How to Keep a Tankless Water Heater From Freezing?

How Built-In Freeze Protection Works and Why Power Matters

Modern tankless water heaters come equipped with electric ceramic heating pads positioned directly against the internal heat exchanger (the metal coils inside that warm the water). When incoming pipe temperatures drop toward 36 degrees Fahrenheit, internal temperature sensors signal these pads to warm up. This gentle heat keeps the small amount of resting water inside the heat exchanger coils from turning into solid ice. In gas-fired models, some units will also briefly fire the gas burner to push warmth through the internal loop when ambient air drops far below freezing.

This automatic protection depends entirely on an active supply of electricity. Even if your tankless system burns natural gas or propane, its computer board, sensors, and heating pads run on standard 120-volt household current. If you unplug the power cord to plug in a tool, or if the circuit breaker trips, the internal freeze protection shuts off instantly. The heater then has zero defense against the cold, even if the gas line is wide open.

Built-in heating pads also have geographic limits. Most factory systems are engineered to guard the interior cabinet down to roughly minus 5 degrees Fahrenheit in calm conditions, though strong winter winds can lower that protection threshold. More importantly, these internal elements only protect the metal coils inside the cabinet. They do not send warmth into the water lines entering the bottom of the unit or the plumbing running through your exterior walls.

Pipe Insulation and Draft-Proofing Checklist

External pipes freeze much faster than the components inside the insulated heater cabinet. If the cold water line feeding the unit freezes solid, water stops moving, which can trap static water inside the heater and trigger an emergency shutdown code. Protecting your exterior plumbing requires sealing out freezing air and wrapping every exposed inch of copper, brass, and plastic.

• Closed-cell foam pipe insulation: Slide thick foam sleeves over both the cold water inlet and hot water outlet lines. Make sure the foam covers the pipes completely from the point where they emerge from your wall or ground all the way to the heater connections. Secure every seam tightly with heavy-duty exterior tape so winter air cannot slip through the slits.
• Self-regulating pipe heating cable: In climates where winter temperatures drop below 20 degrees Fahrenheit for extended periods, slip electric heat tape along the underside of exposed pipes before putting on the foam insulation. Use heating cables listed for plumbing pipes, and plug them directly into a weatherproof outdoor outlet. The cable turns on automatically as pipe temperatures fall, keeping the incoming supply line liquid.
• Air draft sealing around penetrations: Cold drafts blowing through wall gaps can freeze an indoor tankless heater mounted in a utility closet or basement. Inspect where the intake pipe, exhaust vent, and water lines pass through your exterior siding or foundation. Fill every visible gap and crack with silicone caulk or expanding waterproof foam sealant.
• Exhaust and intake vent protection: High winds can force sub-freezing air backward through the exhaust vent directly onto the heat exchanger coils. Ensure your vent termination cap has an operational backdraft damper (a small swinging flapper that lets exhaust air blow out but snaps shut against incoming wind).
• Protection for unheated garages: Tankless water heaters installed in unheated garages face biting drafts and concrete chill whenever the overhead garage door opens. You can shield the plumbing by building an insulated cabinet around the heater pipes or maintaining safe baseline room temperatures using a reliable electric heater for garage with thermostat controls set just above freezing.

The Running Faucet Trickle Method

Moving water freezes at a significantly slower rate than stagnant water. When bitter cold weather threatens your plumbing, opening a faucet inside your home creates a steady current through the water lines and straight through the heater cabinet. This constant movement carries ground warmth into the unit and prevents ice crystals from latching onto the inner walls of the plumbing.

The secret to using this method with a tankless water heater is controlling the flow rate. Tankless units use an internal water flow sensor that requires a specific threshold (often between 0.4 and 0.6 gallons per minute) to trigger the gas burners or electric heating banks. You do not want the burner firing continuously all night, which wastes fuel and creates unnecessary wear. Instead, you want just enough movement to stop ice from forming.

Turn on a hot water faucet at the fixture furthest from your tankless water heater. Adjust the handle until you get a continuous, unbroken stream of water roughly the thickness of a matchstick or the lead in a pencil. Check the display panel on your tankless water heater to confirm that the flame icon or in-use indicator light is not on. If the heater fires up, dial back the faucet handle slightly until the burner shuts off while maintaining a steady, flowing stream.

Use the trickle method whenever outside temperatures are forecast to drop below 20 degrees Fahrenheit, especially during windstorms or when your heater is mounted on an exterior wall. If your home loses electrical power during a winter storm, set the trickle immediately. Without electricity, the internal heating pads cannot turn on, making this stream of moving water your primary line of defense.

Steps to Drain Your Tankless Water Heater During an Outage

When winter weather knocks out your electrical power, the internal freeze protection pads stop working immediately. If temperatures drop below 32 degrees Fahrenheit during an outage, the water trapped inside the narrow copper or stainless steel coils can freeze solid in under two hours. As water freezes, it expands with tremendous force, splitting the coils open and destroying the heater. Draining the unit is the only guaranteed way to prevent catastrophic rupture when you have no electricity.

