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How to Keep a Tankless Water Heater from Freezing

Jul 5, 2024 | Plumbing, Water Heaters and Tanks


If you’ve been wondering how to keep a tankless water heater from freezing, there are different methods for both indoor and outdoor tankless heater installations. Various methods include insulation, backup power, sealing for airtightness, heat tracing pipework, adding insulated housings, and other active preventative measures.

Knowing which to choose depends on your tankless water heater model and installation location.

Model-specific ambient temperature limitations

Most tankless water heaters have on-board freeze protection, such as internal ceramic heaters.

Generally, this allows tankless water heaters to perform in ambient temperatures as low as -25°C.

You can verify this by checking your model and brand serial number with your unit’s manufacturer.

This acts as a safety feature for tankless water heaters.

Draw hot water in your home

If the power to your tankless heater might go out, drip your faucets on the hot side.

A small trickle of hot water is advised to keep your unit circulating water and prevent lines from freezing.

Drain the system

If you are going away for a long time, you could drain your system.

Some tankless water heaters have T&P valves allowing you to discharge the water.

A tankless water heater that does not have water in it cannot freeze.

Indoor tankless heater freezing concerns

For indoor installations, some tankless water heaters will intake air for combustion directly from outdoors.

This is done by a ‘combustion air’ duct, which draws fresh air from outside to your unit.

If your furnace is in the same room as your tankless water heater, that furnace can create negative air pressure in the room. (your furnace also needs an air supply)

Poor duct seals or tankless water heater unit sealing could cause your tankless water heater’s combustion air duct to draw in cold air due to the negative pressure your furnace creates.

In turn, the cold air sucked backward through the concentric combustion/flue duct could freeze the water remaining in the tankless water heater.

Gas-fed appliances such as boilers and tankless heaters are covered under gas building codes, which regulate venting and supply air requirements based on an appliance’s BTU/h output.

Sealing up direct vent tankless water heaters

If you have a direct vent or concentric style (combination intake air with vent), you will want to ensure your unit is airtight.

Use standard duct sealing (aluminized) tape to seal up the ductwork attached to your tankless water heater. You could also use air duct mastic.

Seal up the joints around your tankless water heater perimeter to completely seal your heater body, too.

This will prevent any backdraft(negative air pressure) in your mechanical room from air leaks in your tankless water heater, where you have another gas-fed appliance sucking air, such as a furnace.


separate vent and combustion air shown on direct vent tankless water heater

Never unplug your tankless water heater

Keep your tankless heater plugged in. If it is not plugged in, it can’t heat water, which can lead to ambient temperatures cooling or freezing the water in your unit.

You will not only not have a hot water supply in your home, but you will also risk freezing your tankless water heater.

Backup power

Loss of power during an outage can cause all the loss of heating operations.

Some backup power options for your tankless water heater include a generator or battery setup.

If you have heat traced your pipes feeding to and from your tankless water heater, connect the heat trace to backup power.

Heat trace

Heat trace or heat tape is an electrical heating element that provides heat to a plumbing line, pipe, or drain route.

Use heat trace or tape on your tankless heating supply and return lines. This will prevent your pipes from freezing and is a step forward in preventing your tankless water heater from freezing.

You should consult an electrician for the installation.

Insulating tankless heater pipes

The most basic prevention to keep your tankless water heater from freezing is to insulate pipes and the water heater itself.

Fiberglass, polyurethane, phenolic, mineral wool, or other glass fibers are examples of domestic piping insulation.

Sheltered enclosures

Placing your tankless heater inside an insulated box is a great way to protect it from freezing.

The insulated box will prevent frost, cold air, and heat transfer.


If you’ve been wondering how to keep a tankless water heater from freezing, we hope we’ve answered your question.

Remember, gas-fed plumbing appliances should only be worked on by a licensed plumber.

If you found this post helpful, please consider sharing it.



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