Pilot light turns on and off.

Just making sure you do know the unit will only heat while the water is running. make sure blower is working for proper exhaust.

Hello
Here is some diagnistic information on this unit. Matt Ace technician.
6.4 Pilot light, but burners will not
come on.
• 1. Verify gas type indicated in the rating sticker
located on the cover’s right hand side, coincides with
the gas type you are using. NG is a natural gas unit
and LP is for liquid propane (See chapter 2.2).
• 2. Pilot flame should be blue in color and completely
engulfing the flame sensor. If not, the pilot orifice may
be dirty or clogged. See chapter 5.3 on how to clean
the pilot.
• 3. Flame sensor not recognizing pilot flame. Verify
wire connection clip on wire from the bottom of the
flame sensor is secure. If flame sensor is fully
engulfed by the pilot flame, it may be dirty. Clean
entire surface area of flame sensor with a pencil
eraser.
• 4. Inspect the water path for obstructions. Make sure
all outlets (i.e. showerheads, faucet aerators and
whole house filters) are clear of debris. Inspect and
clean the water heater’s inlet filter screen. See
chapter 5.2.
• 5. Confirm wire connection to the burner electro
valve is secure (see components diagram for location
of burner electro valve). Inspect terminals for
corrosion. To clean terminals, remove the spade
connectors and clean with a pencil eraser.
• 6. The water valve assembly, which allows gas flow
to the burners, requires periodic maintenance (every
2-5 years depending on water quality and use). Visit
http://www.boschhotwater.com to purchase water valve
rebuild kit to service the water valve
• 7.Electric control box, burner electro valve or flame
sensor maybe damaged/defective. Verify adequate
voltage at the spade connection of the burner electro
valve (see components diagram for location of burner
electro valve). After the pilot lights, voltage should
read 1.6VAC on the burner electro valve. If voltage
is proper, the burner electro valve is defective. If
voltage is not proper, electronic control box or the
flame sensor is damaged/defective.
Visit http://www.boschhotwater.com to purchase
replacement part, or call Bosch Water Heating at
800-642-3111 for replacement part if still under
warranty.
Fig. 25 Electrical wiring diagram
2 Pilot electrode
3 Flame sensor
23 Electronic control box
25 Burner electro valve
27 Gas control valve
28 Pilot electro valve
31 Temperature limiter
32 Flue gas safety device
34 LED indicator main burner operation
50 Hydro-generator
51 On/Off switch
110 LED failure control box
111 Overheat limiter
6.5 Main burners go out during hot
water use
• 1. Hot water flow rate is dropping below required
activation flow rate. 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 water heater. If
flow rate is not sufficient, the water heater’s inlet filter
should be inspected and cleaned. See chapter 5.2.
• 2. 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 local plumber
or service person for help correcting a plumbing
crossover.
• 3. The minimum water pressure for the home should
be 30psi or greater. For installation on a private well
system with use of a pressure tank, the lowest
pressure range setting recommended is 30-50 psi
(2.07-3.45 bar).
6 720 608 032
Troubleshooting
21
• 4. 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 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 mixed by turning the
power adjustment knob clockwise for lower hot
water temperatures. See chapter 4.4 and 4.5 for
details in lowering temperature. If the problem
persists, go to http://www.boschhotwater.com for a more
detailed service bulletin on this problem.
• 5. The overheat sensor (ECO) is tripped. The water
heater does not have a thermostat. If the inlet water
is preheated, the unit will overheat, stopping the flow
of gas. Plumb inlet with cold water only.
• 6. Flue gas sensor is tripped. Lack of adequate
combustion air or failure to vent properly by reducing
pipe diameter, improper use of elbows or exceed
maximum vent length will interrupt the flue gas sensor
circuit. Confirm venting and combustion air is in
accordance with requirements in this installation
manual
• 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 overheat and trip the
overheat sensor (ECO) shutting all gas flow. To
descale the heat exchanger, consult service bulletins
on http://www.boschhotwater.com
• 8. The water valve assembly, which allows gas flow
to the burners, requires periodic maintenance (every
2-5 years depending on water quality and use). Visit
http://www.boschhotwater.com to purchase water valve
rebuild kit to service the water valve.
6.6 Hot water temperature fluctuates
at tap
• 1. 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 local plumber
or service person for help correcting a plumbing
crossover.
• 2. Check for a clogged inlet water filter screen. See
chapter 5.2.
• 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 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 mixed by turning the
power adjustment knob clockwise for lower hot
water temperatures. See chapter 4.5. If the problem
persists, go to http://www.boschhotwater.com for a more
detailed service bulletin on this problem
• 4. The water heater 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 the hot water will fluctuate. The
minimum water pressure for the home should be
30psi or greater. For installations on a private well
system with the use of a pressure tank, the lowest
pressure range setting recommended is 30-50psi
(2.07-3.45 bar). The use of a pressure reducing/
regulating valve before the water heater (directly after
the pressure tank on well systems) is an effective way
to maintain constant water pressure to the water
heater. Watts brand 25AUB-3/4″ or N35B-3/4″
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 water heater modulates its burners
when flow rate is changed. It does not have a
thermostat and will not adjust to changes in inlet
water temperature.
• 6. If the inlet water temperature to the water heater is
above 70 F, the heater may produce outlet
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 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
specifications in the installation manual. 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.
6.7 Water is too hot
• 1. Verify gas type indicated in the rating sticker
located on the cover’s right hand side, coincides with
the gas type you are using. NG is a natural gas unit
and LP is for liquid propane (See chapter 2.2).
• 2. Adjust the power adjustment knob clockwise for
cooler temperatures. If the water is still too hot, the
temperature adjustment knob maybe set too high.
Lower the setting and test the water temperature.
