Tankless Water Heater Error Codes: What They Mean and How to Fix Them

A flashing error code on your tankless water heater display is the unit telling you exactly what is wrong. Most codes point to specific components and some can be resolved with a simple reset. Others require a licensed plumber or HVAC technician. This guide covers the most common error codes for Rheem, Navien, and Rinnai units — the brands we service most frequently in Tampa Bay.

General Rules for Tankless Error Codes

Before looking up your specific code, keep these points in mind:

• Write down the exact code. Error codes on the same brand can have very different meanings — Error 11 and Error 12 on a Navien are completely different problems. Note the exact number or letter combination before resetting.
• Try a manual reset first. Many codes will clear after shutting off the unit for 30 seconds and restarting. If the code comes back within a few minutes or hours, the underlying problem needs service.
• Check for simple causes first. Gas supply valve off, low water pressure, or a closed cold-water supply valve causes many error conditions that show up as unit codes.
• Do not ignore persistent codes. Some codes indicate a safety shutdown. Continuing to use the unit while ignoring a persistent safety error can damage the unit or create a safety hazard.

Rheem Tankless Water Heater Error Codes

Rheem uses a lettered code system on their RTGH series and similar units. We install Rheem tankless systems across Tampa Bay and see these codes regularly.

Navien Tankless Water Heater Error Codes

Navien units use a numbered error system. Navien NPE series units are common in newer Tampa Bay construction.

Rinnai Tankless Water Heater Error Codes

Rinnai units use a two or three digit numeric system. Rinnai RU and RL series units appear frequently in Tampa Bay homes and condos.

Tampa Bay-Specific Error Causes

Several conditions common in our area drive specific error codes more than the national average:

• Scale buildup (codes related to overheating). Tampa Bay’s 7 to 11 GPG water hardness deposits calcium inside the heat exchanger faster than in softer water regions. Heat exchangers in homes without water softeners or conditioners can develop significant scale within 3 to 5 years. Regular flushing with white vinegar solution or descaling service is the fix.
• Blocked vent termination from pests. Tankless unit intake and exhaust pipes typically terminate on an exterior wall or roof. Insects and small animals in Tampa Bay — particularly paper wasps and mud daubers — can build nests at vent terminations, triggering combustion air and exhaust error codes.
• Low water pressure errors. High demand from irrigation systems, multiple simultaneous uses, or aging supply pipes can drop water pressure below the unit’s minimum flow rate, causing flow or heating errors.

When to Call for Professional Service

Call Home Therapist when:

• The error code returns within a few cycles after resetting
• The code relates to ignition, gas, combustion, or venting
• You see error codes related to overheating or thermal fuse trips
• The unit is more than 5 years old and has never been descaled
• Any code that says professional service is required in the table above

Frequently Asked Questions

• What is the most common problem with tankless water heaters?

  The most common tankless water heater problem is ignition failure, which triggers a no-hot-water condition and an error code (typically Code 11 or 111 on Rheem and Navien units). It is usually caused by a closed gas valve, low gas pressure, or a dirty flame sensor. Scale buildup from Tampa Bay's hard water is the second most frequent culprit, restricting flow and triggering overheat codes.

• What does code 12 mean on a tankless hot water heater?

  Code 12 on most tankless water heaters indicates a flame failure after ignition was attempted. The burner lit but could not sustain a flame. Common causes include insufficient gas supply, a partially closed shutoff valve, or a fouled flame rod. Turn off the unit, verify the gas valve is fully open, then attempt one reset. If Code 12 returns, call a licensed plumber for diagnosis.

• How do I reset my tankless hot water heater?

  To reset a tankless water heater, press and hold the Reset button on the unit's front panel for three to five seconds until the display clears, then restore power. If there is no dedicated reset button, power the unit off at the breaker for 30 seconds and restore it. Always note the error code before resetting. If the same code reappears after one reset, the underlying fault needs professional repair.

• What does an E5 code mean on a tankless water heater?

  An E5 code typically signals a water flow sensor error or insufficient flow rate. The unit detected water moving through the heat exchanger below the minimum activation threshold, so it shut down to prevent overheating. Check for a partially closed inlet valve, a clogged inlet filter screen, or a failing flow sensor. In Tampa Bay homes, mineral scale on the filter screen is a frequent cause. Home Therapist offers FREE diagnosis on every visit to pinpoint the exact fault.