Navien NHB-055 Installation & Operation Manual page 18

Low Water Cut Off (LWCO) Device
Internal LWCO
The Navien NHB boiler is equipped with a factory-installed,
pressure-sensing type low water cutoff (LWCO) device. The
minimum operation pressure for this device is 6 psi by default.
When the built-in water pressure sensor detects
Note
insufficient water level in the boiler, error code E302 is
displayed on the front panel requiring a manual boiler
reset.
Refer to local codes to determine if a LWCO device is required
for your system and ensure that the built-in device meets the
requirements.
External LWCO
If local codes require a separate LWCO device, install one
separately. Combining the LWCO with the air vent simplifies the
installation. The following illustration shows an example of a
separately installed external LWCO.
The external LWCO must be installed at least 6 in (152 mm) above
the top of the heat exchanger. Refer to "3.6.2 Wiring Diagram -
External LWCO" on page 31 for typical wiring connections of the
LWCO to the boiler PCB.
18 Installing the System Piping
External
LWCO
Backflow Preventer
Install a backflow preventer valve in the make-up water supply to
the unit as required by local codes.
Expansion Tank
An expansion tank must be installed in the space heating piping
to prevent excessive pressure from building in the system. See the
examples of system application at the end of this section for the
installation location. Refer to the expansion tank manufacturer's
instructions for additional details.
Follow the guidelines below when installing an expansion tank.
Connect an air separator to the expansion tank only if the air
separator is located on the suction side of the system pump.
Install the make-up water connection at the same location as
the expansion tank's connection to the system.
When replacing an expansion tank, consult the expansion tank
manufacturer's literature for proper sizing.
For diaphragm expansion tanks, always install an automatic air
vent on the top side of the air separator to remove residual air
from the system.
Isolation Valves and Unions
Full port ball valves are required for the boiler system. Failure
to use full port ball valves could result in a restricted flow rate
through the boiler.
Check valves are recommended for installation. Failure to install
check valves could result in a reverse flow condition during
pump(s) off cycle.
Unions are recommended for unit serviceability.