How To Flush A Navien Tankless Hot Water Heater – A Step-By-Step Guide

ByDavid Clark

Ever turn on the shower, expecting that rush of perfectly hot water, only to get a lukewarm trickle? Or maybe you’ve noticed your utility bills creeping up, even though your habits haven’t changed. It’s a common frustration, and more often than not, the culprit is hiding inside your high-efficiency Navien tankless water heater: mineral scale buildup.

Don’t worry—this isn’t a sign of a failing unit. It’s a predictable issue, especially in areas with hard water. We promise to walk you through the entire process, step-by-step, just like a technician would. This is one of the most impactful DIY tasks you can do to protect your investment.

In this comprehensive guide, you’ll learn exactly how to flush a Navien tankless hot water heater safely and effectively. We’ll cover why it’s so critical, the simple tools you’ll need, a detailed walkthrough of the flushing process, and some pro tips to avoid common mistakes. Let’s restore your Navien to its factory-fresh performance.

Why Flushing Your Navien Isn’t Just ‘Maintenance’—It’s Essential

Think of the pipes inside your Navien’s heat exchanger like the arteries of your home’s hot water system. Over time, minerals present in your water supply—mostly calcium and magnesium—precipitate out and stick to the inside of these pipes, forming a hard, crusty layer called scale.

This buildup forces your heater to work harder and longer to heat the water, leading to a host of problems. Understanding the benefits of how to flush a Navien tankless hot water heater is the first step toward a long-lasting, efficient unit.

  • Restored Energy Efficiency: Scale acts as an insulator, forcing the burners to fire more intensely to transfer heat to the water. Flushing removes this barrier, allowing your unit to operate at peak efficiency and lowering your gas bill.
  • Improved Water Flow and Pressure: As scale builds up, it physically narrows the passages for water, reducing flow and pressure at your taps and showers. A good flush clears these restrictions.
  • Prevents Costly Breakdowns: Left unchecked, scale can cause the unit to overheat, leading to flow sensor errors (like the common Navien E012 error) and even premature failure of the expensive heat exchanger.
  • Extends the Unit’s Lifespan: Just like regular oil changes for your car, an annual flush is the single best thing you can do to ensure your Navien heater lasts for its full expected lifespan of 15-20 years.
  • Maintains Your Warranty: Many manufacturer warranties require proof of regular maintenance. Keeping a record of your annual flushes can be crucial if you ever need to file a warranty claim.

Gathering Your Tools: What You’ll Need for a Successful Flush

Before you start, let’s get your toolkit ready. Having everything on hand makes the process smooth and stress-free. You can buy pre-made kits, but assembling your own is easy and cost-effective.

Here’s your shopping list for this how to flush a Navien tankless hot water heater guide:

  • A 3-5 Gallon Bucket: This will hold your descaling solution.
  • A Small Submersible Pump: A simple utility pump with at least 350 gallons per hour (GPH) of flow is perfect.
  • Two Washing Machine Hoses: Standard 3-4 foot hoses with female garden hose fittings on both ends work perfectly. Make sure they are in good condition.
  • Descaling Solution: You have two great options here.
    • Food-Grade White Vinegar: A natural and eco-friendly how to flush a Navien tankless hot water heater option. You’ll need about 2-3 gallons.
    • Commercial Descaler: A solution specifically formulated for potable water systems. It often works faster and more effectively on heavy buildup. Always follow the manufacturer’s instructions.
  • Channel-Lock Pliers or an Adjustable Wrench: You may need these to loosen tight hose connections, but often hand-tight is sufficient.
  • Old Towels: To catch any minor drips and keep the area clean.

The Step-by-Step Guide: How to Flush a Navien Tankless Hot Water Heater Safely

Alright, you’ve got your tools and you’re ready to go. We’ll break this down into simple, manageable steps. Follow this how to flush a Navien tankless hot water heater care guide carefully, and your unit will be running like new in about an hour.

  1. Step 1: Safety First – Isolate the Unit

    Before you touch a single valve, you must safely power down and isolate the water heater. Your safety is the top priority.

    • Turn Off the Power: Unplug the Navien unit from the wall outlet or turn off the corresponding breaker at your electrical panel.
    • Turn Off the Gas: Locate the gas shutoff valve on the gas line leading to the heater (it usually has a red or yellow handle). Turn the handle a quarter-turn so it is perpendicular to the pipe.
    • Close the Main Water Valves: Your Navien has three main water valves. Locate the handles for the hot water outlet (red), the cold water inlet (blue), and the main valve that feeds the unit. Turn all three handles perpendicular to the pipes to shut off water flow.

  2. Step 2: Connecting Your Flushing Kit

    Now we’ll connect our hoses to the service ports. These ports are designed specifically for this maintenance. You’ll see two ports on the bottom of the unit, one on the hot side and one on the cold, each with a small T-handle valve.

    • Remove the caps from the service ports. A little water might drip out, which is normal.
    • Connect one hose to the cold water service port (blue). Place the other end of this hose into your empty 5-gallon bucket.
    • Connect the second hose to the hot water service port (red). Connect the other end of this hose to the outlet of your submersible pump.
    • Place the submersible pump inside the bucket. You’ve now created a loop!

  3. Step 3: Circulating the Descaling Solution

    With the hoses connected, it’s time to introduce the cleaning solution. Open the service port valves by turning the small T-handles so they are parallel with the valve body.

