Rheem Tankless Combustion Codes 13 & 17: Meaning & Fixes

Combustion codes are the ones owners fear most, because they sit next to gas and flame — and this family needs a steady hand and honest sorting. Of the three codes gathered here, only one is truly a combustion fault: 13, the flame rod reporting poor combustion on tankless models. Code 17 turns out to be an impostor on combustion lists — Rheem documents it as a water leak code on WiFi tankless models — and 45 is not a Rheem code at all. This guide gives each code its documented meaning, its causes, the checks an owner may safely make, and the point where combustion work belongs to a licensed technician.

The shared system: how a tankless burner proves it is burning safely

Every code in this article hangs on one safety idea: a gas burner must never run unproven. A Rheem tankless heater lights in sequence — flow sensed, blower purge, gas valve staged open, igniter spark — and then a flame rod sitting in the flame proves combustion electrically, by passing a small current through the flame itself. The control watches that proof continuously. If flame is never proven, you get the code 11 story; if it is proven and then lost repeatedly, code 12; and if the flame exists but burns badly — weak, unstable, starved of air or fuel — the rod's signal degrades and the combustion family in this article begins. Poor combustion is not an abstraction: it means fuel and air are mixing wrongly, which can mean soot, carbon monoxide in the exhaust, flame lifting off the burner, and heat where it should not be. That is why Rheem treats combustion faults conservatively, why the documented owner checks are all external (gas valve position, LP level, vent terminations), and why everything involving the burner, the gas valve, pressures, and the flame rod itself belongs to a licensed technician with a manometer and a combustion mindset. Keep that division in mind through every section below: observe and report as an owner; open and adjust as a technician.

Code 13: flame rod detecting poor or improper combustion

On indoor tankless units of the RTG generation — and as a hard-lockout code on IKONIC-era models — 13 means the flame rod is detecting poor combustion. Rheem's SVC 820 Pocket Manual states it plainly: indoor units only; the flame rod is detecting poor combustion; this is commonly caused by inadequate gas supply and/or venting. Note the difference from 11 and 12: the flame exists and is being sensed, but its quality is wrong — the rod's proving current is weak or unstable because the flame itself is weak or unstable. The documented diagnostic checks are gas supply and venting, with two specifics worth knowing. First, the altitude setting: the manual hints to make sure the DIP2 altitude setting is properly set, because a unit configured for the wrong altitude meters its fuel-air mixture for air density it is not breathing — a unit moved, or a setting never corrected at install, can burn poorly at altitude forever. Second, the hard-lockout behaviour: if the unit shuts down 5 times within a 4-hour period due to code 13, it must be reset by performing the full reset procedure — the control stops offering casual retries and demands the formal one. Owner checks are the external trio: gas isolation valve fully open, LP tank not low, both vent terminations clear and the venting correctly installed — Rheem's related guidance stresses that exhaust must never recirculate into the clean-air intake, a real risk with concentric or closely-spaced terminations, because breathing its own exhaust is a direct recipe for poor combustion. Everything deeper — flame rod removal and cleaning, manifold pressures, blower inspection — is technician work, and a 13 that survives clean vents and confirmed gas supply should go straight there.

Code 17: the leak code hiding on combustion lists

Here is the correction this article owes you: on Rheem's documentation, 17 is not a combustion code at all. Rheem's Tankless Troubleshooting Manual gives 17 its own section titled 'Water leak detected', and the explanation is exact — this error code is displayed only when a WiFi controller is used; if the water leak sensor detects a water leak inside the water heater, 17 is displayed on the built-in or remote controller, and the heater may have water leakage from somewhere. The manual's diagnostic items are the heat exchanger and the water valve, with an honest note that condensation buildup in the cabinet, or condensation entering from the intake venting, can also trigger the sensor without any component leaking. How did a leak code end up filed under combustion? Probably because third-party lists copy each other without reading manuals, and a two-digit code 'should' be about the burner. The practical harm is real: an owner who believes 17 is about combustion will reset and watch the burner while water quietly damages the unit and the room. Treat 17 as what Rheem says it is: turn the power off, close the water shutoff, and look — inside the cabinet bottom, under the unit, at the fittings. Rheem's flow is to dry the sensor and determine whether the water is internal (heat exchanger or valve — a replacement-level problem) or external/condensation. Its companions matter too: 48 means leak detection has been disabled in the app for 24 hours after a 17, and 49 means the leak sensor is not installed at all. Never silence a 17 into a 48 without knowing where the water is.

