Freeze Protection and Winterizing a Tankless Water Heater

Modern tankless heaters have built-in freeze protection: small heaters inside the cabinet that keep the water in the heat exchanger above freezing. It works well, and it has one very large caveat.

It needs electricity. A winter storm that takes out the power takes out the freeze protection with it, at exactly the moment you need it. And it only protects the heater — the external piping and valves around it are on their own. Rinnai’s position on the consequences is blunt: damage due to freezing is not covered by the warranty.

Before winter

• Check for a code 41. That is the freeze protection sensor fault, and on a SENSEI RX it applies to every model, not just outdoor ones. The unit keeps making hot water with that code showing, which is exactly why people ignore it until January.
• Keep the vent terminal and the condensate line clear of snow and ice. A blocked intake or exhaust stops the unit working; a frozen condensate line backs up into the trap and shuts it down with a blocked-air code.
• Insulate the external pipework, not the sensor. Rinnai’s instruction on the freeze protection sensor is that it must be open to outside air — wrapping it defeats it.
• Ask about a drain-down kit if you lose power often. Rinnai offers solenoid valves that dump the water automatically when power is lost. Note that the condensate trap and the pressure relief valve are not covered by them and still have to be drained by hand.

If power is out and it is freezing

Rinnai’s own suggestion when the unit cannot protect itself is to run a low volume of water through it: open a hot tap to a stream about 0.2 in. (5 mm) thick, roughly 0.1 gal/min. Moving water is much harder to freeze than standing water.

Draining the unit

For a long absence, or a house being closed for the season:

1. Shut off the cold water supply and the gas supply.
2. Turn off the controller and disconnect power to the heater.
3. Close the hot and cold isolation valves.
4. Open the pressure relief valve lever.
5. Open the hot and cold drain valves and let the unit empty.
6. Remove the condensate trap drain plug and let it drain.
7. On models with a built-in pump, remove the pump drain plug as well.

To bring it back: confirm the gas is still off and the relief valve lever is closed, refit both drain plugs, close the drain valves, open the isolation valves, run a tap until water flows steadily, reconnect power, confirm the controller is off, then turn the gas back on and switch the controller on.

If it has already frozen

Do not operate the heater. Close the gas and water valves, turn the power off, and wait for it to thaw — then check the unit and the pipework for leaks before it runs again. A heat exchanger that split while frozen will not show it until there is pressure behind it.