How to Flush a Rinnai Tankless Water Heater in 7 Steps

To flush a Rinnai tankless water heater, connect a hose to the service valve, use a descaling solution, and run the pump for 30 minutes, then rinse thoroughly.

Flushing your Rinnai tankless water heater removes mineral buildup that reduces efficiency and lifespan. This complete guide covers everything from pump selection to troubleshooting common flushing problems.

Flush a Rinnai tankless water heater in easy steps

Why Flushing Your Rinnai Heater Matters

Hard water causes scale buildup in heat exchangers. This reduces heating efficiency by up to 25% and can lead to premature failure. Rinnai recommends annual flushing for most homes, or quarterly if you have very hard water.

Signs You Need to Flush

  • Reduced hot water flow
  • Fluctuating water temperatures
  • Error codes like LC (scale detection)
  • Unusual noises during operation

Required Flushing Equipment

Item Specification
Circulation Pump 3-5 GPM with 18ft head pressure
Hoses 2 x 5/8″ ID, minimum 3ft length
Cleaning Solution 4 gallons undiluted white vinegar
Buckets 2 x 5 gallon capacity

For pump recommendations, see our guide to the best tankless water heater descaling pumps.

Step-by-Step Flushing Procedure

1. Prepare the System

  1. Turn off power at circuit breaker
  2. Shut off gas supply valve
  3. Close cold water inlet and hot water outlet valves
  4. Open hot water faucet to relieve pressure

2. Connect Flushing Kit

Attach hoses to purge valves (cold inlet and hot outlet). The cold side connects to your pump. Use hose clamps to prevent leaks.

3. Circulate Cleaning Solution

Submerge pump in vinegar bucket. Run solution through system for 45 minutes. For severe buildup, extend to 90 minutes.

4. Reverse Flow (Optional)

Switch hoses to reverse flow direction. This helps dislodge stubborn deposits. Never do this with power on.

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5. Rinse the System

Flush with clean water for 5 minutes to remove all vinegar residue.

6. Restart the Heater

Close purge valves, remove hoses, and restore power/gas. Follow the Rinnai startup sequence for your model.

7. Verify Operation

Check for leaks and ensure normal hot water production. The unit may take longer to deliver hot water after flushing.

Troubleshooting Common Flushing Problems

No Flow Through Heater

If solution won’t circulate:

  • Verify all valves are open
  • Check for kinked hoses
  • Try a more powerful pump (minimum 18ft head)
  • Temporarily power unit to open internal valves (then turn off)

Low Flow Rate

Possible causes:

  • Clogged inlet filter (clean before flushing)
  • Undersized hoses (use 5/8″ ID minimum)
  • Pump losing prime (keep submerged)

Maintenance Tips Between Flushes

Extend time between flushes with these practices:

  • Install a whole-house water softener
  • Use a scale inhibitor like Rinnai ScaleStop
  • Clean inlet filter monthly
  • Monitor performance for early signs of scaling

When to Call a Professional

Consider professional service if:

  • Flushing doesn’t restore performance
  • You see error codes after flushing
  • There are leaks at connections
  • The unit requires replacement parts

Proper maintenance keeps your Rinnai tankless water heater running efficiently for years. Regular flushing is the most important service you can perform.

Joye
Joye

I am a mechanical engineer and love doing research on different home and outdoor heating options. When I am not working, I love spending time with my family and friends. I also enjoy blogging about my findings and helping others to find the best heating options for their needs.