Did you just install a new tankless water heater, or is your existing unit switched off after maintenance or repairs? Many homeowners feel unsure about the right way to turn on their tankless water heater—especially with today’s advanced digital controls and safety features. Don’t worry! Whether you have a gas or electric model, this guide walks you through every step to safely power up, set the temperature, and enjoy endless hot water. Plus, you’ll find pro tips, troubleshooting advice, and the best accessories to make startup easier and your system last longer.
Quick Picks: Best Accessories for Tankless Water Heaters
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Best Overall:
My PlumbingStuff Tankless Water Heater Flushing Kit – Must-have for easy annual maintenance and startup. -
Best Budget:
Camco Water Heater Blanket – Simple insulation for exposed units or pipes. -
Best Premium:
Eccotemp EZ-Flush System Descaler Kit – Fast, thorough descaling for top performance.
How to Turn On a Tankless Water Heater: Step-by-Step
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Check the Installation
Make sure your unit is fully installed—plumbing and electrical or gas connections should be complete and secure. Double-check for leaks, loose wires, or missing panels. -
Open the Water Supply Valve
Turn the main cold water valve to the “open” or “on” position. Water should now flow into your tankless heater, filling its internal pipes. -
Turn On the Gas or Electrical Supply
- Gas Models: Open the main gas shutoff valve. Listen and sniff for gas leaks—if you smell gas, stop and fix the problem before continuing.
- Electric Models: Flip the circuit breaker “on” or plug the unit into a dedicated outlet.
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Power Up the Control Panel
Most modern units have a digital control panel. Press the power button (often marked with a “|” or labeled “Power”). You may hear a beep, see indicator lights, or watch the display light up. -
Set Your Desired Temperature
Use the up/down arrows or temperature dial to select your preferred hot water temperature. For safety, most manufacturers recommend 120°F (49°C). Some models require you to hold the button to “lock in” your setting. -
Test the System
Open a hot water tap (sink or shower). You should hear your tankless heater activate—a faint clicking, fan noise, or ignition spark. After a few moments, hot water will flow. Adjust temperature as needed. -
Check for Error Codes or Issues
Watch the display for error codes, flashing lights, or warning sounds. If all is well, you’re set! If not, refer to your manual or the troubleshooting section below.
Tips for Safe & Successful Startup
- Always read your unit’s specific manual—controls and startup steps may vary by brand.
- Don’t turn on a tankless heater with closed water valves; this can damage the heat exchanger.
- If your unit has been off for months, flush it first to remove scale or debris before starting up (use a flushing kit).
- Allow a few minutes for air to clear from new or drained plumbing lines.
- After a power outage or maintenance, always reset your unit before turning on hot taps.
Troubleshooting: Common Startup Problems
- No Power or Display? Check the circuit breaker, outlet, or main fuse. Confirm the power switch is on. For gas units, ensure the pilot or electronic ignition has power.
- No Hot Water? Make sure both gas/electric and water supplies are open. Tankless heaters need a minimum flow—try opening a second tap.
- Error Codes or Flashing Lights? Look up the code in your manual. Most issues relate to gas supply, venting, air intake, or sensor faults. Resetting the unit may clear minor codes.
- Strange Noises? Air in the lines, loose mounting, or debris in the system can cause odd sounds. Run hot water for several minutes to clear air; tighten brackets if needed.
- Water Leaks? Shut everything down and inspect all connections. Fix leaks before using the unit again.
Best Tankless Water Heater Accessories for Easy Startup
My PlumbingStuff Tankless Water Heater Flushing Kit
- All-in-one kit: pump, hoses, bucket, and descaler
- Fast, annual flush keeps your unit running strong
- Easy for first-timers and DIYers
- Works with all brands—Rheem, Rinnai, Noritz, Navien, Takagi, and more
Final Verdict: Keeps startup easy, water quality high, and maintenance simple for years.
See it on Amazon
Camco Water Heater Blanket
- Insulates tankless and traditional heaters against cold
- Reduces heat loss, improves startup speed
- Easy to install—just wrap and tape
- Helps prevent freezing in garages, basements, or crawlspaces
Final Verdict: A low-cost way to boost efficiency and winter startup reliability.
Check price on Amazon
Eccotemp EZ-Flush System Descaler Kit
- Commercial-grade pump and solution for fast, effective cleaning
- Protects against scale, startup errors, and poor performance
- Great for hard water areas or neglected systems
- Simple setup—works on all major brands
Final Verdict: If you want the best possible performance at startup and beyond, this is the kit to get.
See details on Amazon
Comparison Table
| Name | Key Features | Specs/Capacity | Price Link |
|---|---|---|---|
| My PlumbingStuff Kit | Pump, hoses, bucket, descaler | 3.5 gal, 1/6 HP, universal fit | View |
| Camco Blanket | Insulation, easy install, universal | Up to 60-gal, 3” thick | View |
| Eccotemp EZ-Flush Kit | High power, descaler included | Large pump, pro results | View |
FAQ: Turning On a Tankless Water Heater
- Can I turn on my tankless water heater without water?
No—always open the water supply first to prevent damage. Running the heater dry can destroy the heat exchanger. - Do I need to light a pilot on a modern gas tankless unit?
No—most models use electronic ignition. Just turn on gas and power, then follow the digital controls. - Why does my tankless heater show an error code at startup?
Error codes often signal air in the system, gas flow issues, or sensor problems. Reset the unit and try again; if the code persists, consult your manual. - How long does startup take?
Most units power up and produce hot water within 1–2 minutes of turning on. - What’s the safest startup temperature?
120°F (49°C) is standard. Higher temps increase scald risk and energy use.
Conclusion
Turning on a tankless water heater is straightforward when you know the right steps—and it’s the key to instant, energy-efficient hot water all year. With this guide, you’ll avoid common mistakes, start up with confidence, and keep your system running strong. Don’t forget to flush, insulate, and maintain for years of comfort and savings!