Last week, we got a panicked call from a Stroudsburg homeowner whose Rinnai tankless water heater suddenly started producing lukewarm water during their morning shower. “It worked fine yesterday!” they insisted. After 36 years in the business, we knew exactly what was wrong: scale buildup from hard water had finally choked their heat exchanger.
Here’s the thing about tankless water heaters: they’re incredibly efficient and space-saving, but they need regular maintenance that most homeowners completely ignore. Unlike traditional tank water heaters, you can’t just drain these units. You need to flush a tankless water heater with a proper descaling solution to remove mineral deposits that accumulate over time.
We’ve serviced hundreds of tankless units across the Poconos, and the ones that get regular flushing last 15+ years. The neglected ones? We’re replacing them at 8-10 years because scale buildup destroyed their efficiency and components.
Why You Need to Flush Your Tankless Water Heater
What Makes Tankless Water Heaters Different
Tankless water heaters heat water on demand as it flows through a compact heat exchanger. This design makes them incredibly efficient, but it also makes them vulnerable to mineral buildup in ways that traditional tank water heaters aren’t.
You can avail our tankless water heate repair services in Stroudsburg by reaching out to us on call or email.
Effects of Hard Water and Scale Buildup
Hard water contains dissolved minerals like calcium and magnesium that stick to metal surfaces when heated. In a tankless system, these minerals accumulate inside narrow passages of the heat exchanger, restricting water flow and reducing heating efficiency.
We’ve opened up neglected Rinnai tankless water heaters where the heat exchanger looked like it was packed with concrete. The mineral deposits were so thick, water could barely flow through the system.
Benefits of Regular Water Heater Flushing
Regular tankless water heater flush maintenance keeps your system running at peak efficiency, extends equipment life, and prevents expensive emergency repairs. A properly maintained tankless heater maintains consistent hot water temperature and flow rate for years.
We recommend flushing at least once a year for most Poconos homes, but areas with particularly hard water may need more frequent attention. The small investment in annual maintenance saves thousands in premature replacements.
Our Stroudsburg water heater service team sees this constantly: homeowners who assume tankless units are maintenance-free until they suddenly lose hot water pressure or temperature consistency.
Tools and Materials Needed for Tankless Flushing
Essential Equipment for the Job
You’ll need specific tools to properly flush a tankless water heater. Don’t try to improvise with regular garden hoses and expect good results.
Required Equipment:
- Two washing machine hoses (¾ inch connections work best)
- Small submersible pump (like a sump pump, but designed for this purpose)
- 5-gallon bucket for your descaling solution
- Water heater flushing kit (makes connections easier)
The submersible pump is crucial because it creates the circulation needed to push descaling solution through your heat exchanger effectively.
Descaling Solution Options
White Vinegar: The most common DIY choice. Use 2 gallons of undiluted white vinegar for most residential tankless units. It’s safe, effective, and environmentally friendly.
Commercial Flushing Solutions: Professional-grade descaling products work faster and more aggressively than vinegar. We use these for heavily scaled systems, but they require more careful handling.
For routine annual maintenance, white vinegar works perfectly. Save the commercial stuff for problem situations or call us for professional service.
Preparation for Flushing a Tankless Water Heater
Turning Off the Water Supply and Power
Safety first: turn off electrical power to your tankless heater at the breaker panel. For gas tankless water heater units, turn the gas valve to the “off” position.
Next, shut off both the cold water inlet and hot water outlet valves to isolate your tankless heater from your home’s water supply. These are usually located directly above and below the unit.
Bypass Valves Setup
Most modern tankless units have service valves (also called flushing valves) specifically designed for maintenance. These bypass valves allow you to circulate cleaning solution through the heat exchanger while isolating the unit from your home’s plumbing.
If your unit doesn’t have service valves, you’ll need to disconnect the water inlet and outlet connections directly. This is trickier and might be a good time to call a professional plumber.
Connecting Hoses to Inlet and Outlet Valves
Attach one hose to the cold water inlet service valve and run the other end to your bucket of vinegar. Connect the second hose to the hot water outlet service valve and run it back to the same bucket, creating a closed circulation loop.
Make sure all connections are tight. A loose connection will create a mess and prevent proper circulation through your system.
The Flushing Procedure Step by Step
Start the Submersible Pump and Circulation
Place your small submersible pump in the bucket of vinegar and connect it to the hose leading to the cold water inlet. Turn on the pump and let the vinegar begin to circulate through your tankless heater.
