Facing a water shutoff for repairs, vacation, or seasonal closing? Wondering if you should power down your tankless water heater when the water supply is off? It’s a common concern—and the answer is more important than you might think. This guide explains exactly what to do, why it matters, and how to keep your tankless water heater safe and in top shape—no matter how long your water will be off.
Quick Picks: Best Products to Protect and Maintain Your Tankless Water Heater
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Best Overall:
My PlumbingStuff Tankless Water Heater Flushing Kit – Keep your system running smooth before and after downtime. -
Best Budget:
Camco Water Heater Blanket – Insulate and protect your unit in garages or unheated spaces. -
Best Premium:
Watts Lead Free Tempering Valve – Adds anti-scald safety and peace of mind when restoring water.
Should You Turn Off a Tankless Water Heater If Water Is Off?
Yes—you should always turn off your tankless water heater if the water supply is shut off for more than a few minutes. Here’s why:
- Prevent Damage: If the heater turns on without water, it can overheat and damage the heat exchanger—sometimes beyond repair.
- Avoid Error Codes: Running dry can trigger safety shutdowns and error codes, which may require a reset or professional service to fix.
- Save Energy: There’s no need to waste power on a system that can’t heat water while the supply is off.
- Safety: Protects against accidental dry firing or leaks, especially during extended absences or seasonal shutdowns.
This applies to both electric and gas tankless water heaters, whether for a few hours, overnight, or much longer.
When to Turn Off Your Tankless Water Heater
- Plumbing Repairs: If your main water is shut off for home repairs or city work.
- Extended Vacation: Leaving for more than a couple of days? Always shut off both water and heater.
- Seasonal Homes: Shutting down a cabin, RV, or vacation property for the season.
- Freezing Weather Risk: If water will be off and freezing is possible, shut off, drain, and insulate for safety.
Step-by-Step: How to Safely Turn Off Your Tankless Water Heater
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Turn Off the Heater
- Electric Models: Flip the breaker to “off” or unplug the unit from the wall outlet.
- Gas Models: Use the power button or on/off switch. Also, shut the gas valve if you’ll be away for a long time.
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Shut Off Water Supply
Close the main cold water valve leading to your tankless heater. This is usually a lever or round valve right above or below the unit. -
Drain the System (for Extended Absence or Winterization)
- Open the drain ports or service valves on your heater (consult your manual).
- Open a hot tap to release pressure and allow air in.
- If freezing is possible, fully drain both hot and cold lines to prevent pipe bursts.
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Insulate (if needed)
For garages, basements, or unheated spaces, wrap the unit and exposed pipes with a water heater blanket or pipe insulation.
When You Might Leave the Heater On
- Short Outages: If water is off for only a few minutes (like quick repairs), it’s usually safe to leave the heater powered—but don’t use any hot taps during this time.
- Built-In Freeze Protection: Some models have built-in heaters to prevent freezing. If the heater is in a cold space, you might need to leave it on just for this feature, but only if water is in the system. If in doubt, consult your manual.
Always: Never operate or leave your heater powered when dry, even for freeze protection—there must be water in the system.
Restarting Your Tankless Water Heater After Water Is Restored
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Check All Valves
Ensure the cold water supply and any hot side valves are fully open. -
Restore Power or Gas
Turn the breaker back on, plug in the unit, or reopen the gas valve. -
Bleed Air From Lines
Open several hot water taps to clear any air or debris from the lines. -
Check for Leaks
Inspect all plumbing connections for drips or leaks before leaving the heater unattended. -
Set Temperature
Use your digital controls to select your desired hot water temperature (120°F is ideal for most homes). -
Test System
Run hot water at a tap and confirm your heater is igniting or powering up as normal.
Best Accessories for Tankless Water Heater Care
My PlumbingStuff Tankless Water Heater Flushing Kit
- Keeps your system clean and running strong after downtime
- Easy DIY descaling and flushing
- Includes pump, hoses, bucket, and solution
Final Verdict: The simplest way to protect your heater and ensure safe restarts.
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Camco Water Heater Blanket
- Protects against freezing when the system is powered down
- Fits tankless and tank-style heaters
- Easy wrap-and-tape installation
Final Verdict: A low-cost way to guard against weather-related problems.
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Watts Lead Free Tempering Valve
- Ensures safe, steady water temp after restoration
- Anti-scald safety, simple to install
- Ideal for families with children or elderly members
Final Verdict: Adds extra safety and comfort when you turn your system back on.
See details on Amazon
Comparison Table
| Name | Key Features | Specs/Capacity | Price Link |
|---|---|---|---|
| My PlumbingStuff Kit | Descaling, flush pump, hoses, bucket | 3.5 gal, fits all tankless units | View |
| Camco Blanket | Insulation, easy install, freeze protection | Up to 60-gal, fits all | View |
| Watts Tempering Valve | Anti-scald, easy to install, lead-free | 1/2” or 3/4” NPT | View |
FAQ: Turning Off a Tankless Water Heater When Water Is Off
- Can I leave my heater on if water is only off for an hour?
For very short outages, you can usually leave it on—but don’t use any hot taps until water is restored. - What happens if my heater runs without water?
It may overheat, trigger an error code, or permanently damage the heat exchanger. Always turn it off if in doubt. - Do I need to drain my heater for a weekend trip?
No, just turn off water and power. Drain only if you expect freezing or will be away for weeks or months. - Should I turn off the gas, too?
For extended absences, yes—shut off both gas and water supply for full safety. - Is it safe to turn my heater back on myself?
Yes, just follow the startup steps and check for leaks, power, and temperature as outlined above.
Conclusion
Whenever your home’s water is shut off, turning off your tankless water heater is the safest, smartest move. You’ll protect your system, avoid costly repairs, and enjoy worry-free restarts when water service is restored. With the right steps and a few helpful accessories, your tankless water heater will deliver hot water comfort for years to come—no matter how long you’re away.