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Troubleshooting; Introduction; Burner Does Not Ignite When Hot Water Is Turned On; Water Is Too Hot - Bosch 940 ESO Manual

940 eso - natural gas / 940 eso - liquefied petroleum (lp) gas
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34 | Troubleshooting

12
Troubleshooting
CAUTION:
▶ If you are unable to perform the tasks listed below, or
need additional assistance please contact a qualified
gas technician.
12.1

Introduction

Many of the questions customers ask regarding operation of this unit can
be answered by following the troubleshooting steps as outlined below.
Visit our web site at www.bosch-climate.us for more detailed
troubleshooting. For best results, perform each step before proceeding
to the next. The suggested solutions may require that the cover be taken
off. (See fig. 3, page 8).
12.2

Burner does not ignite when hot water is turned ON

It is recommended to use "Control board diagnostics" page 33, chapter
11.6 as a tool for problem solving. If the display is showing a blinking two
digit code, refer to this section.
1. If the display is blank, verify power to outlet. (120VAC/60Hz
properly grounded circuit required). Verify that the heater power
button is pressed ON.
2. Verify the fuses in the control board are good. To access fuses, the
control board must be removed. See chapter 9.2, page 24.
3. Make sure cold water inlet connection is plumbed to the right side of
heater when facing unit. See Fig.25, page 20.
4. A minimum of 0.5 gallons per minute (1.9 l/m) of hot water demand
is required to activate the heater. Confirm at least 0.5 gpm flow by
timing how long it takes to fill a container. A quart container should fill
in 23 seconds or less, from hot water tap only, to activate heater. In
unusual circumstances with high inlet water temperatures and low
set temperatures, the heater may increase the activation rate above
0.5 gpm to avoid overheating at low flows. Gradually increase water
flow (hot water demand) through the heater to determine if the
heater activates with higher flow.
5. Clean inlet filter screen per chapter 8.2, page 19.
6. Inspect the water path for obstructions. Make sure all shower heads,
faucet aerators and whole house filters are clear of debris.
7. Check for plumbing crossover. A crossover in the hot and cold
plumbing pipes creates back pressure on the water flowing through
the heater. Therefore, a higher flow rate than normal is needed to
force the heater to activate. To check for a plumbing crossover, shut
off the cold water supply to the water heater. Then open all of the hot
water taps served by the heater. Wait 10 minutes and check for water
flow at each tap. There should be no water flowing. Any continuous
flow of water, small or large, indicates a crossover is present and
must be corrected. Consult a professional plumber for help in
correcting a crossover. Failing single lever faucet mixing valves are
common causes of plumbing crossovers.
8. With the power button pressed switch turned to OFF and the power
supply cord unplugged, remove the unit's front cover (See fig. 3,
page 8). Check wire connections between the water valve, control
unit and electrode set. See chapter17.2, page 44 for location of
these parts.
9. Water heater in solar mode. If inlet water temperatures exceed the
water heater's set temperature, the burners will not ignite and the
solar mode indicator will show on the display. See fig. 42, page 25.
12.3

Water is too hot

1. Selected temperature on the unit is too high. To lower output
temperature, see chapter 10.4, page 26.
2. Clean inlet filter screen per chapter 11.1, page 29 to increase flow
through the heater.
6 720 644 956 (2016/05)
3. Inspect the water path for obstructions. Make sure all shower heads,
faucet aerators and whole house filters are clear of debris.
4. Confirm the heater's gas type coincides with the type of gas being
supplied. See fig. 2, page 8 for location of rating plate.
5. Avoid restrictive outlets. Clean all shower heads and faucet aerators.
It may be necessary to upgrade to higher flow rate shower heads if
allowable by local code.
6. In areas where the water has a high mineral content, periodic
descaling may necessary. See chapter11.3, page 30 for directions.
12.4

Water is not hot enough

1. Selected temperature on the unit is too low. To raise output
temperature, see chapter10.4, page 26.
2. Clean inlet filter screen. See chapter11.1, page 29 to increase flow
through the heater.
3. Inspect the water path for obstructions. Make sure all showerheads,
faucet aerators and whole house filters are clear of debris.
4. Confirm the heater's gas type coincides with the type of gas being
supplied. See fig. 2, page 8 for location of rating plate.
5. Check inlet gas particle screen for blockage at gas inlet connection on
bottom of unit.
6. Verify gas pressure is in accordance with specifications in
chapter8.6, page 23. A gas pressure reading is needed to proceed
further. Contact your original installer or a local certified gas
technician to obtain this reading.
7. Cold water is mixing into the hot water lines (plumbing crossover). A
plumbing crossover can unintentionally mix cold water with the hot
water leaving the heater. The end result is a cooler water temperature
than desired. To check for a plumbing crossover, shut off the cold
water supply to the water heater. Then open all of the hot water taps
served by the heater. Wait 10 minutes and check all taps for water
flow. There should be no water flowing. Any continuous flow of water,
small or large, indicates a crossover and must be corrected. Consult
a professional plumber for help in correcting a crossover. Failing
single lever faucet mixing valves are common causes of plumbing
crossovers.
12.5

Low water flow/pressure

1. Too many hot water applications are being used simultaneously or
too much flow is demanded. The water heater will effectively support
two 2.0-2.5 GPM shower heads simultaneously or multiple sink
applications. Greater draws will result in a water pressure drop and
reduced flow at taps.
2. Ensure that gas pressure is in accordance with specifications in
chapter8.6, page 23. A gas pressure reading is needed to proceed
further. Contact your original installer or a local certified gas
technician to obtain this reading. If gas pressure is inadequate, the
water heater will close its motorized water valve, reducing the hot
water flow rate in an attempt to reach the selected output
temperature.
3. If selected temperature on the unit is set too high for the demanded
flow rate, the water heater will close its motorized water valve,
reducing the hot water flow rate in an attempt to reach the selected
output temperature. Lowering the selected temperature will allow
the motorized water valve to open up for increased water flow rate.
4. Clean inlet filter screen per chapter11.1, page 29.
5. Inspect the water path for obstructions. Make sure all shower heads,
faucet aerators and whole house filters are clear of debris.
940 ESO

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