Code 45: the code that is not Rheem's

Some lists of Rheem tankless codes include a 45, usually defined as a water leak inside the unit. Rheem's manuals do not. Code 45 appears nowhere in Rheem's Tankless Troubleshooting Manual, the RTGH service chart, the Rheem Australia fault chart, or the older RTG manual — and there is a reason it sounds like a leak code: it is one, but on a different brand. Code 45 is a Noritz code (the NRCP series uses it for a water leak detected inside the unit), and it migrated onto Rheem lists through the copy-and-paste ecology of third-party code pages, each citing the last. This matters beyond pedantry. An owner with a Rheem showing a leak symptom who searches '45' will find confident instructions for a heater they do not own, while their actual Rheem leak codes — 17 on WiFi tankless models, with warnings 48 and 49, and A107 on hybrid models — go unread. If your Rheem display genuinely shows 45, the honest possibilities are a misread display (a partially lit digit can make a 43 or 95 look like 45), a remote controller fault garbling its output, or a non-standard controller fitted at some point — all of which argue for photographing the display and calling a technician or Rheem support rather than acting on a borrowed meaning. This site lists 45 as unverified for Rheem, and documents what Rheem actually uses for leaks, so the next owner lands on the truth first.

Diagnosis order when combustion is the complaint

Whether the display shows 13 or the symptom is sooty, smelly, unreliable burning, work the same order Rheem's manuals work — from the outside in, from the free to the instrumented. First, fuel at the boundary: gas isolation valve fully open, LP tank level read with your own eyes, other gas appliances burning with strong flames. Second, air at the boundary: both vent terminations inspected from outside, the intake confirmed unobstructed with its required clearance, and no sign that exhaust can recirculate into the intake. Third, the unit's own story: read the fault history if your model offers it, because a pattern of 13s building over weeks says 'degrading' — fouling rod, sagging supply — while a sudden 13 after roofing work says 'check the termination someone disturbed'. Fourth, one documented reset and a watched test run: does the burner light cleanly, does the flame through the sight glass look steady, does the code return in seconds (sensing) or under load (supply)? Only after those four steps does diagnosis move inside the cabinet — gas pressures static and dynamic, gas filter, flame rod condition, blower performance, control outputs — and that is the line where the work changes hands. Skipping to step five is how parts get replaced on heaters whose only fault was a blocked termination.

What poor combustion looks, sounds and smells like

You cannot meter a mixture without instruments, but combustion announces its health through senses every owner has, and Rheem's diagnostic culture assumes you will report these observations. Sight first: through the sight glass, a healthy tankless flame is a steady, even blue sheet across the burner that lights promptly and sits still; a flame that rolls, lifts at the edges, burns yellow and lazy, or dances when the blower changes speed is a mixture or pressure problem made visible. Sound second: ignition should be a soft whoomph, not a bang, a rumble, or a delayed pop — delayed ignition means gas accumulated before it lit, which is poor combustion's dangerous cousin and a stop-and-call symptom on its own. Smell third: a faint exhaust note at the termination outdoors is normal; combustion odour indoors, at the cabinet, or around vent joints is never normal and means exhaust is escaping where it should not. Touch and residue complete the picture: soot streaking at the termination or cabinet seams, and unusual heat on the vent pipe, both say the exhaust is carrying unburned fuel or running too hot. None of these observations replaces a combustion analyser, and a carbon monoxide alarm sounding in the home overrides every code in this guide — leave and call emergency services. But a precise sentence ('yellow flame, lights with a pop, soot at the termination since March') is worth an hour of a technician's guessing, and it costs nothing but attention.

Reading 11, 12 and 13 as one story

The three flame codes are less like separate faults and more like chapters of one deterioration, and reading them together — especially in the fault history — tells you where a heater is heading. A unit usually begins with 13s: combustion is still happening, still proven, but the flame rod's signal is marginal because the mixture is slightly wrong, the rod slightly fouled, the pressure slightly low. Nothing feels broken; the water is hot; only the history knows. As the cause deepens, marginal flames start dropping out entirely and the story becomes 12s — flame proven, then lost, three in a row before the code even displays — and the household starts reporting cold bursts mid-shower. Left alone, a supply or sensing fault that produced 13s and 12s matures into 11s, when the flame can no longer be established at all, and the unit goes from unreliable to dead. This progression is why a technician who finds a clean 11 today will still ask about yesterday: weeks of 13s in the history point at mixture and fouling, while a sudden 11 with no ancestors points at an event — a valve closed, a tank emptied, a vent blocked. It is also why clearing history casually is a loss: the DIP-switch procedure that flashes CL exists for after the repair, not before the diagnosis. Treat the three codes as a timeline, record which ones appear and in what order, and you have done half the diagnostic work before anyone opens the cabinet.

