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Bosch 125B LP Installation And Operating Instructions Manual page 12

Automatic instantaneous type water heaters for use with natural and liquefied petroleum gas
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Although the filters can lengthen the cleaning intervals,
they can never completely prevent such clogging. In
consequence the gas jet issuing from the pilot orifice is
reduced and or the air mixture is reduced. The pilot flame
is weak and thus can no longer heat the thermocouple
sufficiently. For cleaning purposes, the air filter screen is
pulled off, washed and blown out. The pilot orifice has
likewise to be cleaned or exchanged.
4. Air in the Gas Line
Note: Normally this is a problem only at the time of initial
installation, after the pipes have been worked on, or after
a propane tank has been allowed to empty, or after the
heater has been shut down for a long time.
Bleed all the air trapped in the gas line. Because of the
very small pilot orifice (especially on LP gas models),
bleeding out all the air could take several minutes. Slide
the gas valve button to pilot position ( ) and depress this
button until all the air has escaped, and the gas has arrived.
During this process, press repeatedly on the piezo ignition
button until the pilot flame has ignited.
1. Pilot push button was not pushed in far enough or
was not held in long enough
Once pilot flame has been lighted. Hold button pushed in
for at least 15 seconds to give time for the pilot flame to
properly heat the tip of the thermocouple.
2. Pilot flame improperly aimed or is too weak so it is
not properly heating the tip of the thermocouple.
The Pilot flame should be a sharp blue flame and aimed
at the tip of the thermocouple so that it envelops 10 mm
(3/8 ") of the thermocouple tip. Pilot flame has to be properly
aimed at the thermocouple. See Fig 9.
3. Poor
Note: Electromagnet is part #8707201012 located on the
right side of the gas valve behind the piezo push-button
assembly. Check the tightness of the thermocouple
connection nut at the electromagnet: The Electro-magnet
connection is a large aluminum 17mm hex head nut. The
thermocouple end is a 5 mm brass nut which screws into
the 17 mm nut. Tighten the thermocouple nut snugly but
not too tight.
4. Poor circuit connections at the ECO. (Energy Cut-
Off overheat protection)
Oxidation or looseness of the ECO terminal connections
can result in millivolt current loss through the thermocouple
safety circuit. Clean terminals with very fine sand paper or
an eraser and reconnect ECO leads.
5. Faulty ECO (part #8707206040)
If cleaning the terminals attached to the ECO did not fix
the problem, connect a jumper wire between the two wires
and try to relight the pilot. If the pilot flame now remains
on, replace the ECO. If the flame still goes out when the
button is released, the ECO is not defective. Go to next
6. Faulty thermocouple (part #8747202083) or
electromagnet ) Unless these 2 parts are at least 8
to 10 years old, it is very unlikely that they are faulty.
Before testing, reconfirm that #2 is absolutely
correct, and that all connections are clean and
To test the thermocouple, disconnect the thermocouple lead
to the ECO. Insert a multi-meter probe into the thermocouple
lead and attach or hold the other lead to the metal gas
valve (DC common). Light the pilot flame and hold button,
meter reading should be 24 mVDC or more. If the reading
is 24 mVDC or more the thermocouple is good. To test the
electromagnet, re-connect the thermocouple lead to the
ECO, light pilot and hold button while taking a reading
between the ECO leads and the metal gas valve (DC
common). The reading should drop to 19 mVDC or less. If
it does not, replace the electromagnet.
1. Pilot is not on.
Light the pilot. See lighting instructions.
2. Gas valve button not positioned on Burner Position
Be sure pilot flame is on. Slide gas valve control to burner
position (
3. Cold incoming water connection made to wrong
side of heater
Make sure cold water inlet connection is on the right side
of heater when you are facing heater.
4. Water flow rate at hot water tap is too low.
Note: When the flow control is turned all the way clockwise,
the AquaStar models 125B require 1/2 gallon per minute
flow to activate the burners. This is a flow which would fill a
quart jar in 30 seconds. If the flow control knob is turned
fully counterclockwise, a flow rate of 1.1 gpm is required to
activate the burners.
5. Cold water inlet filter on heater is dirty.
Remove filter and clean. This screen filter is located at the
inlet side of the brass water valve (fig. 12, #28). Check and
clean faucet aerators too.
6. Crossover in household plumbing
The AquaStar burner activates when there is sufficient water
pressure drop in the AquaStar water valve assembly — i.e.
when a hot water faucet is opened. If there is a crossover in
the plumbing, the necessary pressure drop in the AquaStar
will be insufficient, or totally eliminated. A plumbing
crossover can be caused by a bad washer at a single lever
faucet or incorrect plumbing or a mixing valve in the line,
etc. which permits hot and cold water to mix in the
plumbing. The crossover will create a back pressure in the
system preventing the pressure drop in the Aquastar —
i.e. cold water is entering the water heater from both sides
and the burners will not come on. To confirm there is no
crossover in the plumbing, shut off the cold water supply to
the AquaStar. Open your hot water taps. There should not
be any water flowing. If there is water flowing, there is a
crossover in the plumbing. This is a plumbing problem, not
an Aquastar problem. Please contact your plumber.

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