Rinnai Tankless Water Heater Frozen: What to Do

The Short Answer: If your Rinnai or its pipes are frozen, do not operate it. Rinnai’s manual says to close the gas and water valves, turn off the power, let the water thaw, then check the heater and piping for leaks. Never thaw it with a torch or open flame. If anything drips after thawing, keep it off and call a technician; Rinnai does not cover freeze damage under warranty.

Rinnai’s steps for a frozen heater

The RSC SENSEI manual (opens in a new tab) gives this sequence when the heater or its external piping has frozen:

1. Do not operate the water heater.
2. Close the gas and water valves and turn off the power.
3. Wait until the water thaws. Check by opening the water supply valve.
4. Check the water heater and the piping for leaks.

Let it thaw from the surrounding air. A space heater warming the room (kept well away from the unit and anything combustible) is fine; a heat gun, torch, or flame on the cabinet or pipes is not.

Checking for damage after the thaw

With the water supply back on and the heater still off, look and feel for water at:

• The cold and hot connections and service valves under the unit
• The pressure relief valve and its discharge line
• The bottom of the cabinet and the condensate drain (condensing models)
• The wall and floor below the heater

A small drip from a fitting is a plumbing repair. Water coming from inside the cabinet points to a split heat exchanger or internal water valve. Rinnai’s professional guide notes that liquid coming from the condensate drain while the heater is not operating is a common sign of a leaking heat exchanger.

If everything stays dry, turn the power on, then the gas, then the controller, and run hot water while watching for leaks or error codes. If you are unsure, have a technician inspect it before you restart.

How Rinnai freeze protection works, and why it failed

Rinnai heaters use small electric heaters inside the cabinet to protect themselves. For the RSC models, the manual says that with gas and 120-volt power connected, the heater will not freeze down to -22°F for indoor models or -4°F for outdoor models, when protected from direct wind. Wind reduces that protection.

Common reasons a unit freezes anyway:

• Power outage or a tripped GFCI during a cold snap
• Strong wind on an outdoor unit
• Exposed pipes and valves. The manual is explicit that freeze protection does not protect external piping and valves; they need insulation.

Preventing the next freeze

If you expect a power or gas outage in freezing weather, Rinnai recommends draining the heater, including the condensate trap and relief valve on condensing models. It also describes running a hot tap at a small stream (about 0.1 gallon per minute) with the heater off and gas closed as a way to keep water moving. Rinnai sells a drain-down kit that empties the heater automatically when power is lost; ask your installer whether it fits your model.

Related: Rinnai not working after a power outage, tankless leaking after a freeze, and water at a Rinnai exhaust after shutdown.

Frequently Asked Questions

Q: Will my homeowner’s or Rinnai warranty cover freeze damage? A: Rinnai’s manual states that freeze damage is not covered by its warranty. Homeowner’s insurance policies vary; check yours.

Q: Is a frozen heat exchanger repairable? A: A cracked exchanger is replaced, not patched. On an older heater, that cost often makes a new heater the better choice. A technician can price both.