Bosch GWH 635 ES Reference manual

Service Bulletin Booklet
For Bosch model water heaters
GWH 635 ES, GWH 635 ESO,
GWH 2400 E, GWH 2400 EO,
AQ250SX, AQ250SXO

Data subject to change without notice | Printed in the USA 4.2007 Bosch Water Heating
2
|TWH-G2-02 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Table of Contents
TWH-G2-02 Overcoming fluctuation temps 3
TWH-G2-03 Checking gas pressure 5
TWH-G2-04 Remote instruction 7
TWH-G2-05 Checking and replacing temperature sensors 9
TWH-G2-06 Winterizing and freeze prevention 11
TWH-G2-07 Control board diagnostics 13
TWH-G2-11 Unlocking stuck water valve 15
TWH-G2-12 Measuring and adjusting CO217
TWH-G2-13 Descaling 19
TWH-G2-22 Replacing heat exchanger 21
TWH-G2-23 Replacing and cleaning electrodes 23
TWH-G2-24 Replacing control unit 25
TWH-G2-25 Gas conversion 27
TWH-G2-26 Replacing water valve 29
TWH-G2-27 Attenuator installation 31
TWH-G2-28 Replacing gas valve 33
TWH-G2-29 Flue gas limiter retrofit 35

Service bulletin
Overcoming temperature fluctuations
TWH-G2-02 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
Temperature fluctuations from hot to cold during use are typi-
cally caused by a restriction in the hot water flow from the
tankless heater. This slows the flow within the tankless heater,
decreasing it below the activation flow rate, which shuts off the
burners. The end result is nothing but cold water coming out of
the outlet. Follow each step below before proceeding to the
next. After each step, test the hot water flow to see if it remains
constant without turning cold.
Tools needed:
Various wrenches
Container with known volume (like a quart container)
Clean faucet aerators and shower heads
1. Check for restrictions in plumbing outlets, which could limit hot
water flow and may contribute to heater deactivation. For sinks,
remove faucet aerator. Flush and clean screen and reinstall. For
showers, remove showerhead and flush. If plugged with mineral
deposits, clean according to manufacturer’s suggestions or
replace showerhead. (If showerhead is wand style/hand held,
corrugated tube connecting to head may be too restrictive. Use a
larger tube or replace with a normal shower head.)
Clean heater’s inlet filter screen
1. Inlet filter screen is located just inside the cold water inlet
(right) at the bottom of the unit.
2. Remove filter and clean. If filter is damaged, replace filter.
Check for plumbing crossover
NOTE: A plumbing crossover can be caused by a failed washer at a
single lever faucet, incorrect plumbing or a faulty mixing valve in
the plumbing. The crossover will create back pressure on the hot
water flow and prevent an adequate flow of water through the
heater.
1. Close the installer supplied cold water shut off valve (if none
installed, install before proceeding).
2. Open all hot water taps supplied by the heater. Wait 5 minutes
and check all taps. The water flow should come to a complete
stop at every tap.
3. Any water running is a sign of a plumbing crossover. Consult a
local plumber or service person for help. This condition must be
corrected before the heater can operate properly.
Confirm water pressure
Water pressure must stay above 30psi during heater operation.
For installation on a private well system with the use of a pres-
sure tank, the lowest pressure range setting recommended is
30-50 psi (2.07-3.45 bar). The use of a pressure reducing/
regulating valve directly after the pressure tank is an effective
way to maintain constant water pressure to the water heater.
Watts brand 25AUB-¾” or N35B-¾” pressure reducing/regulating
valvesor equivalent is suggested.
Check temperature balancing valves
Heater deactivated by temperature balancing valves. If the outlet
water temperature is set too high, the heater can produce
temperatures that are too hot. A temperature balance shower
valve will automatically mix in cold water to reduce such hot
water temperature. In the event of any temperature instability at
a fixture using a temperature balancing valve, refer to the valve
manufacturer for instructions on internal adjustment setting. An
adjustment should be made to minimize the amount of cold
water the valve is adding. Additionally, the temperature setting
on the heater can be lowered to prevent the temperature
balance valve from mixing in too much cold.
Confirm activation rate of the heater
1. Fully open one hot water tap.
2. Return to heater and shut cold water supply valve.
3. Slowly open cold water supply valve just until the burners ignite.
4. Return to hot water tap and measure flow rate by timing how
long it takes to fill a quart container. A fill time of 19 seconds
indicates a proper activation rate of 0.8 gallons per minute
(GPM).
5. Repeat steps 4 and 5 a few times to check the accuracy of the
test.
NOTE: A fill time less than 19 seconds indicates an activation rate
above the required 0.8 GPM and a possible problem within the
heater‘s water valve.

