Rinnai tankless water heater maintenance: a yearly checklist

A Rinnai tankless water heater can last many years, but only if scale stays out of the heat exchanger. Most Rinnai problems that turn into expensive repairs, such as overheating codes and failed sensors, start with skipped maintenance.

How often

Let the heater tell you

Newer Rinnai models help with timing:

• SS (Service Soon) is a reminder the installer can set for every six months, one year or two years. Rinnai says it means the heat exchanger should be flushed.
• LC means the heater has detected scale buildup. Flush it soon.
• Code 58 on older condensing models means scale in the secondary heat exchanger has tripped a safety switch.

How to flush a Rinnai

Rinnai heaters are usually installed with isolation valves that have service ports for flushing. The general sequence is:

1. Turn off the heater at its controller, disconnect power and close the gas supply valve.
2. Close the hot and cold isolation valves under the unit.
3. Connect a small submersible pump and hoses to the service ports, with the pump sitting in a bucket of white vinegar or a descaling solution approved by the manufacturer.
4. Open the service ports and circulate the solution through the heat exchanger for the time your manual specifies, usually about an hour.
5. Drain the solution, then flush with clean water.
6. Close the service ports, clean the inlet filter, open the isolation valves and purge air from a hot tap before restoring gas and power.

Follow your owner's manual for the exact steps and timing for your model. Our guide to descale a tankless water heater walks through the job in more detail.

The inlet filter

A small screen at the cold water inlet catches debris. When it clogs, flow drops and the heater may not fire at low flow. Clean it with the water off and the unit drained, then reinstall it carefully.

Venting and condensate

• Keep the intake and exhaust terminals clear of leaves, nests, snow and ice. A blocked vent can cause combustion and overheating codes such as 16.
• On condensing models, make sure the condensate line flows freely. A clogged or frozen line causes code 25.
• Replace the condensate neutralizer media when it is used up.

Hard water

If you have hard water, flushing alone may not keep up. Rinnai warns that hard water must be treated to prevent heat exchanger damage. A water softener or a scale-reduction filter ahead of the heater can stretch the time between flushes.

Winter

Outdoor Rinnai models protect themselves from freezing only while they have power. See our guide to tankless water heater freeze protection before the first hard freeze.

Flushing is also where many warranties draw a line: scale damage is commonly excluded. Keep a simple log of each service. See all codes on our Rinnai error codes page.