    • Pour your descaling solution (vinegar or commercial descaler) into the bucket, ensuring the pump is fully submerged. Add a little water if needed.
    • Plug in the submersible pump. You should hear it start to hum and see the solution being pumped from the bucket, through the hot water service port, into the heater, and back out the cold water service port into the bucket.

  4. Step 4: Let the Solution Work Its Magic

    Let the pump circulate the solution through the water heater for at least 45 to 60 minutes. For very hard water or a long-overdue flush, you can go up to 90 minutes.

    You’ll quickly see the solution change color—turning milky, blue, or gray—and you may even see small flakes of scale returning to the bucket. This is a great sign! It means the descaler is dissolving the mineral buildup. This is one of the most satisfying how to flush a navien tankless hot water heater tips: watching the gunk come out.

  5. Step 5: Rinsing the System Thoroughly

    This step is critical for ensuring no acidic solution is left in your system. Do not skip this!

    • Unplug the pump and remove it from the bucket.
    • Dump the used, dirty solution.
    • Take the hose that was connected to the pump (from the hot water port) and leave it in the bucket or sink to drain.
    • Keep the other hose (from the cold water port) pointed into the bucket.
    • Slightly open the main cold water inlet valve you closed in Step 1. Let fresh water run through the heater for 5-10 minutes, flushing out all remaining debris and descaling solution. Close the valve once the water runs perfectly clear.

  6. Step 6: Pro Tip – Clean the Inlet Water Filter

    While the system is isolated is the perfect time for this often-forgotten step. The small filter on the cold water inlet catches sediment before it enters the unit.

    Unscrew the cold water line from the bottom of the unit. Inside, you’ll see a small screen filter. Carefully pull it out with needle-nose pliers, rinse it thoroughly under a faucet until it’s clean, and pop it back in. This simple action solves a surprising number of flow-related common problems with how to flush a Navien tankless hot water heater.

  7. Step 7: Restoring Your System to Normal Operation

    We’re on the home stretch! It’s time to put everything back the way we found it.

    • Close the service port T-handle valves (turn them perpendicular to the ports).
    • Disconnect your hoses and screw the service port caps back on.
    • Very slowly, open the main hot and cold water valves you closed earlier. Listen for the sound of water filling the unit.
    • Open a hot water tap somewhere in your house (like a bathtub) to bleed out any air in the lines. Let it run until you have a steady, air-free stream.
    • Turn the gas valve back on (handle parallel to the pipe).
    • Plug the Navien unit back in or flip the breaker on.
    • Your unit will power on and be ready to deliver clean, efficient hot water!
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Common Mistakes & Pro Tips for a Flawless Flush

Following the steps above will give you a great result, but keeping these how to flush a navien tankless hot water heater best practices in mind will make you feel like a seasoned pro.

  • Mistake 1: Forgetting the Inlet Filter. As mentioned, this tiny screen is a major chokepoint. Cleaning it during every flush prevents low-flow error codes.
  • Mistake 2: Not Rinsing Long Enough. Leaving acidic descaler in the system can slowly corrode internal components over time. A 5-10 minute fresh water flush is non-negotiable.
  • Mistake 3: Using the Wrong Chemicals. Never use harsh chemicals like CLR that are not rated for potable water systems. Stick to food-grade vinegar or a certified potable water descaler to protect your health and your heater’s components.
  • Pro Tip: Adjust Your Frequency. The “once a year” rule is a guideline. If you have very hard water (you see lots of white spots on your faucets), consider flushing every 8-9 months. If you have soft water, you might stretch it to 18 months.
  • Pro Tip: Neutralize Before Disposing. For a truly sustainable how to flush a navien tankless hot water heater process, add a cup of baking soda to your used vinegar solution before pouring it down the drain. It will fizz as it neutralizes the acid, making it safer for your pipes and the environment.

Frequently Asked Questions About Flushing a Navien Water Heater

How often should I flush my Navien tankless water heater?

The standard recommendation is once every 12 months. However, this depends heavily on your local water hardness. Navien’s control panel can even be set by a technician to display a service reminder based on water hardness and usage. If your water is very hard, every 6-9 months is a better schedule to maintain peak performance.

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What happens if I never flush my tankless water heater?

Ignoring this maintenance will lead to a slow but certain decline in performance. You’ll experience lower water pressure, fluctuating temperatures, and higher energy bills. Eventually, the scale buildup will become so severe that it can clog the heat exchanger, leading to system lockouts, error codes, and a very expensive repair bill that could have been easily avoided.

Can I use CLR or another harsh chemical to flush my Navien?

No, you should absolutely not. Harsh chemicals not rated for potable (drinkable) water systems can damage the sensitive copper and stainless steel components inside your heater and can be unsafe for your household water supply. Always use food-grade white vinegar or a descaler specifically designed for tankless water heaters.

My Navien is still not working right after a flush. What’s next?

If a thorough flush (including cleaning the inlet filter) doesn’t resolve your issue, the problem likely lies elsewhere. It could be a faulty flow sensor, a gas pressure issue, or another component failure. At this point, it’s best to call a licensed and certified Navien technician to diagnose the problem safely and accurately.

Congratulations! You now have all the knowledge needed to perform this crucial maintenance task. Taking an hour once a year to complete this how to flush a Navien tankless hot water heater process is the single best investment you can make in your home’s comfort and your unit’s longevity.

You’ve got this! Taking care of your Navien is the best way to ensure it takes care of you with endless, efficient hot water. Stay safe and stay comfortable!

David Clark
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