4 |TWH-G2-02 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Data subject to change without notice | Printed in the USA | 03.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Lower temperature
Use the controls on the front on the heater to lower the output
temperature. Most people use hot water in the range of 100-
110F.

Service bulletin
Measuring gas pressure
TWH-G2-03 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
In order to meet design capacities of this unit, the gas supply
must meet the design requirements.
It is important to size the gas line to meet the requirements of
ALL gas appliances. Please consult the National Fuel Gas Code
NFPA 54, or a locally licensed gas technician if you are unsure of
the gas line size.
While testing for adequate gas pressure, all gas appliances must
be operated at full capacity.
Tools needed:
Manometer
Philips head screw driver
Small flat head screw driver
Connecting Manometer
1. Shut off gas supply with installer supplied shut off valve.
2. Remove front cover and locate inlet gas pressure measuring tap
on gas valve.
Gas pressure measuring tap (left tapping)
3. Loosen screw inside the tap with a small flat head screw driver
(do not remove) and connect manometer tube on test tap (if
manometer fitting goes inside the tap, remove screw.)
Static Pressure Test
1. Turn gas supply back on.
2. Operate all other gas appliances on same gas piping system at
maximum output.
3. Gas pressures lower than 5” W.C. for Natural Gas or 11” W.C. for
Liquid Propane will result in insufficient degree rise to the hot
water being used and must be corrected.
Operating Pressure Test
NOTE: Reset any error codes with reset button first.
1. Put ON/OFF switch in the OFF position.
2. Press and hold the “Program” (M) button and turn ON/OFF
switch to ON position.
3. As soon as ‘188’ is displayed, release “Program” button and the
display should read ‘P2’.
Program mode
4. Press the ‘+’ button, until P1 appears.
5. Turn on a high volume of hot water flow (at least 6 gpm) and the
heater will ignite.
6. Gas pressures lower than 5” W.C. for Natural Gas or 11” W.C. for
Liquid Propane will result in insufficient degree rise to the hot
water being used and must be corrected.
7. Remove manometer from measuring tap and tighten screw

6
|TWH-G2-03 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Data subject to change without notice | Printed in the USA | 03.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Common causes of low gas pressure
Appliance gas connector is not at least ¾”.
Gas line was not sized large enough. Most often found in retrofit
applications.
The gas filter on the bottom of the unit is clogged with pipe
dope or other debris.
Gas meter or regulator is not set to deliver enough gas or is
defective. Have gas or propane supplier verify proper operation.

Service bulletin
Remote instruction
TWH-G2-04 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
The remote control initialization instructions vary based on the
version of control board installed in your heater. Follow these
instructions to determine the control board version in the water
heater and the corresponding manual to use.
Tools needed:
None
Control board version
1. Turn power switch on the water heater to the off position.
2. When turning power switch to the on position, note the first two
letters that appear on the display (either AF or SU).
3. Make note of the two letters that appeared.
Manual version
1. In the lower left hand corner of the remote control manual, there
are a series of numbers. Both manuals have the numbers: 6 720
606 990 followed by numbers in parantheses.
Manual version
6 720 606 990 (04.03) JS
2. The numbers in parantheses indicate the version of the manual.
Use the following table to determine which instruction manual to
use.
Instruction manual table
Software code Manual version
AF (05.11)
SU (04.03)

Data subject to change without notice | Printed in the USA | 03.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Service bulletin
8 | TWH-G2-04 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
TWH-G2-04 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO

