Rinnai Water Heater No Hot Water: Error Codes & Fixes
Quick Answer
Rinnai tankless not producing hot water: check the error code on the controller. Code 11 = no ignition (check gas supply valve is open, gas pressure at least 4 inches water column). Code 25 = scale surging in the heat exchanger (annual descaling with vinegar solution required). Code 10 = blocked air intake or exhaust - check for bird nests or debris.
Most Rinnai no-hot-water calls I get are one of three things: dirty flame rod, scale buildup in the heat exchanger, or something blocking the vent. Ignore this long enough and you're looking at a cracked heat exchanger, which is a $600+ repair. Catch it early and it's usually a $50 cleaning. The error code on that controller tells you exactly where to start.
Look, Rinnai makes a genuinely great water heater and these things usually last 20 years if you take care of them. But when they stop making hot water, the fix is almost always cheap if you catch it fast. I've seen units throw a no-hot-water condition because of a spider web over the intake screen. Check the controller display first. Whatever code it's showing you is basically the unit telling you exactly what's wrong.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Code 11 - ignition failure35%
Code 12 - flame loss mid-operation25%
Code 25 - heat exchanger scale buildup20%
Code 10 - air/exhaust blockage15%
Flow sensor fault5%
Symptoms You May Notice
You turn on a hot tap and get cold water that never warms up, or it runs warm for about 8 seconds then goes cold again.
The controller display shows a number like 11, 10, 25, or LC, and the unit makes a clicking sound but the burner never actually catches.
You hear the fan spin up and water flowing through, but there's no ignition sound and nothing but cold at the tap.
Hot water works fine at full blast but disappears when you turn it down to a trickle, because you're falling below the minimum flow threshold.
Water temperature surges between scalding and cold mid-shower because scale buildup in the heat exchanger is causing the unit to cycle erratically.
Can you reset a Rinnai waterheater to clear the NO-HOT-WATER code?
Press the On/Off button on the controller to turn it off, then back on. That clears most codes. For a hard reset, unplug the whole unit from the outlet for a full 60 seconds, not 10, not 30, a full minute. When you plug it back in, the control board re-initializes all the sensors fresh and a lot of those glitchy communication errors just disappear on their own.
Tools Required for Diagnosis
Phillips #2 screwdriverFlathead screwdriverMultimeter (for resistance and voltage testing)400-grit sandpaper or green Scotch-Brite padAdjustable wrenchFlashlight or headlampSmall bucket and towels (for inlet filter removal)Utility pump and hoses (for descaling)
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range10000–14000 ohms
ConditionIf Open (OL) or infinite, replace component.
Frequently Asked Questions
How much does Rinnai water heater repair cost?
A standard service call usually runs $150 to $300, which covers cleaning the flame rod, descaling, and cleaning the filters. If you need a new gas valve, that's another $150 to $250 in parts. A bad control board can run $300 to $500 just for the part. Heat exchanger replacement, if it's cracked or leaking, is usually $600 to $1,200, and at that point you've got to decide whether to repair or replace. Honestly, most no-hot-water calls I go on are under $200 because it's a cleaning issue, not a parts issue.
Worth repairing or replacing?
Rinnai units are built to last 20 years and almost everything in them is replaceable. If the unit's under 15 years old, fix it. The only time I tell people to seriously consider replacing is when the heat exchanger is leaking, because that's the core of the unit and repair costs start pushing toward the price of a new one. Otherwise, a gas valve, a flow sensor, a control board, all of those are straightforward repairs that'll buy you another 5 to 10 years easy.
DIY or should I call a pro?
Cleaning the inlet filter, polishing the flame rod, and doing your annual descaling are all solid DIY jobs. None of those involve gas connections or high voltage. But if you're suspecting a gas valve issue, the igniter module, or the main control board, call a pro. Not because it's dangerous to look, but because diagnosing those parts correctly requires a manometer for gas pressure and specific resistance readings. Getting that wrong means you might replace a $400 board when the actual problem was a $15 solenoid.
What does Error Code 11 mean on a Rinnai?
Code 11 is the most common call I get. It means the unit tried to ignite and failed, either no spark got to the burner, gas didn't flow, or the flame didn't sustain long enough for the flame rod to detect it. Start with the flame rod and clean it. Then check gas supply. Then check for a cracked igniter. In that order. Probably 60% of Code 11 calls I go on are just a dirty flame rod that takes 5 minutes to fix.
How often should I descale my Rinnai?
Every year if you're in a hard water area. Every two years if your water's soft. You'll know it's time when you start seeing Code 25 or the LC indicator, or if your hot water flow has gotten noticeably weaker over time. The descaling process takes about an hour, you pump a white vinegar solution through the heat exchanger using a small utility pump and hoses. Rinnai sells a flush kit, part number RKFL001S, or you can rig one up yourself for about $30 in parts from any hardware store.
Why does my Rinnai work on high flow but cut out on low flow?
That's almost always a minimum activation threshold problem. Rinnai tankless units need at least 0.4 GPM to trigger ignition, so if you've got a low-flow fixture or you're barely cracking the tap, the unit doesn't sense enough movement and won't fire. Could also be scale buildup in the heat exchanger causing restriction at lower flow rates. Clean the inlet filter first, then check if the flow turbine is spinning freely by watching it while you run water through.
Models Known to Experience NO-HOT-WATER Errors
This repair applies to most Rinnai waterheaters with this error code. Common model numbers include: