Rheem Tankless Smells Like Gas? Leave

The Short Answer: Rotten-egg odor at a Rheem tankless — RTG, RUTG, RTGH, Prestige, Paloma twin — is mercaptan or unburned fuel from a failed trial until a fitter says otherwise. If the room smells, leave. Do not keep resetting 11 or 12. Cool PVC bite on a condensing Prestige is not gas. Sulfur only at the hot faucet is not this emergency.

Search Intent & The Real Problem

You walked past the outdoor RTG or opened the closet on an indoor RUTG / Prestige and caught a matchbook smell when it fired — or when it sat. People call that “normal tankless exhaust.” Combustion at a legal outdoor cap can have a faint burnt edge for one second at light-off. Mercaptan in the room is not that. Paloma-based Rheems that throw 11 (never lit) or 12 (lit, then died) dump a timed shot of gas, then spark. That smell is the lockout. Keep cycling it and the cabinet fills.

Generic exhaust odor: tankless exhaust smells like gas. Navien twin: Navien exhaust. Closet idle: closet smells like gas. Odor only on a heat call: when it turns on. This page is Rheem / Ruud / Richmond tankless.

What other guides get wrong

They tell you a puff at the concentric is leftover air. Air after a swap is real — and it is a licensed purge, not your remote. They call every sharp odor at Prestige PVC “normal high-efficiency smell” so you stay in a filling room. They paste Rinnai soap-test steps onto a Paloma burner with the cover off. They confuse sulfur in hot water with fuel in the air. They still describe cracking the union. Do not purge it.

Step-by-Step Troubleshooting

Step 1: If indoor air smells like gas — you are done

• What to look for: Rotten-egg odor in the closet, laundry, or garage when the Rheem fires or sits. Hissing. Headache. Kids in the house.
• Required tools: Your feet, a phone outside, the utility emergency number.

Evacuate. Do not operate the unit, furnace, lights, or the garage door opener. The utility shuts the meter. A plumber leak-tests after the line is safe. Wet cabinet after this scare is still leave-first — leaking inside.

Step 2: Name where the odor is — flue vs room vs water

• What to look for: Indoor vs outdoor. Idle vs firing. Cold water vs air. Soot at the cap. A vent joint that walked. White vapor on a 20°F morning is often steam — white smoke.
• Required tools: None on the gas train.

Need it off while you wait? Apartment shutoff.

Step 3: What you may look at without touching gas joints

• What to look for: From a clean-air doorway after the utility, or from the yard at an outdoor RTG that is off: appliance shutoff handle position. Flex rusted, kinked, or 1/2” on a 180–199 kBTU burner. Missing sediment trap. Termination full of snow, a nest, or a contractor cap still on the intake. Indoor Category III or PVC joint that pulled apart. Combustion fan that did not prove before the valve opened — blower dead is a CO path if you keep firing.
• Required tools: Flashlight, rating plate (BTU/hr, NG vs LP, RTG / RUTG / RTGH / Prestige). No wrenches. No dish soap in an occupied house that smelled.

A starving burner lights late and smells. Typical manifold is about 3.5–7 in. w.c. natural gas / 10–11 in. w.c. LP — confirm the plate. That measurement is licensed. Wrong-fuel DIP after a move dumps raw gas out the flue on every try.

Step 4: Failed 11/12 trials you must not reset through a smell

• What to look for: Photograph 11 vs 12 vs 10 vs 13 before you kill power. Each failed trial is unburned gas in the cabinet, then the next spark, then odor at the exhaust. 10 is air/exhaust so it never burns clean. A bang is loud bang — shut down. Rain in an outdoor burner: dead after rain.
• Required tools: Photo of the code. Hand on the switch to leave it off.

Do not keep resetting a 12 through a smell. That is how Paloma cabinets fill. New unit the night after a swap: after replacement, after lighting.

Step 5: What a licensed fitter actually does at the exhaust

• What to look for: Standing-pressure test first, then a fire test: sniff at the inlet union, flare, valve, burner, and the termination while it fires. Manometer against the rating plate. Combustion: stable blue, CO at the cap in spec, no rollout, vent attached. Flame-rod microamps. Condensate trap primed on Prestige so the fan is not shoving flue products out a backup path. Control-box odor after the utility: control box leaking gas.
• Required tools: On their truck. Not yours.

A leak that is tight at rest and wet at fire is still a leak. Dish soap in a house that already smelled is how people stay in a filling room.

When DIY Ends: Calling a Licensed Pro

It ended the first time mercaptan tracked the Rheem flue or the closet. You may leave, call the utility, photograph a code from a clean-air doorway, and hire a licensed plumber to leak-test at fire and check combustion at the termination. You may not soap the burner while it runs, hold the remote to “reproduce” the smell, or keep resetting an 11. Same-day licensed repair is the product. Watching it fire one more time is not.

Frequently Asked Questions

Q: Why does my Rheem tankless water heater smell like gas? A: Unburned fuel from a failed 11/12 trial, delayed ignition, a vent dumping into the space, incomplete combustion, or a leak that only shows when the valve opens. If the room smells, leave and call the utility.

Q: Is a gas smell at a Rheem tankless exhaust normal on startup? A: No. A clean burner should not put mercaptan in the room. A brief hot-metal smell on a brand-new unit is not gas. A whoosh is delayed ignition — shut it down. Cool PVC bite on a Prestige is not automatically mercaptan; if you cannot tell, leave.

Q: The smell is only outside at the RTG cap. Can I keep using it? A: Not if you can smell mercaptan, it bangs, or 11/12 keeps returning. A faint burnt edge for one second at an outdoor cap with a clean house still gets a licensed combustion check if it repeats. Indoors is always leave.