Service bulletin
Checking and replacing temperature sensors
TWH-G2-05 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
The inlet and outlet temperature sensors work together to
ensure that the set-point temperature on the control board can
be reached accurately. Incorrectly placed sensors, corroded
sensor connections or defective sensors may result in tempera-
ture overshooting/undershooting, fluctuating temperatures and
in some cases a lock-out error code on the control board that
requires resetting. These error codes are typically A7, A9, E1,
E2, and in some cases E9.
Tools needed:
Multi-meter
Thermometer
Philips head screw driver
Emery cloth
Preparation
1. Remove the front cover of the water heater. For instructions on
removing the front cover, refer to the installation/owner’s
manual of the water heater.
2. Locate the temperature sensors clipped onto their respective
hot and cold ½" copper pipes inside the water heater. The hot
water temperature sensor should have two red wires connected
from the control board and should be located on the horizontal
pipe. The cold water temperature sensor is connected with two
blue wires and should be clipped to the diagonal pipe. See
below.
Hot sensor
Cold sensor
Hot senso
r
Co
l
d
se
n
sor
Temperature sensor location
3. If the either sensor is not in it’s respective location, move to the
proper location immediately. Ensure that the sensor is firmly
clipped and making good contact with the copper pipe.
Removing, cleaning and/or replacing temperature sensor
1. Remove the wire spade connections on the temperature sensor
in question.
2. Unclip temperature sensor from the copper pipe. Wipe the
inside of the sensor clean with a pencil eraser where it makes
contact with copper pipe. Wipe down the ½" copper pipes
where sensors are placed with a clean rag.
3. Clip the cleaned or new temperature sensors firmly onto the
pipe. (see Temperature sensor location figure) Make sure
sensor is making good contact with copper pipe.
4. Clean and polish spade connections on temperature sensors
with emery cloth. Reconnect wire connections to their respec-
tive sensor ensuring that the spade connectors are fully seated.
NOTE: There is no polarity between the sensor connections
or wires.
5. Repeat these steps for both sensors as needed.

10
|TWH-G2-05 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Data subject to change without notice | Printed in the USA | 1.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Testing temperature sensors
1. Turn the on/off switch on the water heater to off. Display should
be blank.
2. Remove the wire spade connections on both temperature
sensors.
3. Remove the hot water temperature sensor from the outlet pipe
and place this directly adjacent to the cold water sensor on the
inlet pipe inside the water heater. Make sure that both sensors
are seated firmly on the copper pipe.
4. Turn on a hot water fixture such as a sink and allow water to run
steadily through the water heater for 5 minutes. Using a thermo-
meter, measure the temperature of the water flowing out of this
tap. Continue running water and go back to the water heater.
5. Using a multi-meter set on kilo-ohms (kΩ), measure the
resistance between the two connections of each sensor and
record readings.
6. Refer to the table below for resistance readings in accordance
with water temperature readings. NOTE: Readings are approxi-
mate. If readings are inconsistent between sensors and/or do
not match approximate readings, replace the sensor with impro-
per readings. If sensor readings are accurate according to the
table but still give inconsistent temps or an error code, contact
Bosch Water Heating Technical Support at 800-642-3111.
Temperature sensor readings
Temperature Approximate resistance reading
32 °F 40 kΩ
55 °F 22.5 kΩ
65 °F 13.5 kΩ
100 °F 7.5 kΩ
120 °F 4.5 kΩ
140 °F 3.5 kΩ

Service bulletin
Winterizing and freeze prevention
TWH-G2-06 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
Please note that installation instructions state that the water
heater must not be installed in a location where it may be
exposed to freezing temperatures. If the heater must be left in a
space which is likely to experience freezing temperatures (less
than 36° F), all water must be drained from the heater. If precau-
tions are not taken, resulting damage will not be covered under
the warranty.
NOTE: Use of agents like anti-freeze is not recommended as they
may cause damage to the water heater’s internal components.
Tools needed:
Bucket
Adjustable wrench
Air compressor/compressed air
Preparation
1. Turn on/off switch on the water heater to the off (O) position
and unplug power supply cord. The display should be blank.
2. Shut off gas supply to heater.
3. Shut off the water supply to the water heater using installer
supplied shutoff valve.
4. Open hot water taps to drain and relieve pressure from the
plumbing system. If water continues to flow after 5 minutes, a
plumbing crossover is present and must be corrected before
proceeding.
5. Disconnect inlet and outlet water pipes from the water heater.
Place a small bucket underneath the water heater to catch
residual water remaining inside the water heater. Save washers
for future use.
6. Using an air compressor, blow short bursts of air through the
inlet water connection until there is no water present coming
through the outlet water connection of the heater.
7. Reconnect water fittings and return heater to service when
danger of freezing has past.

