How to Winterize a Tankless Water Heater and Protect the Plumbing System

Rinnai RUR199 tankless water heaterRinnai tankless water heater

Did you know that your tankless water heater can freeze even if it is installed inside your home?

If you have a tankless water heater and not planning to use it for some time, and the forecast shows incoming cold weather and freezing temperatures, it is essential to protect the unit and a plumbing system.

How to winterize a tankless water heater: Tips to get ready for cold winter weather

In order to avoid unit's failure, use this step-by-step guide and learn how to winterize a tankless water heater properly.

Most of the new tankless water heaters, coming from the manufacturers like Rheem, AO Smith, Rinnai, Noritz, Takagi, Bosch, Paloma, and others, utilize the advanced technology and features for the freeze protection - protecting the system from freezing, down to -30 F, even lower.

Note: Installing a tankless water heater in a heated room with a wind-resistant features, is the best way to prevent freezing during the winter.

However, you, as the owner of a new tankless water heater, should not rely on such features.


Because all the units use electric power, even the protection system (which is usually a heating element or sensor), if there is a power outage, guess what... your tankless unit will fail. Moreover, this is what many people experienced at the end of 2017, during the "arctic" winter.

This is why you should properly winterize a tankless water heater and be ready for the cold days.

Note: Almost every manufacturer of tankless heaters specifies that the heater must not be installed in the area where the temperature can reach 36 F or lower. If the precautions are not taken, like correct draining, damages to the tankless unit and its components will not be covered under warranty. Adding the anti-freeze solution is also not recommended as it can cause damage to the heater's components.

This is especially important if the tankless water heater is installed in places like cabins, garages, and if you are absent for a long time.

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Can tankless water heaters freeze?

Yes, they can, but less than before and if certain precautions are not followed. And the reason is simple; many modern tankless models are equipped with the freeze protection systems, such as electric heaters.

Electric heaters are installed on the heat exchangers and other internal parts, and they turn on when the temperature drops below a specific temperature, preventing freezing and damage.

Some tankless models are also designed to fire up the system and heat a small amount of water, keeping it frost free.

Note that both systems rely on an uninterrupted power supply, otherwise, the freeze protection will be disabled.

Some manufacturers also recommend installing a flue damper to reduce the cold freezing air from entering through the venting system, so as insulating the exposed pipes.

Most models from Rheem, Takagi, Noritz, Rinnai, and others have systems that protect the unit from freezing. Indoor models are protected when the outside air temperature is up to -22F (no direct wind exposure, known as the wind-chill effect), while outdoor models have protection up to -5F.

Winterizing a tankless water heater: Step-by-step guide

One of the main steps when winterizing a tankless water heater is to drain and unplug the unit. If there is no water in the heater's pipes, especially in the heat exchanger, standing water inside the unit won't turn into ice, expand, and make a damage.

So, how to prepare a tankless water heater for winter?

  • Turn the power off. First, you have to turn the electrical power and gas supply OFF. Turn the gas OFF by using the main valve on the gas supply line. For the electricity, unplug the power supply cord or turn the power OFF on the breaker panel. If the unit has the display at the front of the unit, it should be black, with no lights.
  • Wait until water cools down. Keep in mind that there still might be some hot water inside the unit. If the unit was operating before draining, wait for a while until the hot water cools down.
  • Turn off water. Shut OFF the water supply on the main shut-off valve.
  • Drain the system. Open the taps so the system can drain and release the pressure from the plumbing system.
  • Disconnect the pipes. Disconnect the incoming and outgoing water pipes from the tankless heater.
  • Remove the inlet water filter.
  • Ensure all water from the unit is removed. Use the bucket to collect the residual water. While draining, make sure that it fully drains from the system.

Some manufacturers recommend using air compressors - to blow short bursts of air through the inlet connection to ensure no water is left inside the heater.

Now the tankless water heater doesn't contain any water, and it is fully winterized.

Can I fix a frozen tankless water heater?

If your tankless water heater is frozen, you have to repair it as soon as possible. The build-up ice can damage internal components, especially those made of plastic, also sensors and electronics.

These are the steps to thawing out your tankless water heater:

  • First, check whether your tankless unit or pipes are frozen.
  • If the pipe is frozen, use the hair dryer to defrost it.
  • Close the gas and water supply to the unit.
  • Turn off the power by flipping the breaker to the off position.
  • Disconnect the unit and move it to a warmer room or use heat from the hair dryer. Avoid putting too much heat on sensible elements such as a heat exchanger.
  • Open the hot water tap to allow water to flow.
  • Wait until the system thaws.
  • Check if there are any signs of leaks and damages.
  • Drain all the water.
  • If everything is ok with the unit, reinstall it.
  • Open gas and water valves. 
  • Turn the power back on.

Talk to your plumber if there are damages on a heater, if you are not comfortable working on the unit, or if it needs replacement.


While many tankless manufacturers offer models with built-in protection against freezing, it is still recommended to winterize the unit, especially when away from home or during an extended period of inactivity. As opposed to tank-type models, tankless water heaters require more attention and extra care due to their delicate features.

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