Rheem tankless gas water heater code 05: meaning and exact next steps

Model-specific

What is true of this model and not of its siblings

A warning, not a stop

Code 05 is the early warning. The unit keeps running while it reports poor combustion; if the condition worsens, it becomes an error such as code 13 and the unit stops.

Indoor only

The code applies to indoor units, whose venting and air supply are more likely to be restricted.

Act early

Clearing a vent or having the gas supply checked at the warning stage avoids a shutdown.

A warning stage

Rheem separates a warning, code 05, from the error that follows if the condition worsens. The unit keeps heating while it reports that the flame rod sees improper burning. That is your chance to act before it stops.

Where to look

Rheem names the vent and the gas supply. From outside, look at the vent and air intake terminals for blockage. Inside, check that nothing stands against the unit. If a gas appliance elsewhere also burns poorly, suspect the gas supply.

When it becomes code 13

If the warning is ignored and combustion worsens, the heater stops and shows code 13. The cure is the same, and a technician should look at it before then.

Questions for the installer

Ask whether the vent run is within the length and bend limits in the manual, whether the gas line is sized for the total load and whether DIP switch 2 matches your elevation. A short visit to check these three things often stops the warning for good.

Keeping a log

Note the date and the weather when the warning appears. Wind can push exhaust back into a vent, and a heater that warns only on windy days points at the vent terminal, which a technician can fit with a wind guard.

A seasonal pattern

Venting problems often follow the seasons. In autumn, leaves and spider nests find their way into terminals, in winter, snow and ice can cap an intake, and in spring, birds nest in warm vent terminals. If the warning comes and goes with the weather, the vent terminal is the first place to look, and a guard fitted by the installer prevents most of it.

Gas appliances together

If you have a gas range or dryer on the same line, run them while the heater runs to see whether the flame at the stove changes. A weak stove flame while the heater fires points to a gas line that is too small for the combined load, which only the gas supplier or installer can correct.