12
|TWH-G2-06 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Data subject to change without notice | Printed in the USA | 1.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]

Service bulletin
Control board diagnostics
TWH-G2-07 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
Control boards having software versions 3003 and newer are
equipped with full diagnostic features. Older model control
boards are not capable of running software version 3003 and
newer. Contact Bosch Water Heating at 1-800-642-3111 with
questions.
Tools needed:
None
Determining diagnostic mode availability
1. Turn on/off switch on water heater to off (O) position then back
to the on (I) position. Quickly observe the first set of letters on
the LCD display of control unit. If the heater displays the letters
“SU”, then diagnostics are not available with this software
version. If the heater displays the letters “AF”, then diagnostics
are available and you may proceed to the next step.
Accessing diagnostic mode
1. Turn on/off switch on water heater back to off (O) position.
2. Press and hold the program (P) button (M on older models)
while turning the on/off switch to the on (I) position. The
display will cycle through a start-up procedure including soft-
ware version.
3. Release the ‘P’ button when ‘188’ appears on the display. The
display should read ‘P2’ when the program button is released. If
not, repeat process.
4. Press the ‘+’ button on the control panel until the display reads
‘P4’. You are now in the diagnostic mode of the control board.
5. When the display reads ‘P4’, press the ‘P’ button once again and
the display should read ‘E’.
6. Use the ‘+’ and ‘-’ button on the control board to cycle through
different diagnostic modes available.
NOTE: Each diagnostic mode that appears on the LCD display
will have a number on the left and a letter on the right. If the
letter on the right is an upper case P, then refer to Software
Version 3003 of the diagnostics table on the back page. If the
letter on the right is a lower case “d”, then refer to Software
Version 3005 of the diagnostics table on the back page.
7. Once in the selected diagnostic mode of your choice, press the
‘P’ button to display the diagnostic information.
EXAMPLE(using software version 3005): To read the flow rate in
gallons/minute while the unit is flowing water, cycle to the ‘3d’
mode and press the ‘M’ button. A reading of 25 on the display
would indicate the heater is reading a flow rate of 2.5 gallons/
minute.
8. Once the information is obtained, press the ‘P’ button again to
return to the diagnostic mode menu and scroll to additional
diagnostic information.
9. To exit the diagnostic mode of the heater, use the ‘+’ or ‘-’
button until the display reads ‘E’.
10. Press the ‘P’ button once again and the display should read P4.
11. Turn the on/off switch off (O) and back on (I) again to return
heater to normal function.

14
|TWH-G2-07 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Data subject to change without notice | Printed in the USA | 1.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Software version 3003 diagnostic codes
E Entry/Exit into sub-modes
0P Set-point temperature
1P Inlet water temperature
2P Outlet water temperature
3P Water flow (liters/min)
4P Water flow (gallons/min)
5P Fan speed (Hz)
6P Burner power (%)
7P Maximum power (kW)
8P PWM cycle (signal to fan)
9P Appliance selection
1F Most recent error/failure
2F 2nd most recent error
3F 3rd most recent error
4F 4th most recent error
5F 5th most recent error
6F 6th most recent error
7F 7th most recent error
8F 8th most recent error
9F 9th most recent error
10F 10th most recent error
Software version 3005 diagnostic codes
E Entry/Exit into sub-modes
0d Set-point temperature
1d Inlet water temperature
2d Outlet water temperature
3d Water flow (gallons/min)
4d Gas type (LP or NA)
5d Fan speed (Hz)
6d Burner power (%)
7d Maximum power (kW)
8d PWM cycle (signal to fan)
9d Appliance selection
1F Most recent error/failure
2F 2nd most recent error
3F 3rd most recent error
4F 4th most recent error
5F 5th most recent error
6F 6th most recent error
7F 7th most recent error
8F 8th most recent error
9F 9th most recent error
10F 10th most recent error

