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Cold Weather and Your Tankless Water Heater: How to Prevent Freezing


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When the weather turns cold and the temperatures begin to plunge, your tankless water heater is prone to freezing and experiencing other weather related issues. Whether you need to winterize your hot water heating system as you close down a summer cabin, or you live in an area with cold winters, your tankless system is going to require a little additional attention.

This article will give you the information you need to keep your tankless water heater from freezing during cold weather. However, it's important to recognize that recommendations may vary between manufacturers and you should always check your owners manual for details for your specific tankless system.

Thermometer in Snow

Can a Tankless Water Heater Freeze?

During cold weather tankless water heaters need extra care. Even water heaters installed indoors could be prone to cold weather complications. Most manufacturer warranty's do not cover damage caused by freezing. 

However, many manufacturers, such as Rinnai and Rheem have built-in some type of freeze protection which allows the unit to protect itself when temperatures fall below freezing.

Rinnai tankless water heaters have two lines of defense to protect them from cold weather and prevent freezing. The primary protection for a Rinnai heater, is the addition of ceramic heaters which are placed on the heat exchanger and other internal parts, as well as water lines. These ceramic heaters fire-up when the temperature drops into the low 30's and prevent the internal parts from freezing.

If the ceramic heaters fail for any reason, or the temperature drops too fast, a secondary freeze protection method is also built-in. This system utilizes a sensor inside the unit which determines if the tankless is below freezing. If the sensor is triggered it'll automatically turn the unit on for a few seconds and continue to cycle the heater on-and-off until the threat of freezing has past. 

Both of these methods provide freeze protection to the Rinnai unit as long as both electrical power and gas is available to the unit. Be sure to check your owners manual for details on your specific tankless.

Frozen Pipe

How to Thaw a Frozen Tankless Water Heater

Cold weather can cause frozen pipes and a frozen water heater. If the external piping to the water heater has frozen, or the water heater itself as frozen DO NOT OPERATE THE UNIT. Follow these steps:

  • CLOSE the Gas and Water Shutoff Valves and turn OFF the Electrical Power.
  • Allow the tankless unit to thaw. To check if the unit has thawed, open the Water Supply Valve. If water flows, the tankless water heater has thawed.
  • Carefully inspect the external pipes, internal components, and the plumbing for leaks.
  • If everything looks good, you can OPEN the Gas and Water Valves and turn the Electrical Power ON. 

Protecting External Pipes from Freezing

Many homeowners forget to protect the external plumbing leading to and from the tankless water heater, but these pipes and valves are prone to freezing if they're left unprotected. Fortunately, wrapping the external pipes with either a pipe wrap insulation or a pipe heating cable can protect them from freezing and prevent a major headache. Both methods are simple, inexpensive and readily available.

Many pipe heating cables are rated for external use and will prevent pipes from freezing in temperatures up to -50°. Some homeowners choose to wrap their pipes first with the heating cable and then add the pipe wrap insulation, but you should always check the manufacturer's recommendations for the specific products.


How to Prevent a Tankless Water Heater from Freezing

Even with built-in freeze protection, there are times when you need to drain your tankless water heater. If you ever lose power your tankless will be left defenseless to the cold temperatures and could freeze. We highly recommend, particularly for cold climates, to drain your tankless system if you'll be away for an extended time.

Although, tankless heaters can always be drained manually, installing a set of freeze protection solenoid valves is an excellent way to give you peace of mind during the cold winter weather. These valves automatically drain the water from your tankless unit if there's ever a power outage. 

Freeze protection solenoid valves are sometimes called drain down solenoid valves or "normally open" drain down solenoid valves. They're designed to always be open and require an electrical current to keep the valve closed. When there's a power outage, the valves automatically open and release the water within the tankless water heater.

Installing freeze protection solenoid valves is an excellent way to protect your tankless heating system, especially in cold regions. However, if you choose not to add this extra layer of protection, it's a best practice to do the following to protect your tankless water heater in the event of a power outage during cold weather:

  • Remove/manually open the Pump Drain Plug
  • Remove the Condensate Trap Drain Plug
  • Remove the Water Drain Plug 
Rinnai 104000059 Freeze Protection Solenoid Valves Kit

Rinnai Drain Down Solenoid Valves

Installing a freeze protection solenoid valve kit on your outdoor unit is an excellent way to safeguard against freezing.

