The “in use” light on a Rinnai tankless water heater signals that the unit is actively heating water. If this indicator is not illuminated when hot water is demanded, it can indicate a sensor, electrical, or venting issue, or a fault code stored in the system memory. This guide provides practical, step-by-step troubleshooting, safety considerations, and when to contact a professional. It covers common models used in the United States and aligns with typical Rinnai diagnostics and service procedures.

Recognize Symptoms And Safety First

Before diving into troubleshooting, verify the exact symptoms. Common signs include the “in use” light not turning on, inconsistent hot water, error codes flashing on the display, or unusual burner sounds. For safety, switch off power to the unit at the disconnect switch or breaker before inspecting electrical components. Do not bypass safety switches or assumptions about gas supply—if there is a gas smell, evacuate and contact emergency services.

Check The Power And Electrical Connections

The in use light on a Rinnai unit relies on a stable electrical supply. Start with the basics:

Ensure the circuit breaker for the water heater is not tripped and the fuse is intact.
Inspect the external power switch, if present, and confirm it is in the ON position.
Verify the disconnect switch near the unit is set to ON and that wiring is intact without signs of damage or loose connections.
Look for any recent wiring changes or upgrades that could have affected the control circuit.
Check the unit’s control board for visible damage or burnt components. If you see signs of overheating, turn off power and call a technician.

If power cycling is safe, reset the unit according to the user manual (often a combination of holding a reset button or powering off for a set period). If the in use light returns during a reset, monitor performance to see if the issue recurs.

Inspect The Venting System And Combustion Air

Improper venting or restricted combustion air can prevent normal operation and trigger fault codes. Follow these steps:

Check that vent pipes are free of obstructions, with proper slope and no crushing or kinks.
Confirm vent terminations are secure and sealed as required by local code.
Ensure exterior air openings are unobstructed to provide adequate combustion air to the unit.
Inspect for any signs of condensation, rust, or corrosion at joints, which may indicate leaks or exhaust backdrafts.

Note: Do not attempt vent modification beyond the manufacturer’s approved configuration. For ventilation-related issues, a licensed technician should assess ductwork and vent integrity.

Evaluate Water Flow Sensors And Flow Rate

Modern Rinnai units use flow sensors to detect hot water demand. If the sensor fails or is dirty, the system may not register usage, and the in use light may not illuminate even when hot water is drawn. Actions include:

Check for proper water flow at faucets; a restricted aerator or clogged faucet can produce low flow signals that confuse the sensor.
Inspect the water inlet screen for debris. A clogged screen can reduce flow and affect sensor operation.
Ensure no downstream valves are closed that could starve the unit of water flow.

If cleaning screens, use mild detergent and water, then reassemble. Test hot water to observe if the in use light responds appropriately.

Review Error Codes And Diagnostic Procedures

Rinnai models display fault codes that help pinpoint the problem. When the in use light is not on, check the control panel for any blinking LEDs or error codes. Common areas include:

Gas pressure or ignition issues that may prevent the burner from firing.
Water temperature sensor fault or miscalibration.
Electrical board fault or failed relay that interrupts the heating cycle.
Overheat or safety shutoffs that disable the heating cycle even if demand exists.

Consult the owner’s manual or Rinnai’s official online diagnostics to interpret codes. If the unit provides a specific fault code, follow the recommended steps for that code. Do not ignore repeated codes, as persistent faults can indicate looming failures.

Gas Supply And Burner Functionality

For gas-powered models, issues with gas supply or burner ignition can cause the in use light to stay dark. Consider these checks:

Confirm the gas valve supplying the unit is fully open.
Check for gas pressure adequacy as per the unit’s specifications. Low pressure can prevent ignition.
Inspect the burner for cleanliness and proper ignition; a dirty burner can fail to ignite reliably.

Due to the dangers of gas work, if gas supply or ignition concerns are suspected, contact a licensed HVAC professional to diagnose and service the burner and gas connections.

Maintenance And Filter Considerations

Regular maintenance helps prevent the in use light problems. Key tasks include:

Schedule annual professional service to inspect burners, heat exchangers, and venting systems.
Replace or clean any air or sediment filters per the model’s maintenance schedule.
Flush the system to remove mineral buildup if the unit’s maintenance guidelines require it, especially in hard-water regions.
Inspect seals and gaskets around the heat exchanger for leaks or wear.

Many issues stem from neglect; a proactive maintenance plan can minimize unexpected outages and keep the in use light functioning as intended.

Reset, Recalibrate, And When To Call For Service

If basic checks do not restore the in use light, a reset or recalibration can resolve minor software glitches:

Power down the unit, wait 60 seconds, then restore power and test operation.
Recalibrate temperature settings to manufacturer recommendations and verify hot water delivery at multiple fixtures.
Document any fault codes, symptoms, dates, and steps taken for reference when speaking with a technician.

When the light remains off after resets, or if there are frequent error codes, noise, water leaks, or signs of gas issues, schedule service with a certified Rinnai technician or a licensed plumber/HVAC professional. Proactive professional diagnostics can prevent air, gas, or electrical hazards and protect the unit’s efficiency and lifespan.

Choosing The Right Professional And Safety Considerations

Engaging a qualified service provider ensures problems are diagnosed accurately and repaired correctly. Look for:

Technicians with current Rinnai certification and experience with the specific model family.
Proper licensing and insurance as required by state or local codes.
Clear estimates, parts pricing, and adherence to safety standards for gas and electrical work.

Always prioritize safety. If there is a gas odor, gas smell, or suspected gas leak, evacuate the premises and contact emergency services immediately. Do not attempt risky repairs yourself.