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Maintenance And Service; Maintenance Intervals; Main Burners; Water Valve - Bosch Pro Tankless GWH-425-HNO-N Instructions For Use Manual

Gwh 425 hno outdoor model for exterior use only
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Maintenance and service


Maintenance intervals

The 425 HNO requires periodic maintenance. The
below time maintenance intervals should kept the unit
operating for many years.
Every year
B Inspect inlet water filter screen ( 5.2)
B Inspect pilot assembly and flame ( 5.3)
B Inspect burner assembly ( 5.4)
B Inspect vent assembly ( 5.6)
Every 2 years
B Lubricate and clean water valve ( 5.2)
Every 3 to 5 years
B Rebuild water valve ( 5.2)
B Clean pilot assembly and clean or replace orifice
( 5.3)

Water valve

The water valve is the main control that tells the heater
to fire. If the periodic maintenance is neglected, more
costly damage may occur over time.
Inspecting inlet water filter screen
Shut off the installer supplied cold water supply valve.
Disconnect water pipe going to the back of the water
valve. Remove filter from back of water valve. Clean the
filter and inspect for damage. Replace if the filter is at all
damaged. Inspect the top of the water valve for signs of
a water leak or corrosion. If present, rebuild the water
valve. (See below).
Lubricating the water valve
A more detailed instruction is available on our website
at Shut off all water to the unit and
disconnect water pipe going to the back of the water
valve. Disconnect water pipe going to heat exchanger
by removing pin and pulling pipe out. Loosen two set
screws at the base of the gas valve and drop the water
valve down. Separate the two halves by removing 5
screws on the top of the water valve. Apply lithium or
faucet and valve grease to the pushrod (see Fig 14,
pos. 31). Inspect and clean the venturi (see Fig 14,
pos. 49).
Rebuilding the water valve
Several parts within the water valve should be replaced
every 3-5 years. The parts list and instruction can be
found on our website at
8 716 473 148
Inspecting pilot
The pilot should burn with a clean sharp blue flame. The
flame should fully envelope the bend of the flame sensor
rod. If it does not, clean the pilot assembly per
procedure below.
Cleaning pilot assembly
Disconnect the flame sensor rod cable and the igniter
cable by pulling gently down. Remove two screws hold-
ing pilot assembly in place. Loosen compression fitting
and remove pilot assembly. NOTE: The orifice sits on
top of the compression fitting DO NOT LOSE. Polish
flame sensor rod and igniter tip with emery cloth. Clean
out pilot assembly and orifice with carburetor cleaner. If
heater is an LP unit, replace orifice. Caution: Do not
enlarge orifice.

Main burners

The main burner flames should be blue, with a more
intense blue cone in the center core. Yellow flames
could be a sign of wrong size gas orifices or dirty
burners, or a blockage on the heat exchangers fins. If
some burners have yellow flames while others have
good flames, it is likely that dust, lint or spider webs
have partially clogged the burner venturis. To clean the
burners, contact a gas service person.

Vent assembly

Inspect the draft hood and heat exchanger fins for signs
of soot build-up or any other foreign material such as
spider webs. Clean out any debris found in the vent
hood. Signs of soot indicate insufficient combustion air
or exhaust draft. Check for vent assembly blockage or
combustion air blockage on the underside of the unit.

Mineral scale build-up

The 425 HNO, when operated at lower temperatures
settings, does not accumulate mineral build-up. If
however, the heater is used at the higher temperature
settings and the water has a high mineral content,
periodic descaling may be necessary. The heating coils
should be flushed with a descaling solution. Consult
your dealer or BBT North America for instructions.
Consult your service person.
Maintenance and service



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