Tankless Water Heater Error Codes: Troubleshooting Guide

Your tankless water heater is flashing a cryptic code, and suddenly there’s no hot water. Is it something serious? Can you fix it yourself? Do you need to call someone immediately?

At H&H Heating and Air Conditioning, we’ve diagnosed every error code in the book. Most have straightforward causes, and some you can even fix yourself. This guide will help you understand what your unit is telling you.

How Error Codes Work

Unlike traditional tank heaters that fail silently, tankless units have built-in diagnostic systems that monitor performance in real-time. When something goes wrong, the display shows a code indicating the specific problem—helping you (or your technician) pinpoint the issue quickly.

Some codes are warnings that allow continued operation. Others trigger safety shutdowns that require attention before the unit will restart.

Common Rinnai Error Codes

Since H&H specializes in Rinnai installations, here’s a breakdown of the most common codes we encounter:

Ignition Problems:

• Code 11: No ignition—unit won’t light
• Code 12: Flame failure—flame went out during operation

Venting/Airflow Issues:

• Code 10: Air supply or combustion problem
• Code 61: Fan/combustion fan failure

Overheating:

• Code 14: Thermal fuse triggered (overheating detected)
• Code 16: Over-temperature warning

Sensor Issues:

• Code 32: Outgoing water temperature sensor problem
• Code 33: Incoming water temperature sensor problem

Other Common Codes:

• Code 25: Condensate drainage issue (condensing units)
• Code 65: Water flow control problem
• Code 79: Water leak detected
• LC: Scale buildup—maintenance/flushing needed

Codes You May Be Able to Fix Yourself

Code 11 (No Ignition):

1. Check if gas is on (valve fully open)
2. Verify other gas appliances work
3. Reset the unit (power off, wait 60 seconds, power on)
4. If recently installed, air may need purging from gas lines

Code 12 (Flame Failure):

1. Check for gas supply interruption
2. Verify gas valve is open
3. Reset the unit
4. If outdoor unit, check for wind interference

Code LC (Maintenance):

1. Scale buildup detected in heat exchanger
2. Schedule flushing service (or DIY if experienced)
3. Unit may continue operating temporarily—don’t ignore long-term

General First Step: Try resetting the unit. Turn off power, wait 60 seconds, restore power. If the code clears and doesn’t return, the issue may have been a temporary glitch.

How to Reset Your Tankless Water Heater

1. Turn off power to the unit (breaker or dedicated switch)
2. Wait 30-60 seconds
3. Restore power
4. Check if error clears
5. If error returns, the underlying problem needs diagnosis

Some units have a dedicated reset button—consult your manual for model-specific instructions.

Codes That Require Professional Service

Call immediately for these codes:

Code 10 (Combustion Problem): Gas and air mixture issue. Safety concern—professional inspection needed.

Code 14/16 (Overheating): Could indicate restricted airflow, scale buildup, or component failure. Don’t ignore—overheating damages expensive components.

Code 25 (Condensate Issue): Drainage problem in condensing units. Blocked condensate can damage the unit and potentially cause water damage.

Code 61 (Fan Failure): Combustion blower problem. Unit can’t safely vent exhaust gases without proper fan operation.

Code 79 (Leak Detected): Water leak inside the unit. Shut off water supply and call immediately.

Any Persistent Code: If resetting doesn’t clear it, or it keeps returning, professional diagnosis is needed.

Venting Errors: A Safety Priority

Codes related to venting (10, 61) deserve special attention. Your tankless water heater produces combustion gases that must be safely vented outside. Blockages can cause dangerous backdrafting and carbon monoxide risk.

Common causes: Bird nests, debris, ice buildup (winter), improper installation, or damaged vent pipe.

What to do: Do not operate the unit until professionally inspected. Check outdoor vent termination for visible obstructions, but leave internal inspection to a technician.

The LC Code: What It Really Means

LC stands for “Lime scale buildup” and is one of the most common codes we see. It’s not an emergency—it’s your unit telling you it needs maintenance.

What’s happening: Mineral deposits have built up in the heat exchanger, reducing efficiency and potentially causing overheating.

How urgent: You can often continue using the unit temporarily, but don’t ignore it. Prolonged scale buildup causes permanent damage.

The fix: Flush/descale the unit. This can be DIY with a flushing kit or scheduled as professional maintenance.

Prevention: Annual flushing prevents LC codes. If you’re getting this code frequently, consider a water softener.

Preventing Error Codes

Most error codes are preventable with proper maintenance:

• Annual flushing prevents LC codes and overheating issues
• Keep venting clear of debris, bird nests, and seasonal obstructions
• Clean filters every 6-12 months to prevent flow restrictions
• Don’t block airflow around the unit
• Address small issues before they become big problems

The H&H Approach

We’ve diagnosed every Rinnai error code—and most problems have straightforward solutions. When you call us about an error code, we’ll often help you troubleshoot over the phone first. If it’s something simple you can check yourself, we’ll tell you. If it needs professional attention, we’ll explain why and schedule service.

Our goal is getting your hot water back quickly—not selling unnecessary repairs.

Staring at an error code and not sure what to do? Call H&H—we’ll help you figure out what it means and whether you need service.

Frequently Asked Questions