Rinnai In Use Light Not On
The Short Answer: The In Use (or Priority) lamp lights when the Rinnai has a call — flow proven, board firing or about to. Dark lamp plus cold water means the unit never started: controller Off, no 120 V, flow below minimum, isolation valves closed, or a dead remote. It is not a 12 lockout. You cannot heat what the board never saw.
Licensed local technicians
Rinnai In Use light stays dark?
Connect with a licensed 24/7 technician in your zip code right now.
Search Intent & The Real Problem
You opened a hot faucet. The controller stayed idle. No fan, no spark, In Use dark. That is a no-call diagnosis. If In Use is on and water is still cold, you are on ignition/gas — no hot water and code 12. If water will not move at all, no water flow. This page is the lamp.
What other guides get wrong
They tell you to descale because the light is out. Descale is for a unit that ran. They mix In Use with a status LED on a tank gas valve. They replace the PCB because the bathroom remote is unplugged. And they skip activation flow: a 0.3 GPM trickle on a water-saver head never turns the lamp on. That is physics, not a failed heater.
Step-by-Step Troubleshooting
Step 1: Split dark In Use from lit In Use
| Lamp / behavior | Meaning | Next |
|---|---|---|
| In Use dark, no fan, no code | No call — power, Off, flow | This page |
| In Use on, fan runs, still cold | Ignition / gas / heat exchanger | Codes, gas path |
| In Use on, then dark mid-shower | Min-flow dropout | Goes cold |
| Priority light on another remote | That controller stole priority | Set Priority on the one you are using |
| Blank display | No 120 V or dead PCB | GFCI / disconnect |
- What to look for: MC-91, MC-195, or SENSEI onboard pad. Some labels say In Use, some Priority. Same idea.
- Required tools: Eyes on the controller during a hot-faucet open.
Step 2: Power and the On button
- What to look for: Controller Off after an outage or a child hitting the button — dead after a power outage. Outdoor GFCI popped. Disconnect off. Bath-fill locked. Display alive but Off is not a flow sensor.
- Required tools: Turn On. One GFCI reset. If the GFCI trips again, leave it off.
Step 3: Flow that never reaches activation
- What to look for: Isolation valves half-closed after a “handyman flush.” Inlet screen packed with grit or Teflon. Fixture mixing so much cold that net hot flow is under ~0.4–0.6 GPM. Recirc crossover so the sensor never sees a delta. Well or PRV so weak the unit never wakes — gauge the hose bib; under ~40 psi static is a house problem.
- Required tools: Fully open service valves. Rinse the cold-inlet screen. Open a tub spout (no mixer) as a high-flow test.
If the tub lights In Use and the shower does not, the shower is below activation. Low pressure if the tub is also a trickle.
Step 4: Remote, cable, and priority fights
- What to look for: Bathroom remote unplugged, thermostat wire nicked, MC-91 showing a communication fault. Two remotes and Priority sitting on the one in a guest bath. Kitchen controller Off, bathroom On — some setups will not fire until the master is On.
- Required tools: Factory cable seated. Do not splice thermostat wire with a wirenut and hope.
A dead remote with a blank pad and a live PCB at the unit: you can often fire from the onboard pad. If the onboard pad is also dark, you are back on 120 V.
Step 5: Flow sensor and the board — the licensed end
- What to look for: Valves open, screen clean, tub-spout flow, 120 V, controller On, still no In Use and no fan. Inlet water-flow sensor stuck (turbine packed with sand) or unplugged after a flush. PCB that lost the sensor input. That is a licensed pull of the sensor, not a screwdriver in the turbine with water on.
- Required tools: None once the easy checks passed.
Do not bypass the flow sensor. The board uses it to shut gas off when you close the tap.
When DIY Ends: Calling a Licensed Pro
You may turn the controller On, reset one GFCI, open isolation valves, rinse the inlet screen, and test a tub spout. Stop at a repeating dark lamp after those, a wet board, gas odor, and any jumper on the flow sensor. Sensor replacement, recirc design, and PCB work are licensed. A dark In Use with a packed screen is a $0 rinse. A dark In Use with a dead turbine is a service call, not a new RU199.
Frequently Asked Questions
Q: Why is the In Use light on my Rinnai not on? A: The unit never got a call. Controller Off, no 120 V, flow below activation, closed isolation valves, a packed inlet screen, or a remote/priority fault. It is not a flame-failure code.
Q: Does a dark In Use light mean the Rinnai is broken? A: Not by itself. Open a tub spout with the controller On. If In Use lights there, the shower is below minimum flow. If it never lights, hire a licensed tech after you rinse the screen.
Q: In Use is on but I still have no hot water. Is that this problem? A: No. The lamp proved the call. You are on ignition, gas, or a lockout code — different repair.