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Maintenance And Service; Troubleshooting; Burners Do Not Ignite When Hot Water Is Turned On; Water Is Too Hot - Bosch 250SX LP Manual

Temperature modulated with electronic ignition
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Maintenance and service

Maintenance and service
Warning: Always turn off the electrical
power supply, turn off the manual gas
valve and turn off the manual water
control valves whenever servicing.
The unit should be checked once a year by a gas
technician. If repairs are needed, the repairs should be
done by a gas technician
To remove front cover
B Remove plastic decals on front panel (Page 5, Fig.
B Loosen the two Philips head screws located behind
B Lift front cover panel upward and remove.
Systems and parts that should be checked at
least once a year
Reference diagrams on pages 29 and 30.
• Venting system
• Burners -see page 29 for observation window
• Manual operation of the pressure relief valve to insure
correct operation
• Periodic cleaning of the water filter screen, see Fig.
24, chapter 2.13.
• Flushing the heat exchanger with a descaling
solution if mineral build up is evident. Scale build up
will shorten the life of the water heater, descale heat
exchanger thoroughly and repeat annually depending
on mineral content of ground water.


The 250 SX burner is activated by a water flow valve.
Numerous water related problems can cause this water
valve to malfunction such as: Insufficient water flow
volume to activate the burners at its minimum flow
requirement; Dirt in the water flow valve causing it to
malfunction; Sediment build-up in faucet aerators, or
shower heads; Uneven pressures between cold and hot
(with single lever mix valves); and Plumbing crossovers.
These water flow related problems can cause the
heater to deliver less than its full output, or to fail to
ignite or to shut down completely.
Problems stated below in upper case and in bold face
should be investigated when there is a problem with the
performance of the water heater but there is NO error
code on the heater's LCD display or wireless remote
Any failures that result in a displayed error code are
referenced on page 26. A Functional Scheme of the
250 SX is detailed on page 28. If there is a problem
with the installation, venting or operation of the unit, the
heater will communicate the source by these error
• Cold incoming water connection made to
wrong side of heater
Make sure cold water inlet connection is plumbed to
the right side of heater when facing unit
• Water flow rate at hot water tap is too low
A minimum of 0.8 gallon/minute (3 l/m) is required to
activate the heater
• Cold water inlet filter screen is dirty
Remove water inlet filter screen and clean. The
screen is located at the inlet connection to the water
heater, disconnecting the main cold supply line at the
heater is required. Check and clean faucet aerator
screens too.
• Crossover in plumbing
The heater activates when there is sufficient water
flow through its water valve, a minimum of 0.8 gallon/
minute (3 l/m) is required. If there is a crossover in the
plumbing, the necessary hot water flow rate through
the unit may not be reached, even though the flow at
the hot water tap is sufficient. A plumbing crossover
can be caused by a failed washer at a single lever
faucet, incorrect plumbing or a faulty mixing valve in
the piping. The crossover will create a back pressure
on the water heater and prevent an adequate flow of
water through it. To confirm there is no crossover in
the plumbing, shut off the cold water supply feed to
the water heater. Individually open each of the hot
water taps. There should be no water flowing under
a constant pressure. If there is a constant flow of
water then the plumbing crossover must be
corrected before the heater can operate properly.


• Temperature selection too high
Lower temperature adjustment on heater or wireless
remote control accessory, see chapter 3.2.


• Temperature selection too low
Increase temperature adjustment on heater or
wireless remote control accessory, see chapter 3.2
• Btu input is too low due to inadequate gas line
See specifications in chapter 2.10 - 2.12. The 250
SX requires adequate gas pressure to reach desired
temperatures, to compensate for inadequate gas
supply or lack of gas pressure the 250 SX will
effectively reach the selected output temperature by
reducing the hot water flow rate. Its motorized water
valve makes this adjustment
• Cold water is mixing with the hot water outlet
6 720 607 057

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