Service bulletin
Unlocking stuck water valve
TWH-G2-11 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
If there is little or no flow coming out of the heater, the inlet
screen is clean and the unit is plumbed correctly, then the water
valve may be stuck closed. To cycle the water valve to the open
position, follow the steps below.
Tools needed:
Very small flat head screwdriver
Unlocking water valve
1. Turn the on/off button on the front of the heater to the off
position (to the left).
2. Locate the water valve inside the lower half of the heater where
the cold inlet water comes in. Locate the plug connection that
comes from the control board and plugs into a connection
coming from the water valve. This plug has 8 multi colored wires
coming into each connection.
3. Disconnect the multi colored wire connections coming from the
water valve.
4. On the connector coming from the control board and starting at
one side you will see a black wire, then a red wire, and then a
green wire (on older models, this wire was brown). Using a very
small screwdriver, push down on the silver tab to release and
disconnect the green wire (or brown for older models) from the
connector.
5. Leaving the green wire (or brown wire for older models) discon-
nected, plug the 2 connectors back together.
6. Turn the heater on (to the right). You should hear the water valve
opening up. Leave the heater on for 5-10 seconds and then shut
the heater off (to the left).
7. Unplug the wire connections and replace the green wire (brown
for older models) from where it was pulled out.
8. Plug the 2 connections back together and turn the heater on (to
the right) and the heater should be back in operation and the
water valve to its full open position.
9. Make sure inlet water is on, gas is on, and power is on and run a
hot water tap and the heater should be operating fine now.
10. If the problem persists, please contact Bosch technical support
at 800-642-3111 for further assistance.

Data subject to change without notice | Printed in the USA | 03.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Service bulletin
16 | TWH-G2-11 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
TWH-G2-04 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO

Service bulletin
Measuring and adjusting CO2
TWH-G2-12 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
NOTE: The CO2levels can only be adjusted by a certified gas techni-
cian with a calibrated combustion gas analyzer.
Tools needed:
Combustion gas analyzer
Flat head screw driver
Philips head screw driver
Measure gas pressure
One factor that may affect CO2levels is improper gas pressure.
Please see bulletin TWH-G2-03 for the procedure to measure gas
pressure. Correct any deficiencies in gas pressure before
proceeding.
Preparation
1. Turn ON/OFF switch to the OFF (O) position.
2. Remove brass flat head screw on the exhaust collar as seen
below.
Measuring port
3. Insert CO2 analyzer probe into the measuring port. The tip of
the probe should be in the center of the flue pipe (approx 1.5”
inserted).
4. While holding the Program (M) button, move the ON/OFF switch
to ON (I) position. As soon as ‘188’ flashes on the display,
release the program button. The display should now read P2.
Program mode
Measuring CO2
1. Open hot water taps to achieve a flow rate of at least 4 gallons
per minute. (1 tub and 2 sinks should be sufficient)
2. Record the P2 CO2reading below. (Analyzer reading may take
several minutes to stabilize)
P2 CO2reading: % CO2
3. Press the ‘+’ button until P1 appears. Unit will ramp up to high
fire and the water flow should increase.
4. Record the P1 CO2reading below.
P1 CO2reading: % CO2
5. Compare your readings to those found in the following table. If
CO2readings are off make adjustments as outlined in the next
section.
Proper CO2readings
Natural Gas LP Gas
Max input P1 9.7 ± 0.3 % 10.7 ± 0.3 %
Max input P2 9.5 ± 0.5 % 10.5 ± 0.5 %

