Rheem Tankless Water Heater Troubleshooting

The Short Answer: A Rheem tankless that will not heat is four folders: no 120 V, no flow call, a two-digit lockout, or freeze/gas. Photograph the remote before you kill power. 11 never lit. 12 lit, then died. 14 is a thermal fuse. Do not vinegar-flush a dark controller. A cold tank Rheem is a different machine.

Search Intent & The Real Problem

Every hot tap is ice. The RTG / RTGH / EcoSense / Performance remote is blank, flashing two digits, or sitting there with no fan. Paloma-based Rheem, Ruud, and Richmond tankless share this board. The internet said “reset it” and “descale with vinegar.” Resetting an 11 into a dark burner glazes the igniter. Descaling a unit with a closed isolation valve or a packed inlet screen does nothing except flood a mixer. Sister no-heat tank: Rheem not heating. Ruud badge: Ruud troubleshooting. Mid-shower dropout with no code: hot to cold. This page is the dead tankless hub.

What other guides get wrong

They paste a Rinnai 10 table onto a Paloma board and call every dead Rheem “scale.” Scale is 14 / 16. They mix electric RTE / RTEX E-codes with gas 11. They send you at a manometer. They skip isolation handles left 45° after a flush. They treat 13 remaining-flame as a reset. And they still describe cracking the union — do not purge it.

Step-by-Step Troubleshooting

Step 1: Name the machine, then split power, flow, and code

• What to look for: Rating plate (RTG-64/84, RTGH-90/95, indoor RUTG twins, Prestige condensing). Photo of the code and the plate. Closest hot faucet as the test.
• Required tools: Phone camera. No wrenches.

Failed-trial language on a tank Honeywell LED is failed ignition lockout, not this Paloma chart.

Step 2: 120 V — then one hot call

• What to look for: Outdoor GFCI popped. Disconnect off. Display woke Off. Switch off 10–30 seconds, on. Fan must spin and prove before the valve opens. No whir, no fire — same CO rule as a dead blower. After a storm: tankless after rain. A whoosh then a slam is delayed ignition — shut it down.
• Required tools: Eyes. Receptacle tester on a GFCI. Ears on the cabinet.

Reset a GFCI once. If it trips again, leave it off. Freeze protection is electric. Do not boot an outdoor unit that iced.

Step 3: Flow — isolation valves and the inlet screen

• What to look for: Service valves left 45° after a flush. Cold-inlet screen packed so the board never sees min GPM — no code, not 11. Isolation shut, bucket, rinse. Open both handles fully. A packed screen that never calls is not a thermal fuse. Sister flow physics: Rinnai no hot water as a method, Rheem digits as digits.
• Required tools: Isolation shut, bucket, needle-nose. No vinegar yet.

Do not jumper a flow sensor to force a trial. House static over ~80 psi lifting a T&P is pressure, not a dead Paloma.

Step 4: Read the two-digit code — then stop guessing

• What to look for: 11 = never proved flame (gas cock, LP, spark, thermal fuse open, drowned burner). 12 = lit, then lost flame (rod, wind, pressure). 10 = air/exhaust or false flame — your manual wins; vent and fan first. 14 is overheat / thermal fuse, not an igniter. Photograph, fix the obvious cause, one reset. Repeated 11 is how ceramics crack. Gas cock parallel to the pipe. Other appliances lighting. Empty LP. No gas at other loads is not getting gas, not a board.
• Required tools: The photo. Eyes on the cock. No manometer.

Inlet and manifold pressure are licensed: NG often ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the rating plate. 4500 W / 240 V ~18.8 A / ~12–13 Ω is tank-element math. This chassis is 120 V plus gas. Thermocouple ~20–30 mV numbers belong on a tank valve, not a Paloma flame rod (licensed microamps).

Step 5: Condensate, leak, and when one reset is enough

• What to look for: Trap packed, drain looped uphill, frozen outdoor condensate. Water in the cabinet is a leak first, not a 11. Exhaust odor indoors: leave. Unit fires in the trial window, stable blue, no bang, code gone on the next shower. Same digit on the next call is licensed. Need it off while you wait? Apartment shutoff.
• Required tools: Eyes on the trap and the cap from the ground. Paper towel. Power off before any cover.

Do not steel-wool a flame rod every weekend. Do not hold a tripped breaker On.

When DIY Ends: Calling a Licensed Pro

Stop at gas odor, remaining-flame 13, delayed-ignition bangs, a silent fan, water on the PCB, and any gas-pressure or gas-valve work. You may photograph the code, restore a GFCI once, open isolation valves, rinse the inlet screen, and power-cycle once. Combustion, manometer, igniter or flame-rod replacement inside the burner, thermal-fuse replacement on a scaled exchanger, and PCB work are licensed. A Rheem tankless that never called for fire is usually power or flow — not a new heat exchanger by default.

Frequently Asked Questions

Q: Why is my Rheem tankless water heater not working? A: No 120 V, closed isolation valves or a packed inlet screen, or a two-digit lockout. 11 is ignition failure. 12 is flame lost after it lit. 14 is thermal fuse / overheat. Photograph the remote before you clear it.

Q: How do I reset a Rheem tankless water heater? A: Photograph the code, fix the obvious cause (gas cock, LP, GFCI, isolation valves, vent cap), then power off 10–30 seconds. One reset. A code that returns is a licensed diagnosis, not a reset schedule.

Q: Is a Rheem tankless the same as a Rheem tank? A: No. Paloma-based RTG / RTGH boards use two-digit codes. A tank Rheem uses a gas-valve status light or an electric ECO. Do not paste a tank 5-flash chart onto a tankless 11.