Quick Repair Specifications
| Difficulty: | Moderate (Requires mechanical aptitude) |
| Estimated Time: | 60 – 90 Minutes |
| Tools Needed: | Phillips Head Screwdriver (#2), Long-neck Vacuum Attachment, Multimeter, Ladder (for exterior vent access) |
| Estimated Cost: | $0 (Cleaning) to $150 (Replacement parts) |
Error Code 99 Definition: This code indicates a “Combustion Abnormality” or “Venting Blockage.” Specifically, the Rinnai unit’s internal sensors have detected that the exhaust gases are not exiting the system properly or the intake air is insufficient, leading to incomplete combustion. This is a critical safety lockout designed to prevent the accumulation of carbon monoxide.
⚠️ Safety First: Read Before Repairing
For your safety and to avoid voiding the warranty, please check the official docs.
If you are seeing this code, you are likely experiencing a sudden loss of hot water, perhaps preceded by a loud vibrating or “thumping” sound from the unit. The blower motor might be running at high speeds without the burner igniting. While it sounds intimidating, this is often caused by external debris or simple maintenance neglect, and in most cases, it is entirely fixable with a systematic approach.
Troubleshooting & Replacement Instructions
Follow these steps in order. WARNING: Always disconnect the electrical power supply and turn off the gas valve before removing the front cover of the unit.
- Clear the Exterior Vents:
Use a ladder to inspect the termination point of your intake and exhaust pipes outside the building. Look for spider webs, bird nests, or debris. Even a small obstruction can trigger a 99 code. If you have “mesh” screens on your vents, remove them and clean them thoroughly; they often get clogged with dryer lint or dust. - Inspect the Internal Blower Fan:
Remove the front panel by unscrewing the four Phillips head screws (usually at the corners). Locate the large circular blower motor at the bottom. Check for “fuzz” or dust buildup on the fan blades. Use a can of compressed air or a vacuum to clean the fan. If the fan cannot spin freely, it will report a false blockage. - Check the Heat Exchanger Fins:
Look at the top of the unit where the copper heat exchanger sits. Look up into the fins (you may need a mirror). If you see a buildup of black, powdery substance (soot) or white crusty minerals (scale), the air cannot pass through.
Detailed Nuance: If sooted, you must vacuum the fins. If the sooting is heavy, the heat exchanger may need to be pulled and cleaned with water pressure, which is an advanced task. - Verify Gas Pressure:
Using a manometer (if you are trained) or by calling a professional, check the static and dynamic gas pressure. Error 99 often occurs because the gas valve is providing too much fuel, causing “rich” combustion that chokes the system. - Test the Blower Motor Continuity:
With the power off, use your multimeter to check the resistance of the blower motor windings. If the motor shows “Open Loop” (OL), the motor has failed internally and must be replaced to resolve the error. - Reset the Logic Board:
Once the blockage is cleared, plug the unit back in. To clear a hard 99 code, you may need to cycle the power, then press the “On/Off” button on the controller several times. On some older models, you must toggle Dip Switch #1 in the “bank of 8” to the ON position and back to OFF to reset the combustion memory.
What Triggers this Code?
Understanding the “Why” is essential for a permanent fix. Error 99 is a safety failsafe triggered by several nuanced mechanical failures:
- Exterior Vent Obstructions: This is the most common cause. Bird nests, wasp hives, or even heavy snow accumulation can block the intake or exhaust pipes. Why? Because tankless units require a precise air-to-fuel ratio; any resistance in the vent pipe forces the blower motor to work harder, eventually exceeding its programmed parameters.
- Heat Exchanger Clogging (Sooting): Over years of operation, especially if the gas pressure is set incorrectly, “soot” (unburned carbon) can build up between the thin copper fins of the heat exchanger. This acts like a wall, preventing exhaust from passing through to the vent. This is often caused by “Short Cycling” or poor gas quality.
- Blower Motor Degradation: The blower motor is responsible for pushing exhaust out. If the bearings are wearing out or if the fan blades are coated in heavy dust/grease (common in indoor units near kitchens), the motor cannot move enough air volume to satisfy the internal sensors.
- Altitude Settings: If the unit was recently installed and wasn’t programmed for high-altitude operation (above 2,000 ft), the air is thinner than the unit expects. This leads to improper combustion that the system eventually interprets as a blockage.
Symptoms
The manifestation of a Rinnai Error 99 is rarely subtle. Because this code relates to the very “breath” of the machine, you will notice physical performance issues alongside the digital readout:
- The Lockout Flash: The digital controller will flash “99” and the unit will cease heating water immediately. This is a “hard lockout,” meaning the unit will not attempt to fire again until the power is cycled or the error is cleared.
- Audible Strain: You may hear the blower motor spinning at an unusually high RPM. This is the unit attempting to “purge” the combustion chamber of perceived blockages.
- Intermittent Flame: Before the hard lockout, you might notice the water temperature fluctuating wildly as the burner struggles to maintain a consistent flame with limited oxygen.
- Sooting or Discoloration: In severe cases, you may see yellowish staining or dark soot marks around the exhaust vent or on the internal heat exchanger fins.
How to Prevent Error 99
To ensure this safety lockout doesn’t leave you in a cold shower again, implement these maintenance protocols:
- Annual Heat Exchanger Flushing: Use a sum-pump and white vinegar to flush the water side of the heat exchanger. While this focuses on scale, a cleaner unit generally runs cooler and experiences less “sooting” on the combustion side.
- Install Proper Vent Caps: Ensure your exterior vents have “bird-proof” terminations that allow air to flow but prevent nesting. Ensure these are at least 12 inches above the maximum expected snow line.
- Combustion Analysis: Every 2 years, have a technician perform a combustion analysis. They can adjust the “Low” and “High” fire gas settings on the PC board to ensure the unit is burning as cleanly as possible, preventing the soot buildup that leads to Error 99.
Frequently Asked Questions
Q: Can I continue to use the water heater if the error goes away temporarily?
A: No. Error 99 is a safety-critical code. If the unit is struggling to vent, it could be releasing Carbon Monoxide (CO) into your home or damaging the internal components due to excessive heat. You must find the source of the blockage before regular use.
Q: Is Error 99 the same as Error 11?
A: Not quite. Error 11 is an ignition failure (no spark or no gas). Error 99 means the unit *can* ignite, but the quality of the burn is so poor or the venting is so restricted that it is unsafe to continue. Error 99 is generally more serious than Error 11.
Q: Why does my unit make a “boiling” sound right before the 99 code appears?
A: This is often “kettling.” If the heat exchanger is clogged with soot or scale, heat cannot transfer to the water efficiently. The copper gets too hot, causing the water inside to flash-boil. This overheating often triggers the combustion abnormality code.