This dial turns counterclockwise for lower
temperatures and clockwise for higher temperatures.
22 6 720 608 032
Troubleshooting
• 3. Inlet water is preheated. This model does not have
a thermostat and will not modulate flame height in
order to prevent overheating. It is designed for a cold
water feed only. Replumb the heater with cold water
to the inlet.
• 4. Increase flow rate. Restrictions in the water path
can slow the flow of water through the heater,
resulting in very hot outlet temperatures. Restrictive
showerheads and faucet aerators should be cleaned
out or upgraded with less restrictive ones. In
addition, the inlet filter on the water heater should be
cleaned and inspected. See chapter 5.2.
• 5. If water is still too hot, consult service bulletins on
http://www.boschhotwater.com to lower outlet
temperatures further.
• 6. If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing the water to boil and produce
extremely hot temperatures. To descale the heat
exchanger, consult service bulletins on
http://www.boschhotwater.com.
6.8 Water is not hot enough
• 1. Verify gas type indicated in the rating sticker
located on the cover’s right hand side, coincides with
the gas type you are using. NG is a natural gas unit
and LP is for liquid propane (see chapter 2.2).
• 2. Flow at one particular tap is too great or too many
fixtures are running at one time. Lower flow to stay
within heater’s specifications.
• 3. Ensure the power adjustment knob is all the way
clockwise for highest temperatures. In addition, the
temperature adjustment knob maybe set too low.
Raise the setting and test the water temperature.
This dial turns counterclockwise for lower
temperatures and clockwise for higher temperatures.
• 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 local plumber
or service person for help correcting a plumbing
crossover.
• 5. Inspect the water path outside 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 should be
inspected and cleaned. See chapter 5.2.
• 6. The water valve assembly, which allows gas flow
to the burners, requires periodic maintenance (every
2-5 years depending on water quality and use). Visit
http://www.boschhotwater.com to purchase water valve
rebuild kit to service the water valve.
• 7. Gas pressure is too low. Ensure gas pressure is in
accordance with specifications in the installation
manual. 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.
6.9 Burners ignite without water flow
• 1. The water valve may be dirty and sticking,
prohibiting it from fully closing the gas valve. This part
requires periodic maintenance (every 2-5 years
depending on water quality and use). Visit
http://www.boschhotwater.com to purchase water valve
rebuild kit to service the water valve.
6.10 Low hot water pressure
• 1. Confirm adequate flow rate through the unit. With
cold supply shut off to the unit, disconnect hot outlet
side to the unit. Place a gallon jug under hot water
outlet to catch water. Open cold water supply and
measure flow rate out of the unit. With temperature
adjustment knob set all the way clockwise, the flow
rate should be close to 2GPM (filling the jug in 30
seconds or less). If the flow rate is not adequate,
inspect and clean inlet filter screen. See chapter 5.2.
• 2. Confirm the incoming water pressure to the unit is
above the required 30psi.
• 3. Inspect the water path outside the heater for
obstructions. Make sure all outlets (i.e. showerheads,
faucet aerators and whole house filters) are clear of
debris.
• 4. Be sure to run only one major fixture at a time with
this water heater. Opening too many taps at one time
can disperse water flow resulting in diminished flow/
pressure at all outlets.
• 5. If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing diminished water flow. To
descale the heat exchanger, consult service bulletins
at http://www.boschhotwater.com.
6.11 Noise when heater is running
• 1. Confirm adequate flow rate through the unit. With
cold supply shut off to the unit, disconnect hot outlet
side to the unit. Place a gallon jug under hot water
outlet to catch water. Open cold water supply and
measure flow rate out of the unit. With temperature
adjustment knob set all the way clockwise, the flow
rate should be close to 2GPM (filling the jug in 30
seconds or less). If the flow rate is not adequate,
inspect and clean inlet filter screen. See chapter 5.2.
• 2. Inspect the water path for obstructions. Make sure
all outlets (i.e. showerheads, faucet aerators and
whole house filters) are clear of debris. Restrictions
6 720 608 032
Protecting the environment
23
in the water path can slow water flow through the
heat exchanger causing it to boil and make noise
• 3. If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing the water to boil resulting in
noise and overheating. To descale the heat
exchanger, consult service bulletins on
http://www.boschhotwater.com.
6.12 Burners do not burn cleanly, yellow
flames when operating
• 1. Verify gas type indicated in the rating sticker
located on the heater’s right hand side, coincides
with the gas type you are using. NG is a natural gas
unit and LP is for liquid propane (see chapter 2.2).
• 2. Gas pressure may be inadequate resulting in
improper combustion. Ensure gas pressure is in
accordance with specifications in the installation
manual. A gas pressure reading is needed to
proceed further. Contact your original installer or a
local certified gas technician to obtain this reading.
See chapter 3.9.
• 3. For proper combustion at high elevations the inlet
gas pressure should be set at installation to the
values in table 3 on page 11 (while operating at
maximum BTU). A gas pressure reading is needed to
proceed further. Contact your original installer or a
local certified gas technician to obtain this reading.
• 4. Ventilation is not adequate. Ensure adequate
combustion air is being supplied to the unit in
accordance with requirements in the installation
manual. See chapter 3.6.
• 5. Verify the water heater is vented properly. Ensure
the venting is in accordance with requirements in the
installation manual. See chapter 3.7.
• 6. The water heater has burners much like a furnace.
It is essential that they remain clean and are supplied
with adequate air for combustion. If the flames burn
yellow or orange it is strongly recommended that the
burners be removed from the unit and cleaned. Once
the burners are removed, the fins in the top of the
heat exchanger can be accessed and brushed or
vacuumed clean. Consult service bulletins on
http://www.boschhotwater.com for removing/cleaning the
burners.

Hello, I have attached a manual for this unit. Look in section 6 that is the trouble shooting section.