If your Navien tankless water heater isn’t delivering hot water like it used to, mineral buildup could be the culprit. Over time, hard water leaves behind scale that clogs heat exchangers and reduces efficiency. The good news? You can restore performance by flushing the system, a simple maintenance task that takes about an hour and can extend your unit’s lifespan by years.
This guide shows you exactly how to flush a Navien tankless water heater safely and effectively. You’ll learn the tools required, the step-by-step process, and how to avoid common mistakes. Whether you’re troubleshooting poor heating or doing annual upkeep, this walkthrough ensures you get it right the first time.
Essential Safety Precautions Before Flushing
Before touching any component, safety must come first. A tankless water heater operates under high pressure and temperature, so improper handling can lead to injury or damage.
Turn Off Power and Gas Supply
Switch off the unit at the remote control or thermostat. Then, locate the gas shutoff valve near the water heater and turn it to the “Off” position. For electric models, flip the corresponding breaker in your electrical panel. Never assume the system is safe just because it’s not running.
Shut Off Water Inlet and Outlet
Close both the cold water inlet and hot water outlet valves on the unit. This prevents water from flowing into or out of the heater during the flush. If your model lacks built-in service valves, consider installing them for easier future maintenance.
Let the System Cool Down
Wait at least one hour after shutdown to allow internal components to cool. Opening lines while hot can cause scalding or damage to hoses used in the flushing process.
Gather Required Tools and Materials
Having everything ready before starting keeps the job smooth and prevents mid-flush delays.
What You’ll Need for the Flush
- 5/16″ ID clear vinyl tubing (5 feet each for inlet and outlet)
- Submersible utility pump (commonly used for descaling)
- Two hose clamps or quick-connect fittings
- 5-gallon bucket
- White vinegar or commercial descaling solution (5 gallons recommended)
- Funnel
- Adjustable wrench
- Safety gloves and goggles
Pro Tip: Use food-grade vinegar (at least 5% acetic acid) for light scale. For severe buildup, opt for a stronger descaling agent like CLR or a Navien-approved cleaner.
Connect the Flushing System Properly
Now you’ll bypass the water heater’s internal flow to circulate cleaning solution directly through the heat exchanger.
Attach Tubing to Service Valves
Locate the service ports on your Navien unit, typically found on the cold water inlet and hot water outlet sides. Remove the plastic caps and connect one end of the vinyl tubing using hose clamps or quick-connect adapters. The inlet tube goes to the cold side, outlet to the hot.
Submerge Pump in Cleaning Solution
Place the submersible pump in the bucket filled with vinegar or descaling fluid. Connect the other end of the inlet tubing to the pump. Make sure the outlet tubing from the heater drains back into the same bucket to create a closed loop.
Visual Cue: Ensure no kinks in the tubing. A smooth path allows proper circulation.
Start the Flushing Process
With all connections secure, it’s time to circulate the cleaning solution through the system.
Run the Pump for 45–60 Minutes
Turn on the submersible pump and let the solution flow through the heat exchanger. Watch for bubbles or cloudiness in the return line. This indicates mineral dissolution.
Set a timer. Most experts recommend at least 45 minutes, but heavily scaled units may need up to two hours. Do not exceed manufacturer-recommended exposure times when using strong chemicals.
Monitor for Leaks or Blockages
Check all connections periodically. If you notice dripping or reduced flow, pause the pump and tighten fittings. A blockage might require backflushing. Reverse the pump direction or swap tubing positions briefly.
Common Mistake: Rushing the process. Incomplete flushing leaves residue that reaccumulates quickly.
Rinse the System Thoroughly After Descaling
After descaling, residual acid must be flushed out to prevent corrosion.
Flush with Clean Water
Empty the bucket and refill with fresh water. Run the pump again for 10–15 minutes to rinse the internal passages. This step neutralizes leftover vinegar and clears dislodged debris.
Alternative Method: Use a second bucket of clean water and redirect the return line to avoid re-circulating dirty fluid.
Reconnect and Restore Normal Operation
Now that cleaning is complete, return the unit to normal function.
Remove Tubing and Close Valves
Turn off the pump and disconnect all hoses. Replace the service port caps tightly to prevent leaks. Reopen the cold water inlet and hot water outlet valves slowly to avoid water hammer.