Natural gas and LP do not fail the same way

Combustion codes wear different faces on the two fuels, and Rheem's pressure specifications explain why: the manual requires 4.0 to 10.5 inches of water column at the unit for natural gas, and 8.0 to 13.0 for LP — propane runs at higher pressure, delivered from a finite tank through a regulator, while natural gas arrives from a street main that almost never 'runs out'. On LP, the classic combustion story is starvation with warning: as the tank empties — and faster in cold weather, when the liquid vaporises slowly — pressure decays under draw, flames weaken (13), begin dropping (12), and finally will not light (11), all while the gauge still shows a sliver of fuel. The regulator adds its own failure modes, including vent blockage and age-related drift. On natural gas, outright absence usually means a closed valve or utility work, and the subtler fault is dynamic sag: pressure that measures fine at rest but collapses when the water heater, furnace and range draw together through an undersized line — the manual's insistence on testing pressure at maximum combustion exists precisely for this. Altitude overlays both fuels: thinner air at elevation needs the unit's DIP2 altitude configuration set correctly, or the mixture runs rich and sooty on either fuel. When you book a combustion call, the first sentence should be the fuel, the tank state if LP, and whether other appliances were running — those facts route the entire diagnosis.

When combustion work stops being DIY

Combustion is the area of water heating where the DIY line is drawn brightest, and Rheem's manuals draw it consistently. The moment a repair requires opening the burner compartment, breaking a gas joint, measuring or adjusting manifold pressure, replacing a gas valve or flame rod, or altering venting beyond clearing a termination, it belongs to a licensed gas technician — not because owners are incapable, but because the failure mode of a mistake is carbon monoxide in a home. Certain presentations skip the queue entirely: any smell of gas (leave, call the utility), soot streaking at the vent termination or cabinet, a burner that lights with a bang or a rolling yellow flame, and any code in the false-flame family (72, 80) where the control reports flame at impossible times. A hard lockout from repeated 13s also belongs on the professional list even if you can perform the reset procedure — five poor-combustion shutdowns in four hours is the heater insisting, formally, that its mixture is wrong. Book the visit, report the code, the count, the pattern, and what you checked. A good combustion technician would rather hear your termination was blocked and cleared than discover it after an hour of instrument work — and will tell you so.

Key takeaways

• Code 13 is the true combustion code here: think gas supply and venting first, altitude (DIP2) setting second, flame rod third — in that order.
• Code 17 on a Rheem is water, not fire: power off, water shutoff closed, and look for the leak before resetting anything.
• If a website gives Rheem code 45 a meaning, it is quoting Noritz. Rheem's leak codes are 17/48/49 (tankless WiFi) and A107 (hybrid).
• Exhaust recirculating into the intake is a real and fixable cause of poor combustion — check termination spacing after any roof or siding work.
• Five 13-shutdowns in 4 hours trigger a hard lockout by design; a unit in that state needs a technician, not a sixth reset.

Resetting the Rheem water heater

Each Rheem platform resets the way its manual documents, and only that way. Tankless models: close the hot faucet, turn the controller off, switch off the electrical supply, wait 5 minutes, restore power, turn the controller on, and open a hot tap; IKONIC displays also clear alarms via ENTER and 'Clear Alarms?' — Yes; hard lockouts (10, 13, 99) have a DIP-switch procedure that ends with the remote flashing CL (fault history cleared) or UL (reset confirmed). Tank gas controls have no reset button: the status LED clears when the underlying fault clears, and Rheem's chart routes 4, 5, 7 and 8 flashes straight to a qualified technician. Hybrids and smart electric tanks: read and note the alarm first (Next / Down arrows on the hybrid display), fix the cause, then clear; a leak alarm such as A107 can be overridden for 24 hours only by holding Clear Alarm for 5 seconds — time bought to arrange the repair, not to skip it. One reset after fixing the cause is the documented pattern on every platform; repeated resets against a live fault are not a strategy.

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Facts on this page were checked against manufacturer documentation: files.myrheem.com · files.myrheem.com · www.manualshelf.com · www.manualshelf.com · www.manualshelf.com · mans.io · manuals.plus · manuals.plus · manuals.plus