Bosch GWH 425 EF User Manual page 17

The 425 EF pilot and burners are ignited 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
faucets; Plumbing cross overs. 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.
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Pilot does not spark with
water flow
1 Close water tap
2 Remove screw cap and
unscrew till sparking starts
3 Screw in turning one and a
half turns
Pilot always sparks
without water flow
1 Close water tap
2 Screw in adjusting screw
until it stops sparking
3 Continue to screw
turning one and a half
turns more
in
17