Tankless water heaters are reliable, space-saving, and efficient—but they aren’t problem-free. From no hot water to error codes and temperature swings, even the best on-demand systems can hit snags. The good news? Most tankless water heater problems are easy to diagnose and often simple to fix—especially with the right know-how and a few handy tools. This guide explains the most common issues, why they happen, how to fix them, and the best products to keep your system running strong.
Quick Picks: Top Tools & Products for Tankless Water Heater Troubleshooting
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Best Flush Kit:
My PlumbingStuff Tankless Water Heater Flushing Kit – All-in-one, fits every brand, essential for scale removal and flow restoration. -
Best Scale Inhibitor:
Watts Premier Inline Scale Inhibitor Filter – Prevents hard water damage before it starts. -
Best Leak Detector:
Govee WiFi Water Leak Detector – Alerts you to leaks or drips from your tankless unit, even when you’re not home.
Top Tankless Water Heater Problems and How to Fix Them
No Hot Water
- Check for power or gas supply—breaker, gas valve, and unit display should all be on.
- Inspect water shut-off valves—make sure hot and cold lines are open.
- Look for error codes on the display; consult your manual for code meanings.
- Low flow? Tankless units need at least 0.5 GPM to trigger heating. Clean faucet aerators and showerheads if needed.
Fluctuating Water Temperature
- Mineral buildup in the heat exchanger can restrict water flow and cause rapid cycling—flush your unit with a descaling kit.
- Simultaneous use: Running too many taps at once can exceed the heater’s capacity, causing temperature swings.
- Mixing valve issues: Old or failing mixing valves can cause uneven temperatures at fixtures.
Error Codes on Display
- Consult your unit’s manual for specific codes. Most common are related to ignition, flow sensors, scale, or venting.
- Power-cycle the heater (turn it off, wait, and turn back on) to reset minor glitches.
- Persistent codes may require a technician—don’t ignore gas or venting errors.
Low Hot Water Pressure
- Clean or replace inlet screens and filters at the heater and fixtures.
- Flush the system to remove scale or sediment from the heat exchanger.
- Check for partially closed shut-off valves or kinked lines.
Unit Won’t Ignite (Gas Models)
- Check propane/natural gas supply and regulator.
- Clean the flame sensor or igniter with a soft brush.
- Ensure proper venting—blockages can prevent safe ignition.
Water Too Hot or Not Hot Enough
- Adjust the temperature setting (most heaters are best at 120–125°F for comfort and safety).
- Reset or recalibrate the thermostat if possible.
- Low flow or incoming water temperature can also affect output—choose the correct model size for your climate and needs.
Leaking or Dripping Heater
- Inspect for loose or corroded fittings at inlet/outlet connections.
- Replace any worn gaskets, O-rings, or washers.
- If the leak is from inside the unit, call for professional service—heat exchangers rarely leak unless damaged.
Unit Shuts Off Unexpectedly
- Electrical overloads—check your breaker and wiring for capacity issues.
- Overheat safety: Scale or blockages may trigger safety shutoff—flush the heater and clean venting if needed.
- Check for clogged air intake or vent pipe (gas units).
Buyer’s Guide: How to Prevent and Solve Tankless Water Heater Problems
- Annual Flushing: Use a descaling/flush kit at least once a year, or twice in hard water areas.
- Install a Scale Inhibitor: Protects your heat exchanger from hard water minerals—essential for long life.
- Monitor for Leaks: Install a leak detector under your unit for peace of mind (especially in basements or closets).
- Check Water Flow: Clean all inlet screens, aerators, and showerheads regularly for strong, even flow.
- Know Your Error Codes: Keep your manual handy (or snap a photo) for quick troubleshooting.
- Choose the Right Size: Undersized units are stressed and fail sooner—always match your GPM needs.
Top Maintenance Products: Reviews
My PlumbingStuff Tankless Water Heater Flushing Kit
- Includes pump, hoses, bucket, and citric acid descaler
- Universal fit for all tankless brands
- Makes annual flushing fast and simple
Final Verdict: The single best investment for keeping your tankless heater running like new.
See it on Amazon
Watts Premier Inline Scale Inhibitor Filter
- Prevents scale before it forms
- Easy install—fits on the cold water line before your heater
- Extends heater lifespan and cuts maintenance
Final Verdict: A must-have in any hard water region.
Check price on Amazon
Govee WiFi Water Leak Detector
- Instant alerts to your phone if water is detected
- Great for vacation homes or hidden installs
- Simple, reliable, and reusable
Final Verdict: Protects your home from water damage—especially in finished spaces.
See details on Amazon
Comparison Table: Troubleshooting & Maintenance Essentials
| Product | Main Use | Fits All Brands? | Pro or DIY? | Price Link |
|---|---|---|---|---|
| Flush Kit | Descale & clean | Yes | DIY | View |
| Scale Inhibitor | Prevent scale | Yes | DIY/Pro | View |
| Leak Detector | Monitor leaks | Yes | DIY | View |
FAQ: Tankless Water Heater Troubleshooting
- Why won’t my tankless water heater turn on?
Check for power/gas supply, reset the unit, and look for error codes. Low flow or tripped breakers are common culprits. - Why does my hot water keep going cold?
Scale, low flow, or overtaxed units are common causes. Flush your heater, check fixtures, and match usage to your model’s GPM rating. - Can I flush my tankless water heater myself?
Yes—most homeowners can flush their own units with a kit. Always follow your model’s instructions. - How often should I maintain my tankless unit?
Flush yearly, change filters as needed, and check for leaks or corrosion every few months. - Are error codes serious?
Some are minor, but gas or venting errors can be dangerous. Don’t ignore codes—consult your manual or a technician if needed.
Conclusion
Tankless water heaters can hit a few bumps, but most problems are simple to fix with routine maintenance and the right tools. Don’t let common issues ruin your comfort—invest in a flush kit, scale filter, and leak detector, and your on-demand water heater will keep running smoothly for years. Need more troubleshooting help or specific advice for your model? Reach out anytime!