Data subject to change without notice | Printed in the USA | 03.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]
Service bulletin
18 | TWH-G2-12 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Adjusting CO2
NOTE: When making adjustments with the front cover off, CO2levels
will be 0.3 - 0.5 % lower than with the front cover on. Make final
readings with the front cover installed.
NOTE: Adjusting P1 CO2levels will change the P2 CO2levels. Con-
firm the P1 value BEFORE adjusting the P2 level.
Adjusting P1 CO2level
1. Loosen yellow painted Philips screw and cover should rotate
down revealing a brass slotted screw.
Adjusting P1 CO2level
2. Adjusting the brass slotted screw counter-clockwise will raise P1
CO2 levels and clockwise will lower P1 CO2 levels.
Adjusting P2 CO2level
1. Remove yellow painted #40 Torx cover from the front of the gas
valve. A plastic #40 Torx screw will be revealed.
Adjusting P2 CO2level
2. Adjusting the plastic #40 Torx screw counter-clockwise will
lower P2 CO2 levels and clockwise will raise P2 CO2 levels.
NOTE: These screw adjustments are very sensitive and may take
several minutes to stabilize.
Final readings
1. Verify both P1 and P2 are within the ranges specified in the
table below. Continue adjusting the P1 and P2 levels as necess-
sary until CO2values are within the specified ranges.
Proper CO2readings
Natural Gas LP Gas
Max input P1 9.7 ± 0.3 % 10.7 ± 0.3 %
Min input P2 9.5 ± 0.5 % 10.5 ± 0.5 %
2. As a safety precaution, verify the Carbon Monoxide (CO) read-
ings on P1. CO levels should not exceed 300ppm (follow local
codes if more restrictive). If values exceed this limit, inspect
vent system and fincoils on the heat exchanger for blockage. To
access heat exchanger fincoil, refer to service bulletin TWH-G2-
22.
Proper CO readings
Natural Gas LP Gas
Max input P1 300ppm 300ppm
Returning to Service
1. Return slotted cover to original position.
2. Reinstall Torx cover.
3. Remove CO2analyzer probe and reinstall brass flathead screw in
exhaust collar.
4. Turn ON/OFF switch to the OFF (O) position and then back to
the ON (I)position.
5. Heater is ready for normal operation.

Service bulletin
Descaling
TWH-G2-13 | 2400E, 2400EO, 250SX, 250SXO, 635ES, 635ESO
Introduction
Periodic descaling may be necessary in areas with high mineral
content in the water. Scale buildup in the heat exchanger may
result in lower flow rates, error codes of A7 and E9 and boiling
sounds in the heat exchanger.
Tools needed:
Various plumbing wrenches
Bucket for descaling solution
Small circulator or pump
Descaling using a pump
1. Disconnect electrical supply from the water heater.
2. Shut off the water supply to the water heater using (installer
supplied) isolation or shutoff valves.
3. Open hot water taps to drain and relieve pressure from the
plumbing system.
4. Drain water from the unit’s heat exchanger by disconnecting
inlet and outlet water connections from the heater.
5. Connect a line (A) from the outlet of the circulating pump
(installer supplied) to the inlet water fitting on the water heater.
6. Connect another line (B) to the water outlet fitting on the water
heater. Route the other end of this line into a descaling reservoir.
7. Using a 3rd line (C) from the descaling reservoir, connect to the
inlet side of circulating pump. Verify that there is a filter on the
end of the line in the descaling reservoir.
8. Make sure all connections are “hand tight.”
9. Fill tank with descaling solution so both lines inside are submer-
sed. We recommend straight white vinegar. If using a commer-
cial descalant, refer to manufacturer’s instructions on dilution
with water.
11. Operate the circulating pump.
12. Make sure there are no leaks and the solution is flowing from the
descaling reservoir through the heater and returning to the
reservoir.
13. Run solution through the heater until the solution returning to
the descaling reservoir comes out clear. (Changing to a fresh
solution may be necessary during this process)
14. Disconnect all lines and drain all solution from heat exchanger.
Properly discard of solution.
15. Position a container below the hot water outlet and reconnect
cold water supply. Open cold water supply isolation valve and
flush heat exchanger with clean water.
16. Shut cold water isolation valve and reconnect hot water line to
the water heater.
17. Reconnect electrical supply to unit, open water isolation valves,
and return the unit to service.
A
B
C
Pump
Filter
Descaling diagram
A
B
C
Pum
p
Fi
l
te
r

20
|TWH-G2-13 | 2400E, 2400EO, 635ES, 635ESO, 250SX, 250SXO Service bulletin
Data subject to change without notice | Printed in the USA | 03.2007 Bosch Water Heating
BBT North America Corporation
Bosch Group
Bosch Water Heating
340 Mad River Park
Waitsfield, VT 05673
Telephone [phone]
Fax [phone]
[e-mail]