Rinnai Tankless Constantly Running: Causes
On this page (5 sections)
The Short Answer: A Rinnai that sounds like it never stops is usually running a recirculation pump, not the burner. Built-in pump models (RUR, RSC) and external pumps circulate water on a schedule or learned pattern. Freeze protection and a short fan run after each use are also normal. If the burner fires with every tap closed, hot water is flowing somewhere: a leak, a fixture, or a crossover.
First, figure out what is running
| What you notice | Likely cause | Concern level |
|---|---|---|
| Quiet hum, no burner roar, hot water arrives fast everywhere | Recirculation pump doing its job | Normal; adjust the schedule if you want |
| Fan runs for a short time after a tap closes | Post-purge clearing the combustion chamber | Normal |
| Cold night, outdoor or garage unit, no water use | Freeze protection | Normal; keep power on |
| Burner and fan firing with all taps closed and recirculation off | Hot water flowing somewhere | Find it soon |
| Burner firing plus a code on the display | A fault and retry | Note the code |
The controller’s In Use indicator helps. If it shows hot water being supplied while every fixture is closed, the heater detects flow.
Recirculation settings
Rinnai’s built-in pump models can run recirculation in several modes. The RSC SENSEI manual (opens in a new tab) describes a “Smart-Circ” mode that spends its first 24 hours learning household hot-water patterns before it schedules the pump. A setting that keeps the pump always on will make the heater cycle regularly to reheat the loop, especially if the return pipes are not insulated.
To test, switch recirculation off or to a timed mode using your controller or app as the owner manual describes. If the constant running stops, the system is working as set; the fix is a schedule that fits your routine, or insulation on the loop.
Hidden flow that keeps the burner on
Rinnai lists a minimum activation flow of 0.4 gallons per minute for the RSC models. That is more than a slow drip, so if the burner fires with recirculation off, look for something that moves a real stream of hot water:
- A hot-side hose bib, laundry valve, or fixture not fully closed
- A tub or shower valve that does not shut off completely
- A leak in hot piping under a slab or inside a wall (warm floor spots or a meter that keeps moving)
- A failed shower cartridge or mixing valve letting cold and hot mix, which can move water through the heater even with no faucet open
A simple test: close the cold-water valve feeding the heater. If the burner stops immediately, the heater was responding to real flow in the house. A plumber can trace a crossover or leak.
When the heater itself is at fault
If the cold supply is closed, recirculation is off, and the burner still runs, or the fan never stops, turn the heater off and call a technician. They check the water flow sensor, the gas valve and its driver on the control board, and the board itself. Rinnai’s checklists for flow problems also include confirming that hot and cold lines are not reversed on new installations.
Related: Rinnai humming noise, Rinnai In Use light not on, and Navien heater constantly running.
Frequently Asked Questions
Q: Does a running recirculation pump use a lot of gas? A: The pump uses electricity. Gas use comes from reheating water that cooled in the loop. An always-on setting with uninsulated pipes uses the most; a timer, learned schedule, or on-demand button uses far less.
Q: Should I turn off power to stop freeze protection running? A: No. Rinnai’s freeze protection needs power. Cutting it in freezing weather can let the heat exchanger freeze and split.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Rinnai: RSC SENSEI installation and operation manual (100000773) (opens in a new tab)Rinnai RSC SENSEI models with built-in pump: minimum activation flow, recirculation parameters, and Smart-Circ learning period.
- Rinnai: official document library (opens in a new tab)Select your exact model and owner manual before interpreting a fault or reset procedure.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
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