Restore Gas and Power
Turn the gas supply back on and check for odors. If you detect gas, shut off immediately and inspect connections. Once confirmed safe, restore electrical power by resetting the breaker or switching the remote back to “Heat.”
Run Hot Water to Clear Air
Open a nearby hot water tap and let it run for 1–2 minutes. This purges air trapped in the lines and confirms proper flow and ignition.
Warning: Listen for unusual noises. Popping or knocking may indicate remaining air or incomplete rinsing.
Test Heater Performance After Flushing
Verify your work was successful before considering the job done.
Check for Normal Ignition
Use the remote control to call for heat. The unit should ignite within seconds. Watch the display for error codes. None should appear if the system is functioning correctly.
Measure Temperature Rise
Set the faucet to a mix of hot and cold, then increase hot flow. The water should heat steadily without fluctuation. A weak response suggests incomplete descaling or flow sensor issues.
Pro Tip: Record recovery time and temperature output before and after the flush to track improvement.
Prevent Future Scale Buildup in Your Heater
Regular maintenance beats costly repairs. Take steps now to reduce mineral accumulation.
Install a Whole-House Water Softener
If you live in a hard water area, a softener is the best long-term defense. It removes calcium and magnesium ions before they enter the heater.
Schedule Annual Flushes
Navien recommends flushing every 6 to 12 months, depending on water hardness. Mark your calendar. Flush twice yearly if using well water or noticing reduced performance.
Add a Pre-Filter System
Inline sediment filters or scale inhibitors help reduce scaling without salt-based softening. These units trap particles before they reach the heat exchanger.
Troubleshoot Common Flushing Issues
Even with care, problems can arise during or after a flush.
No Flow Through System
Possible causes include:
– Kinked or clogged tubing
– Closed service valves
– Pump failure
Fix: Inspect each segment of the loop. Backflush or replace tubing if needed.
Error Codes After Flushing
If the unit displays codes like “C1” (ignition failure) or “E5” (flame sensing), check the following:
– Gas supply stability
– Vent blockage
– Moisture in electrical components (dry thoroughly)
Reset the unit by powering off for 30 seconds, then restarting.
Low Hot Water Pressure
Could mean:
– Incomplete descaling
– Airlock in lines
– Debris in inlet filter
Solution: Re-flush or clean the cold water filter screen located inside the inlet valve.
Frequently Asked Questions About Flushing a Navien Tankless Water Heater
How often should I flush my Navien tankless water heater?
Navien recommends flushing every 6 to 12 months depending on water hardness. If you use well water or notice reduced performance, flush twice yearly.
Can I use vinegar to flush my Navien water heater?
Yes, white vinegar with at least 5% acetic acid works for light scale buildup. For severe accumulation, use a commercial descaling solution like CLR or a Navien-approved cleaner.
How long does the flushing process take?
The full process takes about one hour, including setup, 45–60 minutes of circulation, and 10–15 minutes for rinsing. Heavily scaled units may require up to two hours of pump runtime.
What happens if I don’t flush my tankless water heater?
Mineral scale accumulates in the heat exchanger, reducing efficiency, lowering water temperature, and eventually causing system failure. Regular flushing prevents costly repairs and extends unit lifespan.
Do I need a professional to flush my Navien heater?
Most homeowners can perform this maintenance themselves using basic tools. However, if you’re uncomfortable with gas or electrical connections, hire a licensed technician for safety.
What should I do if my heater shows an error code after flushing?
Power off the unit for 30 seconds, then restart. Check gas supply, vent blockage, and moisture in electrical components. If the code persists, consult your model manual or call a technician.
Final Thoughts on Flushing Your Navien Tankless Water Heater
Flushing your Navien tankless water heater is a straightforward, cost-effective way to maintain efficiency and avoid premature failure. By following these steps annually, you ensure reliable hot water and protect your investment.
The most critical points to remember are safety first, thorough circulation of the cleaning solution for at least 45 minutes, and proper rinsing before restarting. Always consult your model’s manual for specific details, and when in doubt, call a licensed technician.
Your next step: gather your tools, mark your calendar for the next flush, and enjoy consistent hot water year-round.
