How to Flush a Rinnai Tankless Water Heater in 7 Steps
On this page 6 sections
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.
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
- Turn off power at circuit breaker
- Shut off gas supply valve
- Close cold water inlet and hot water outlet valves
- 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.
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.
Water Heater Owner's ToolkitFix the common faults yourself, keep it running for years, and know when a repair is money wasted.No-hot-water flowcharts, gas and electricFlush, anode, relief valve and descaling stepsRepair-or-replace calculator (Excel)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.
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