Maintenance and service
recommended maintenance may result
in complete failure of the unit over time.
The warranty does not cover failures
The GWH 425 PN requires periodic maintenance. The
below time maintenance intervals should kept the unit
operating for many years.
B Inspect inlet water filter screen ( 5.2)
B Inspect pilot assembly and flame ( 5.3)
B Inspect burner assembly ( 5.4)
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
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 isolation valve
to the heater. If one is not installed, install before
proceeding. Open the nearest hot water tap to drain the
plumbing lines. Position a bucket under the heater's
water valve assembly to catch any water that may drain
from the heater. Remove the filter retaining clip located
at base of water valve (see Fig. 26). Remove the
threaded bushing to access the cylindrical filter inside.
Remove filter, clean with water and inspect for damage.
If the filter is at all damaged, it should be replaced.
Lubricating the water valve
A more detailed instruction is available on our website
at www.boschpro.com. Shut off the installer supplied
cold water isolation valve to the heater. If one is not
installed, install before proceeding. Open the nearest
hot water tap to drain the plumbing lines. Position a
bucket under the heater's water valve assembly to catch
any water that may drain from the heater. Disconnect
the inlet water pipe going to the back of the water valve.
Disconnect the water pipe leading out of water valve to
the heat exchanger. Do this by removing the retaining
6 720 608 034
clip on water valve assembly and pulling the pipe free
from the water valve. Loosen the two set screws at the
bottom of the water valve where it connects to the gas
valve. If the set screws are seized, STOP and call
Bosch tech support before proceeding. Forcing and
snapping seized set screws may cause irrevocable
damage to the water valve and gas valve assemblies.
Remove the water valve by pulling it to the right.
Separate the two halves of the water valve by removing
the five screws on the left side of the water valve. Apply
lithium or faucet & valve grease to the pushrod and o-
ring. (See Fig. 29). Inspect the venturi and clean if
necessary. (see Fig 29).
Rebuilding the water valve
Several parts within the water valve should be replaced
every 3 to 5 years depending on water quality and
usage. The parts list and instructions can be found on
our website at www.boschpro.com. Failure to rebuild
the water valve can result in decreased performance or
damage to the heater.
The pilot should burn with a clean sharp blue flame. The
flame should fully engulf the tip of the thermocouple. If
it does not, clean the pilot assembly per procedure
Cleaning pilot assembly
Shut off gas supply to the heater using installer supplied
manual gas shutoff. Locate pilot gas tubing mounted in
the center of the burner assembly. At the base of the
pilot gas tubing there is a retaining clip. Release pilot
gas tubing from this clip. Push upwards on other end of
pilot gas tubing to release it from burner mount.
Remove brass hex headed pilot orifice from gas valve
(located near base of pilot gas tubing). Soak orifice and
pilot gas tubing in carburetor cleaner for 30 minutes.
Dry off and reinsert orifice in gas valve. If heater is an LP
unit, replace orifice. Reinstall the pilot gas tubing
following removal instructions in reverse. Open gas
supply and return heater to service.
Caution: Do not ream or poke orifice as
it will enlarge orifice opening.
Maintenance and service