Rinnai Constantly Running: Recirc or Leak
The Short Answer: A Rinnai that “never shuts off” is usually the recirc pump or freeze protection, not a burner welded on. Circ-Logic and RUR pumps are supposed to move water. A burner that actually fires for hours with no fixture open is a stuck flow sensor, a dripping faucet, or a leak in the loop. Name pump vs flame before you replace an RU199. Sister complaint: Navien constantly running.
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Search Intent & The Real Problem
The closet whooshes all afternoon. The In Use lamp or the fan never drops. People hear the combustion fan or the recirc pump and swear the heater is constantly running. On an RUR (or an RU with an external Circ-Logic pump and MC-195-10) that pump can circulate on a schedule with the burner off. That is not a lockout. A burner in fire for an hour with every tap closed is flow the board can see: turbine stuck, recirc always-on, a 0.5 GPM drip, or a pinhole in the loop. Fan-only on a power-vent tank is a different machine — fan keeps running.
What other guides get wrong
They tell you the unit is oversized or “short cycling because it is too big,” then sell a tank. A Rinnai modulates. Always-on Circ-Logic is a menu, not a failed heat exchanger. They skip the dripping toilet fill valve that holds the turbine just above minimum fire. They oil the fan. And they confuse freeze-protection whir on a 20°F night with a haunted PCB.
Step-by-Step Troubleshooting
Step 1: Pump, burner, or freeze — they are not the same “running”
| What you see / hear | Meaning | Next |
|---|---|---|
| Pump hum, no burner roar, pipes warm at the recirc return | Circ-Logic / RUR pump doing its job | Check recirc mode, not the exchanger |
| Burner and fan on with no fixture and no recirc scheduled | Flow the board can see | Sensor, drip, leak |
| Fan on low, outdoor or garage unit, night below ~40°F, no water flowing | Freeze protection — leave it powered | Not a defect |
| Cycles every few minutes, then idles | Normal modulation around setpoint or a tiny draw | Time it; do not descale first |
| In Use lamp on with every tap closed | The board still sees a call | This page, not In Use dark |
| Error on the controller, still trying | Lockout / retry | Photograph the code — no hot water |
- What to look for: Flame / In Use vs pump behavior. Rating plate: RUR (onboard pump) vs RU/RU199 (no pump unless external) vs SENSEI. Recirc set to Always On will sound like it never sleeps. A hum that never knocks is fan or pump electrical.
- Required tools: Phone photo of the controller and the rating plate.
Step 2: Circ-Logic, crossover, and the far bathroom
- What to look for: MC-195-10 clock still inside the recirc window. Dedicated return vs a crossover valve under the far sink — a failed-open crossover keeps flow through the Rinnai all day and the burner will follow it. External pump on a timer that failed closed. Intelligent recirc that learned a bad pattern after a party weekend.
- Required tools: Recirc menu without changing gas parameters. Eyes under the furthest fixture for a crossover.
You can set recirc to off as a test. If the “constant run” dies when recirc is off, you found it. Do not start flipping dip switches on the PCB.
Step 3: Drips, leaks, and the flow sensor
- What to look for: Toilet fill valve hissing, shower valve weeping, hose bib that never seats, recirc loop pinhole, or an isolation-valve packing drip that is enough to keep the turbine spinning. Rinnai activation is on the order of ~0.4–0.6 GPM — a leak you would not notice as a “flood” is plenty. Inlet screen packed so the turbine chatters and the board never sees zero flow. Cabinet wet: isolate first — Rinnai leaking from the bottom, tankless draining.
- Required tools: Paper towel on every fixture, ears at the toilet, isolation valves.
Close the cold isolation. If the burner dies immediately, the “run” was real flow from the house, not a haunted PCB.
Step 4: Scale, sensors, and cascade
- What to look for: Outlet temp hunting, rapid fire/quit that feels like always-on, white flakes in the inlet screen. Scale makes overheat / LC history more often than a 12-hour fire, but a fouled inlet thermistor can keep it hunting. EZ Connect / MSA cascade: the secondary coming on because the primary still sees flow. Do not acid-flush through the house plumbing.
- Required tools: Inlet screen service with isolation closed. Licensed descaling if the exchanger is the restriction.
House static over ~80 psi does not make a Rinnai “run forever.” It lifts a T&P (typical 150 psi / 210°F). Different failure.
Step 5: When the burner is actually stuck in fire
- What to look for: Isolation valves closed, recirc off, every fixture confirmed dead, In Use still on and the fan in high. That is a flow sensor that will not open, a stuck gas-valve driver, or a PCB. Licensed. A delayed-ignition bang then a continuous fan is shutdown, not a Circ-Logic setting. Need it off? Apartment shutoff.
- Required tools: None on the gas train. Manometer work is licensed (NG ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the rating plate). Do not purge the line.
When DIY Ends: Calling a Licensed Pro
You may read the controller, turn recirc from Always On to off as a test, close isolation valves, and hunt dripping fixtures. You may not jumper the flow sensor, descale with the gas on, or open the gas valve. Leave freeze protection powered on an outdoor unit. A burner that stays in fire with the house isolated, a wet board, or gas odor is a same-day licensed call. Circ-Logic left in Always On is a setting; treat it like one before you buy an exchanger.
Frequently Asked Questions
Q: Why is my Rinnai tankless constantly running? A: Recirc (Circ-Logic, RUR pump, or an external pump) is the usual noise. An actual burner that never drops is flow the board still sees: a stuck turbine, a dripping fixture, a crossover left open, or a leak in the loop. Outdoor fan on low in the cold is freeze protection.
Q: Is Circ-Logic supposed to run when nobody is home? A: In Always On, yes — the pump circulates on purpose. Timed modes should idle outside the window. If the burner is in fire with recirc off and no fixtures, that is not Circ-Logic.
Q: Will a constantly running Rinnai ruin the heat exchanger? A: Hours of real fire from a leak or a stuck sensor waste gas and can glaze the exchanger. Pump-only recirc does not. Isolate the house. If flame continues, shut it down and hire a licensed tech.