Troubleshooting Rheem Error Code 99: What It Means & How to Fix

If you are seeing this code, you are likely experiencing a total loss of hot water, accompanied by the sound of the blower fan running at high speeds as the unit attempts to clear the vent. You might also hear unusual whistling or vibrating noises from the venting pipes. While this is a serious safety interruption, it is often caused by external debris or lack of maintenance and can be resolved with a systematic inspection.

Symptoms of Rheem Error 99

Detecting Error 99 involves more than just reading the digital display. As a safety professional, I advise you to look for these physical “red flags” that indicate your venting system is failing:

• The LCD Display Flashes “99”: This is the primary indicator. The unit will refuse to ignite as long as this code is active.
• Audible Blower Strain: You may hear the combustion fan spinning significantly faster or louder than usual. This happens because the control board is increasing fan RPM to try and “push” past a perceived blockage.
• Lukewarm Water Followed by Cut-off: The unit may ignite for a few seconds, provide a burst of warm water, and then immediately shut down as the flue temperature or pressure sensors trigger the limit.
• Unusual Odors or Moisture: If you notice a “backdraft” smell or excessive condensation around the intake pipe, the exhaust gases are likely being pushed back into the cabinet.

Why is my Rheem showing Error 99?

Error 99 is rarely a “glitch” in the software; it is almost always a physical reality. Here is a deep dive into the mechanical failures that trigger this state:

1. Exterior Vent Obstruction (The Most Common Cause):
Rheem tankless units require a clear path for both intake and exhaust. Birds often find the warmth of exhaust vents appealing for nesting, and rodents may crawl into intake pipes. During winter, hoarfrost or snow accumulation can block the terminal. Even a spider web in the right spot can disrupt the sensitive air pressure switch readings. If air cannot move, the system enters a hard lockout to prevent Carbon Monoxide (CO) poisoning.

2. Heat Exchanger Fin Clogging (Soot and Scale):
If the gas-to-air ratio has been slightly off for a long period, “sooting” can occur. This is a buildup of carbon on the copper fins of the heat exchanger. These fins are packed tightly; even a small amount of soot acts like a wall, preventing air from flowing through the unit. This is often caused by poor gas pressure or installing the unit in a “dirty” environment (e.g., near a dryer vent or construction dust).

3. Excessive Venting Length or Elbows:
Every vent pipe has a maximum “equivalent length.” Every 90-degree elbow you add creates friction and backpressure. If your installation is on the edge of the manufacturer’s limits, a windy day or a slight buildup of dust can push the backpressure over the limit, triggering Error 99. This is a failure of the initial system design and installation.

4. Blower Motor or Hall Effect Sensor Failure:
The blower fan uses a “Hall Effect Sensor” to tell the control board how fast it is spinning. If the fan motor bearings are wearing out (creating drag) or the sensor is sending faulty data, the board assumes the air is not moving fast enough due to a blockage, even if the pipes are clear.

Comprehensive Repair Guide

Comprehensive Repair Guide for Error 99

WARNING: Before beginning, shut off the electrical power to the unit and close the gas isolation valve. Failure to do so can result in electrical shock or gas explosion. Wear safety glasses to protect against debris in the venting.

1. Inspect the Exterior Vent Terminals:
   Start outside your home. Locate the PVC or stainless steel pipes where the unit breathes. Use your flashlight to look deep into the pipes. Remove any bird nests, leaves, or ice. Ensure that the exhaust is not blowing directly onto the intake pipe, which can cause “re-circulation,” leading to soot buildup.
   Safety Tip: Check for any signs of melting or discoloration on the vent pipes, which indicates excessive heat.
2. Clean the Air Intake Filter:
   Inside the cabinet (usually at the bottom right or top), there is a small mesh screen for the air intake. Unscrew the mounting screw and pull the filter out. If it is clogged with dust or pet hair, the unit cannot “inhale.” Wash it with soap and water, dry it completely, and reinstall.
3. Examine the Blower Motor Assembly:
   Using your Phillips head screwdriver, remove the front panel of the heater. Locate the large plastic or metal blower housing. Check the wiring harnesses leading to the fan to ensure they are seated tightly. If you have a multimeter, you can check for continuity on the fan motor leads, but primarily, check for physical dust buildup inside the fan blades. Use compressed air to blow out any debris that might be slowing the RPMs.
4. Clear the Heat Exchanger Fins:
   If the vents and fan are clear, the blockage is likely inside the heat exchanger. Looking through the sight glass, check for “green” or “black” buildup on the fins. You may need to use a vacuum with a brush attachment to carefully clean the top and bottom of the heat exchanger assembly. Do not use sharp metal objects that could puncture the copper tubing.
5. Check the Vent Pressure Switch Tubes:
   There are small silicone tubes connecting the fan to the pressure switch. Pull these tubes off carefully and check for moisture or cracks. If water (condensation) is trapped in these tubes, the sensor will give a false “Blockage” reading. Blow through them to ensure they are clear, then reattach them firmly.
6. Perform a Hard Reset:
   Once you have cleared all obstructions, restore power and gas. Turn the unit off and on via the remote controller. If the code persists, press and hold the “Min” and “Max” buttons on the control board (consult your specific model’s manual) to clear the history, though usually, a power cycle is sufficient once the blockage is physically removed.

How to Prevent Error 99

As a compliance officer, I emphasize that prevention is the only way to ensure the long-term safety of your household. Error 99 is often a symptom of neglect.

• Install Vent Screens: Purchase manufacturer-approved bird and rodent screens for your exterior vent terminations. This prevents 90% of Error 99 cases caused by external debris.
• Annual Descaling and Cleaning: You must flush your tankless unit with food-grade white vinegar once a year. During this service, you should also use a vacuum to clear the internal cabinet of dust and lint, which prevents the blower motor from becoming unbalanced.
• Monitor Gas Pressure: If your gas pressure is too low, the flame will “soot up” the heat exchanger very quickly. Have a licensed plumber check the manifold pressure during your annual maintenance to ensure the combustion is clean.

FAQ

Frequently Asked Questions

1. Is it safe to run my water heater while Error 99 is flashing?
No. Error 99 is a critical safety lockout. The unit is programmed to shut down because it cannot safely exhaust flue gases. Attempting to bypass this or forcing the unit to run can lead to carbon monoxide leaking into your living space, which is odorless and potentially fatal. Always resolve the blockage before use.

2. Can heavy wind cause an Error 99?
Yes. If your vent terminal is located on a side of the house that receives direct, high-velocity wind, the wind can create “backpressure” that the fan cannot overcome. This results in a temporary Error 99. If this happens frequently, you may need to install a specialized wind-cap or re-route the vent termination.

3. My vents are clear, but the code won’t go away. What now?
If the venting is 100% clear and the fan is spinning freely, the issue is likely a failing **Air Pressure Switch** or a faulty **Main Control Board**. Use a multimeter to check if the pressure switch is closing when the fan kicks on. If the switch stays open despite high airflow, the switch must be replaced.