitor the T&P for 48 hours — dripping should stop. If the T&P still drips, measure supply pressure with a gauge; above 80 PSI, reduce with the PRV.

1. 4Step 4 — Replace the flow sensor O-ring: shut off cold water supply and unplug the unit. Remove the front cover and locate the flow sensor on the cold inlet manifold — it connects via a two-wire or three-wire electrical connector. Disconnect the wiring harness, remove the retention clip or screw holding the sensor, and slide the sensor out of the manifold. Identify the O-ring (a small ring seated in a groove on the sensor body or on the manifold port). Replace with an OEM Rinnai O-ring for your model (part number in the service manual) — do not use generic O-rings, which may not be rated for potable water temperature range. Lubricate the new O-ring with food-grade silicone grease. Reinstall the sensor, reconnect wiring, restore water supply, and check for leaks.
2. 5Step 5 — Perform post-freeze inspection (Code 03 or after power outage in cold weather): if Code 03 appeared or if the unit lost power during below-freezing temperatures, inspect for freeze damage before restarting. Close the cold supply isolation valve. Open a hot tap to relieve pressure. Slowly crack open the cold supply valve just enough to pressurize the unit — do not open it fully. Inspect the heat exchanger body, all water connections, and the union fittings for any weeping or dripping while under pressure. If any leak appears from the heat exchanger body, close the cold supply immediately — a freeze-cracked heat exchanger must be replaced before the unit can be used. Do not operate a Rinnai with a cracked heat exchanger; water can contact the burner assembly.
3. 6Step 6 — Evaluate a condensate leak on RUR/RUS (descale vs. replace): if water appears from the heat exchanger body on an RUR or RUS condensing model, first descale the unit by circulating white vinegar or CLR through the service port isolation valves for 60–90 minutes. After flushing and restoring operation, watch under a hot water draw for 5 minutes. If the leaking stops after descaling, the unit likely had a loose scale deposit blocking a very small weep point that is now visible but self-sealing after descale; monitor over the next few days. If the leak continues after descaling, the heat exchanger has a true perforation. At this point, compare the cost of heat exchanger replacement ($400–$800 parts + labor) against the unit's age — if older than 10 years or outside the 12-year warranty, replacement of the full unit may offer better long-term value.

Repair vs Replace

Recommended Tools & Parts

• PTFE (Teflon) Tape and Pipe Thread Sealant (Pipe Dope)

  Required for re-sealing Rinnai union fittings on cold inlet and hot outlet connections. Use both tape and paste-style pipe dope together — tape alone is insufficient for water connections that are repeatedly tightened. Do not use tape on flare or compression fittings.

  $5–$15

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• Thermal Expansion Tank (2-gallon, potable water rated)

  Required for closed plumbing systems (those with a check valve, PRV, or backflow preventer) to prevent T&P valve dripping from thermal expansion pressure. Pre-charge to match cold supply pressure before installing. Verify sizing with a plumber for systems above 80 PSI.

  $30–$80

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• Rinnai Flow Sensor O-Ring Kit (model-specific)

  OEM replacement O-ring for the Rinnai flow sensor body or manifold port. Fixes slow weep at the flow sensor housing. Use only OEM Rinnai spec — generic O-rings may not match the temperature and chemical resistance requirements. Check the part number in your unit's service manual.

  $10–$25

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• White Vinegar or CLR Calcium, Lime & Rust Remover (Descale Kit)

  Descaling solution for Rinnai RUR/RUS condensing heat exchanger to address scale-related leaks. Use 3 gallons undiluted white vinegar or CLR, circulated via service port isolation valves and a submersible pump for 60–90 minutes. Required before replacing a suspected perforated heat exchanger to confirm the perforation is genuine.

  $10–$25

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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