Rheem 1L code: lime buildup warning

1L is Rheem's scale warning. It does not mean a part failed; it means the heat exchanger needs cleaning before scale causes overheating and damage.

This page uses two Rheem sources: the current Rheem and Ruud tankless troubleshooting manual, and Rheem's error code list for earlier tankless models. Codes can differ between generations, so the meaning for each is shown where they differ. Rheem technical support: (800) 432-8373.

What to do

1. Flush the heat exchanger with a small pump, hoses and a descaling solution or white vinegar, following the Use and Care manual. Rheem's flush kit uses a submersible utility pump.
2. Clean the cold water inlet filter while the unit is drained.
3. Reset the warning: hold MIN and MAX on the remote control together for 10 seconds.

Rheem notes that heavy buildup may take more than one flush. Our guide to descale a tankless water heater covers the steps.

Why it matters

Scale insulates the heat exchanger. Ignored, it leads to overheating codes such as 14 and 15 and can permanently damage the unit. Scale damage is also commonly excluded from warranties, so keep a record of each flush.

Preventing it

• Flush about once a year, more often with hard water.
• Consider a water softener or scale-reduction filter.
• Avoid setting the temperature higher than you need; scale forms faster at higher temperatures.

Before you call

Have the model and serial number from the rating label ready, and note whether your unit is a condensing (high-efficiency) or non-condensing model. Tell the technician the exact code, when it appears and any recent work on the gas, water, venting or electrical supply. That often saves a second visit.

See every code on our Rheem error codes page.