thermocouple tip. If pilot is too small, clean pilot orifice
(see chapter 5.3).
7. If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing the water to over heat which shuts
all gas off to the heater. Instructions for descaling the
heat exchanger are available at www.boschpro.com.
8. If steps 1 through 7 do not correct the problem, then
confirm that the gas pressure is adequate. Ensure gas
pressure is in accordance with manufacturer's
specifications (see chapter 3.9). A gas pressure
reading is needed to proceed further. Contact your
original installer or a licensed gas technician to obtain
Burners do not light with water
1. Verify the pilot is lit.
2. Verify the gas control slide button is in middle
3. Verify the cold water supply is connected to the right
side of the heater.
4. Close installer supplied cold water shut off valve (if
none installed, install before proceeding). Open all hot
water taps supplied by the heater. Wait 5 minutes and
check all taps. Any water running is a sign of a plumbing
crossover. Consult a local plumber or service person for
help in correcting a plumbing crossover.
5. Verify flow through the water heater is sufficient. Turn
temperature adjustment knob all the way clockwise.
Fully open a hot water faucet and fill a quart container.
If the container fills in 30 seconds or less, the flow rate
(1/2 gallon per minute) is sufficient to activate the
6. Inspect the water path outside of the heater for
obstructions. Make sure all outlets (i.e. showerheads,
faucet aerators and whole house filters) are clear of
debris. Also, the water heater's inlet filter screen should
be inspected and cleaned (see chapter 5.2).
7. The water valve assembly, which actuates the
burners, requires periodic maintenance (every 2 - 5
years depending on water quality and use) (see
Hot water temperature fluctuates
1. Close off installer supplied cold water shut off valve
(if none installed, install before proceeding). Open all
hot water taps supplied by the heater. Wait 5 minutes
and check all taps. Water running is a sign of a
plumbing cross-over. Consult a local plumber or service
person for help in correcting a plumbing crossover.
2. Check for a clogged inlet water filter screen (see
3. Hot water is very hot out of the tap, requiring a lot of
cold water to be added with it in order to attain a
useable hot water temperature. The addition of too
much cold will overpower and slow the flow within the
tankless water heater, decreasing it below activation
point, which shuts off the burners. The end result is
nothing but cold water coming out of the outlet.
Reduce the need for cold water mix by setting the gas
slide control button towards the single flame position
for lower hot water temperatures. If the problem
persists, go to www.boschpro.com for a more detailed
service bulletin on this problem.
4. The GWH 425 PN is designed to modulate gas flow
to the burners when water flow varies. If the water
pressure in the home is erratic and the water flow is not
consistent while a tap is opened, then the temperature
of hot water will fluctuate. The minimum water pressure
for the home should be 30psi or greater. For installation
on a private well system with the use of a pressure tank,
the lowest pressure range setting recommended is 30-
50 psi (2.07-3.45 bar). The use of a pressure reducing/
regulating valve before the water heater (directly after
pressure tank on well systems) is an effective way to
maintain constant water pressure to the water heater.
Watts brand 25AUB- ¾" or N35B-¾" pressure
reducing/regulating valves or equivalent are suggested.
5. If the inlet water temperature is not steady then the
hot water temperature from the water heater will
fluctuate. The GWH 425 PN modulates its burners
when the flow rate is changed; it does not have a
thermostat and will not adjust to changes in inlet water
6. If the inlet water temperature to the water heater is
temperatures that are too hot for most users. A
temperature balance shower valve can automatically
mix in cold water to reduce such hot water temperature.
This can slow the flow within the tankless water heater,
decreasing it below activation point, which shuts off the
burners. In the event of any temperature instability with
the use of a temperature balance shower valve, refer to
shower valve manufacturer's instructions for internal
adjustment setting. An adjustment should be made to
minimize the amount of cold water the valve is adding.
7. The gas pressure also needs to be stable and
adequate. Ensure gas pressure is in accordance with
manufacturer's specifications (see chapter 3.9). A gas
pressure reading is needed to proceed further. Contact
your original installer or a local certified gas technician
to obtain this reading.
8. A water valve, which has not been serviced, may be
struggling to keep the gas valve open. The water valve
depending on water quality and use) (see chapter 5.2).
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