1. Turn off the electrical supply switch and unplug the unit.
   Unplugging the power cord from the wall outlet or flipping the dedicated electrical breaker prevents electrical surges from frying the delicate computer control board when utility power suddenly restores.
2. Shut off the gas supply valve or the high-voltage breaker for electric units.
   Turn the manual gas valve handle perpendicular to the gas pipe, or switch off the dedicated 240-volt double-pole breaker on electric tankless models. This ensures the unit cannot accidentally ignite or attempt to heat while the internal chambers are completely dry.
3. Close the cold water supply valve.
   Turn the handle on the incoming cold water pipe (usually marked with a blue handle) until it stops perpendicular to the pipe. This cuts off incoming pressurized water from your municipal water line or well pump so no fresh water can enter the heater.
4. Close the hot water isolation valve.
   Turn the hot water shut-off handle (usually marked with a red handle) directly under the heater to the closed position. This isolates the water heater from the rest of your household plumbing, preventing gallons of water stored in your home upper-floor pipes from draining back down into the unit.
5. Place a five-gallon bucket directly beneath the unit.
   Positioning a sturdy bucket under the plumbing fittings catches the draining water and keeps utility room floors, basements, or siding dry.
6. Unscrew the drain caps from the isolation valves.
   Carefully twist off the drain caps (small removable plugs that let water escape) located on the sides of the service valves under the unit. Water will immediately begin pouring out into your bucket as pressure releases.
7. Open the small service purge valves.
   Turn the small brass lever handles on the service ports parallel to their tiny spouts to allow air into the lines and let the bulk of the water empty out.
8. Remove the cold water inlet filter screen.
   Unscrew the plastic or brass plug holding the cold water filter screen, located near the bottom of the cold water inlet pipe. Pulling this filter completely out breaks the internal vacuum seal, allowing the final, trapped pockets of water to dump out of the heat exchanger (the metal coils inside that warm the water).
9. Leave all drain caps off and service ports open until power returns.
   Keep the drainage points open for the entire duration of the deep freeze. If any tiny moisture droplets remain clinging to the internal metal passages, open valves give the expanding ice room to push harmlessly out into the air rather than cracking the metal coils.

Identifying Frozen Lines and Safe Thawing Procedures

If you turn on a hot water tap on a freezing morning and get zero flow, or if only a tiny trickle emerges and then stops completely, your tankless water heater or its connected pipes have likely frozen. The digital display on the front of the heater may flash an error code indicating water flow failure, abnormal combustion, or sensor errors. In some cases, the control panel may remain completely blank if ice has forced internal safety switches open.

Before you attempt to thaw anything, perform a careful visual check. Inspect the piping directly beneath the cabinet, focusing on elbow joints, brass valves, and the plastic filter housing. Look for bulging copper pipes, split seams, frost rings, or hairline fractures. If you see split metal or cracked plastic, do not attempt to thaw the line. Thawing a cracked pipe while the water supply is active will cause high-pressure water to spray across your home or yard. If you notice structural damage, leave the water supply shut off and contact a licensed plumber.

If the piping appears intact, you can begin thawing the lines gradually. Never use an open flame, propane torch, charcoal lighter, or high-intensity heat gun to thaw a tankless water heater. Direct flames can melt internal wiring, ignite plastic components, boil trapped water into explosive steam, and permanently warp the metal heat exchanger coils inside the cabinet.

The safest thawing method uses gentle, indirect warmth. Start by opening the hot water faucets throughout your home so the melting water has an open path to relieve internal pressure. Next, wrap thick towels soaked in warm (not boiling) water around the frozen pipe sections and isolation valves (small shut-off handles directly under the heater). Alternatively, aim a household hair dryer set to medium heat at the frozen pipes, keeping the nozzle moving back and forth continuously roughly six inches away from the surface. Warm the pipe starting from the point closest to the open faucet and work your way backward toward the heater.

Once water begins trickling out of the open faucets, let the stream run until it reaches full pressure and runs clear. Turn off the taps and inspect every valve, fitting, and connection underneath the unit. Take off the front cabinet cover according to the instructions in your owner manual and shine a flashlight inside to inspect the base pan. If you see water pooling inside the cabinet or dripping from the copper loops, shut off the cold water intake valve immediately and call a certified technician to inspect the heat exchanger.

FAQs

At what temperature will a tankless water heater freeze?

An unprotected tankless water heater can start freezing when outside air temperatures stay below 32 degrees Fahrenheit for several consecutive hours. Units installed outdoors or in drafty unheated spaces can freeze even faster if strong winter winds blow directly against the cabinet. While built-in heating pads offer internal protection down to roughly minus 5 degrees Fahrenheit, the exposed external water pipes can freeze at 30 degrees Fahrenheit without proper insulation.

Will letting a hot faucet drip trigger the gas burner all night?

A slow drip or tiny pencil-thin stream will not trigger the burner as long as the flow stays below the unit activation threshold. Most tankless heaters require a continuous flow rate between 0.4 and 0.6 gallons per minute before the computer commands the burner to ignite. As long as you keep the stream slender and verify that the heating indicator light remains off on the display panel, water will move through the system without burning gas or electricity.

Can I use heat tape on tankless water heater pipes?

Yes, you can apply self-regulating heat tape to the exposed metal and plastic water pipes running into the bottom of the heater. Always select a certified heating cable labeled specifically for water pipe freeze prevention, and wrap it smoothly along the pipe according to the manufacturer instructions before covering it with foam insulation. Never wrap heat tape over itself, and avoid running heating cables inside the metal cabinet of the water heater.

What should I do if the power goes out during a winter storm?

If your area experiences a power outage during sub-freezing weather, act immediately because the heater internal electric freeze protection will not function. If municipal water pressure is still active, open a faucet to maintain a thin, steady trickle of water through the hot lines. If the water supply has stopped or temperatures drop into the single digits, shut off the water valves and follow the manual drain procedure to empty the unit completely.