You should see vinegar flowing out of the hot water outlet hose back into the bucket. This creates a closed loop that allows the descaling solution to work on mineral deposits inside your heat exchanger.
Allow the Vinegar to Circulate Through the System
Let the pump run and circulate the vinegar for 60 to 90 minutes. This gives the acetic acid time to dissolve scale buildup inside the heat exchanger passages.
Don’t rush this step. We’ve seen homeowners try to speed up the process and end up with incomplete cleaning that doesn’t solve their efficiency problems.
Flush Away Sediment and Scale Buildup
After circulation is complete, turn off the pump and disconnect the hoses. You’ll probably notice the vinegar solution looks cloudy or discolored from dissolved mineral deposits.
Replace the vinegar with fresh water and run the pump for another 10 to 15 minutes to rinse cleaning solution residue from your system.
Final Rinse with Fresh Water
Before reconnecting your tankless heater to the house water supply, run fresh water through both service valves to ensure all vinegar is completely flushed out. Nobody wants their hot water to smell like salad dressing.
This final rinse step is crucial for removing any remaining descaling solution that could affect water taste or damage fixtures. For comparison with conventional systems, check our how to drain a water heater guide.
How Long to Flush Your Tankless Water Heater
The entire flushing procedure typically takes 2 to 3 hours from start to finish: 90 minutes for vinegar circulation, plus setup, teardown, and rinsing time.
For heavily scaled systems, you might need to repeat the process or use a stronger descaling solution. If you’re not seeing improvement after a thorough flush, it’s time to call our professional technicians.
We recommend scheduling this water heater maintenance at least once a year. Homes with very hard water or high usage might benefit from flushing every 6 to 8 months.
Post-Flushing Maintenance and Testing
Checking Valves and Connections
After flushing, reconnect all water valves and restore power to your tankless heater. Check all connections for leaks before turning the system back on.
Open a few hot water faucets to purge air from the lines and verify that your tankless heater fires up properly when you demand hot water.
Testing Water Heater's Efficiency
Run hot water at multiple fixtures to confirm that your tankless heater is providing consistent temperature and adequate flow rate. You should notice improved performance immediately after a thorough flush.
If you’re still experiencing temperature fluctuations, reduced flow, or other issues, there might be additional problems that need professional diagnosis.
Keep Your Tankless System Running Strong
Regular tankless flushing is one of those home maintenance tasks that pays huge dividends. A properly maintained tankless water heater provides endless hot water, lower energy bills, and decades of reliable service.
Unlike traditional tank systems covered in our how to flush a water heater guide, tankless units require this specialized descaling process.
Wondering about frequency? Our detailed guide on how often drain water heater covers timing for both tankless and conventional systems.
Ready to skip the DIY hassle? Our licensed technicians have specialized equipment and commercial-grade descaling solutions that work faster and more thoroughly than DIY methods. With our 4.9★ HomeAdvisor rating and 24/7 emergency dispatch, we’ve handled every tankless water heater issue in the Poconos.
Save our emergency number: Tankless problems don’t wait for convenient times.
Download our tankless maintenance checklist to stay on top of your system’s needs.
Frequently Asked Questions
Q: How often should I flush my tankless water heater?
At least once a year for most homes. Hard water areas may need flushing every 6-8 months. Our technicians can test your water and recommend the ideal schedule.
Q: What happens if I never drained my water heater?
Sediment buildup reduces efficiency, shortens lifespan, and causes rust-colored water. We’ve seen 15-year tanks fail at 6 years from neglect while maintained units last decades.
Q: Can I use vinegar to descale my Rinnai tankless water heater?
Yes, white vinegar is safe and effective for routine maintenance. Use 2 gallons of undiluted vinegar and circulate for 60-90 minutes for best results.
Q: What happens if I never flush my tankless water heater?
Scale buildup reduces flow rate, temperature consistency, and efficiency. Eventually, mineral deposits can permanently damage the heat exchanger, requiring expensive repairs or replacement.
Q: Do I need special equipment to flush a tankless?
Yes, you need a submersible pump, proper hoses, and service valve connections. Many homeowners find it easier to have professionals handle this specialized maintenance procedure.
Q: How long does the tankless flushing procedure take?
The complete process takes 2-3 hours: 90 minutes for descaling circulation plus setup, connection, and rinsing time. Don’t rush the circulation step for thorough cleaning.
Quick disclaimer: Always check local building codes before doing major plumbing work. This guide covers routine maintenance—gas line issues or complex repairs need professional attention.