Rheem code 13: poor combustion

The flame rod does more than prove the burner is lit. On Rheem indoor tankless models, it also watches how cleanly the burner is burning. Code 13 means it detected poor combustion.

The meaning and service steps below come from Rheem's tankless gas troubleshooting manual, which covers both Rheem and Ruud tankless water heaters. Rheem technical support: (800) 432-8373.

What causes code 13

• Gas supply. An undersized gas line, low pressure or a gas meter too small for all the home's appliances. Tankless heaters need a lot of gas at full fire.
• Venting. A blocked, damaged or incorrectly installed vent restricts air and exhaust.
• Altitude setting. Rheem notes the altitude DIP switch on the control board must be set correctly.
• Flame rods. Debris or buildup on the rods, or a loose connector.

The lockout

According to Rheem, if the unit shuts down five times within a four-hour period because of code 13, it locks out and needs a specific reset procedure. Repeated resets without fixing the cause are exactly what triggers this.

What you can check

1. Check the vent terminal outside for nests, leaves, snow or ice.
2. Note whether the code shows up when other gas appliances, such as a furnace or dryer, are running at the same time. That points to gas supply.
3. Reset once. If code 13 returns, call a technician.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last serviced. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rheem error codes page.