Navien NPN-160U User's Information Manual page 20

To ensure that the water heater does not freeze,
follow these guidelines:
Do not unplug the power supply cord, except
for routine maintenance. The water heater
has a freeze protection function that requires
electricity. The freeze protection function will
operate regardless of whether or not the power is
turned on or off, as long as the electric supply is
still connected.
Do not close the gas valve, except for routine
maintenance, as this will limit additional freeze
protection.
If the power or gas supplies must be
disconnected for an extended period, drain the
water heater. Freezing damage may occur if there
is a loss of power or an interruption in the gas
supply to the water heater.
If the water heater will not be used for an
extended period, drain the water heater.
If hot water will not flow and you suspect that the
water heater is frozen, follow these steps:
1. Press the Power button on the front panel to
turn off the water heater.
2. Close the gas valve.
Gas
Valve
3. Open the hot water faucet that is closest to the
water heater.
4. Use a hair dryer or a portable electric heater to
heat up both the primary and secondary Heat
Exchangers.
20 Maintaining the Water Heater
Opened Closed
5. Check every few minutes to see if water is
running at the open faucet.
6. When the water starts flowing again, check the
water heater and piping for leaks. If you detect
any leaks or the water heater is not operating
properly, contact an authorized technician or
licensed professional.
CAUTION
If the unit will not be used immediately, close off
all gas and water shutoff valves, drain all of the
water out of the unit and the plumbing system
to prevent the unit and system from freezing,
and bleed the gas out of the gas line.