How to Manually Drain Your Tankless Water Heater

Whether you're draining your tankless unit for cold weather conditions or another reason, if your tankless doesn't have the freeze protection solenoid valve kit installed, you'll need to manually drain the unit. Manually draining is not as easy, but it's just as effective at removing the water from inside the tankless heater.

Here's how to manually drain your tankless water heater:

  • Turn OFF the Cold Water Supply and turn OFF the Gas Supply.
  • Turn OFF the Temperature Control.
  • Disconnect the electrical power to the water heater.
  • We recommend opening a hot water tap/faucet inside the house. This will help relieve the pressure in the heater.
  • Place a bucket under the water heater to catch the water.
  • Remove the Drain Caps on the Hot and Cold Isolation Valves.
  • OPEN the valves. The hot water is under pressure, so it may "shoot out" of the unit. Use caution, the water may be hot.
  • REMOVE the Cold Water Inlet Filter.
  • If your tankless water heater has drain plugs, you should remove them. They are located on the bottom of the tankless unit. (Water Drain Plug; Pump Drain Plug; Condensate Trap Drain Plug; and the Hot Water Drain Plug).

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How To Return Your Tankless Water Heater to Normal Operation

Once you're ready to start using your tankless water heater you'll need to return it to normal operation. This is very easy, and basically the opposite of the draining procedure outlined above.

Here's what to do:

  • Double check that all of the drain plugs are removed from the unit, the hot water taps/faucets within the house are closed, and the gas supply is off.
  • Replace the Water Drain Plug, Pump Drain Plug, and Condensate Trap Drain Plug.
  • Replace the Check Valve Drain Plug.
  • Replace the Cold Water Inlet Filter.
  • OPEN the Cold Water Supply. This will allow the tankless unit to fill with water.
  • OPEN a hot water tap/faucet inside the house. If water flows from the tap/faucet, then it is flowing properly through the tankless water heater and you can CLOSE the tap/faucet.
  • Turn ON the electrical power to the tankless heater.
  • Double check that the Temperature Controller is OFF.
  • Turn ON the gas supply.
  • Turn ON the Temperature Controller.

Plumber Repairing Water Heater

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How to Run a Low Water Flow Through the Tankless 

Although, this method is not recommended for extended periods of time, such as vacations, it can be helpful if there's a loss of power or the temperature drops below the unit's ability to freeze protect itself. Running a low volume of water through a tankless water heating system will prevent the heater and external piping from freezing.

Follow these steps:

  • Turn OFF the electrical power to the tankless water heater.
  • CLOSE the gas supply valve.
  • OPEN a hot water tap/faucet within your house. The water should flow at a rate of 1/10th of a gallon per minute or the water stream should be .20-inches in width.
Preventative Maintenance Frozen pipes

How to Winterize a Tankless Water Heater

If you have a tankless water heater at a vacation home, or plan to be away during the winter, you may want to consider winterizing the unit as a precautionary measure to protect it from freezing. Cold weather conditions can happen quickly and planning ahead can save you a major unexpected headache.

Please keep in mind, that these instructions are provided as a guide, and your individual tankless water heater may have different requirements. You should always consult your owner's manual for directions for your specific heater.

We also recommend hiring a licensed professional to do the work for you. Be sure that they guarantee their work and will cover any expenses if the job is not done correctly. If you take good notes, you'll know exactly what to do next year!

Here's what to do:

Gas Supply 

  • Turn OFF the gas to the water heater.

Water Supply

  • Turn OFF the cold water supply to the tankless water heater.
  • Drain the unit by opening the freeze protection (drain down) solenoid valves on the cold and hot water lines.
  • Open a couple of hot water faucets inside the house.
  • Remove the inlet filter on the cold water line. If your tankless system has a plug or inline filter on the hot water side, it should also be removed to leave room for expansion if there's any water within the lines that freeze.

Electrical Supply - Turn OFF the power supply to the tankless system by either unplugging or flipping the circuit breaker off. This will protect the tankless water heater should there by any power surges or outages.

Venting System - If it can be safely reached, place a cover on the end of the vent, both the intake and exhaust. This will keep debris, leaves, and even small animals such as squirrels from becoming trapped within the vent and causing air flow issues when it's time to start using your tankless water heater again.

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