Rinnai says its tankless fan normally continues briefly after hot-water flow stops, helping manage rapid starts and exhaust residual flue products. That sound alone does not establish burner operation or safe venting. Match the model and symptoms; prolonged or changed operation, a fault code, gas odor or a CO alarm needs a different response.
Brief after-run can be intended operation
Rinnai's residential troubleshooting FAQ directly addresses a fan that continues after the hot tap closes. It says the fan runs for a short time after water flow stops, helping maintain consistent temperatures during rapid starts and stops and exhaust residual flue products. Rinnai says it then turns off on its own after a few moments.
That explanation supports ordinary brief after-run. It does not set a universal number of seconds for every model or diagnose a new grinding sound, continuous operation or a displayed fault. Do not switch power off simply to silence this expected brief cycle or open the cabinet to check it.
Separate fan sound from a heating complaint
Notice when the sound starts and ends relative to actual hot-water use. From a safe position, record the model and any controller message. Note whether the sound is familiar or recently changed and whether hot water otherwise works normally. These observations are more useful than labeling every fan sound a bad motor.
Hearing a fan alone does not prove the burner remains on, that water is overheating or that exhaust is leaving safely. A power-vent tank heater with a blower but no burner involves a different operating sequence; do not borrow its tests for a tankless Rinnai.
If water actually goes cold or heating fails, use the tankless problem guide to describe the separate symptom. A momentary fan continuation after the tap closes is not the same complaint as losing heat during a shower.
A fault code calls for the matching manual
The RSC SENSEI manual, covering RSC199i, RSC160i, RSC199e and RSC160e, identifies code 61 as a combustion-fan condition and requires trained, qualified installation and service. That is a documented fault example, not a code inferred from ordinary fan sound.
Use the installed model's instructions and record the exact display before requesting service. Rinnai code meanings help organize that information, but codes and procedures vary by generation. Leave internal fan, wiring, combustion and vent testing to qualified service personnel. Do not remove vents, run with the cover off, defeat a safety device or repeatedly reset a returning fault.
Emergency signs override the normal explanation
If you smell gas, leave without operating electrical switches or using a phone inside, then contact the gas supplier or emergency services from outside. Do not remain beside the heater waiting for its fan to stop.
For a CO alarm or suspected poisoning, get everyone outside and call 911. CPSC guidance specifically calls for immediate fresh air followed by 911 when CO poisoning is suspected. A fan that sounds normal cannot establish that the building is safe. Keep the appliance out of use until the emergency and required professional checks are resolved.