Bosch AE12 User manual

12 kW Electric Tankless Water Heater
Troubleshooting Guide
Models: AE12, RP12T, US12
Technical Service Bulletin
DANGER: ELECTRIC SHOCK
ELECTRICITY IS EXTREMELY DANGEROUS.
TAKE EXTRA PRECAUTIONS AND
DISCONNECT THIS PRODUCT FROM THE
ELECTRICAL SUPPLY BEFORE CLEANING,
SERVICING OR REMOVING THE COVER.
BOSCH ELECTRICAL APPLIANCES
SHOULD ONLY BE SERVICED BY A TRAINED
TECHNICIAN OR LICENSED ELECTRICIAN.
Bosch technical support is available at 866-330-2729.
Please document results from each troubleshooting step
prior to calling. Without this information documented, we
cannot make a determination as to what is wrong with your
water heater. This will delay a resolution.
Introduction
Who should perform the troubleshooting?
Anyone who has the qualifications to work safely with 240VAC. If you
do not possess the tools or the knowledge to work safely with
240VAC, contact a licensed professional.
What does this troubleshooting guide cover?
This guide covers every test we can advise you to perform on the
location, installation, and water heater. With ALL the information
from ALL the tests in this guide, it is possible to diagnose any water
heater of this type to a point where we can advise a repair or cover
warranty.
Required tools:
Adjustable wrench
Empty gallon container
Digital multi-meter
Phillips head screwdriver
Flat head screwdriver
Step 1 – Document installation details
Record information in Step 1 of the Building and Install
questionnaire on page 5.
DANGER: ELECTRIC SHOCK
FOR STEPS 2, 3 AND 4, SHUT OFF THE
CIRCUIT BREAKER POWERING THE
APPLIANCE, LOCK IT, AND VERIFY THAT
THERE IS NO VOLTAGE AT THE UNIT.
Step 2 – Water supply and water heater settings
Verify the heater is installed in one of the two positions (inlet/
outlet connection on bottom) shown in Fig. 1.
Hot Cold Hot Cold
Hot Cold
Installation #1 (front view)
Installation #2 (front view)
H
ot
C
old H
o
t
C
old
Ho
t
C
old
I
nstallation
#
1
(f
ront view
)
I
nstallation
#
2
(f
ront view
)
Figure 1

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|Troubleshooting Guide | AE12, RP12T, US12
Data subject to change without notice | Printed in the USA | BTC 721002308 A | 09.2013 Bosch Thermotechnology Corp.
Make sure the heater is not being supplied by preheated or
recirculated water.
Verify that the cold water supply is connected to the INLET
(marked in blue).
Check temperature knob setting (see Figure 2 below):
1. Remove the heater cover.
2. Make sure the temperature spindle is set all the way
clockwise.
Figure 2
Temperature Control Knob
3. Document your findings in the Building and Install
questionnaire on page 5.
Verify flow rate
1. Using a measured container (a ½ or 1 gallon container
works well), time how long it takes to fill it water from
the hot side of a faucet connected to the water heater.
Calculate the flow rate by taking the size of the container
and dividing it by the time it took then multiplying the
answer by 60. This will yield a gpm result.
Example:
Water collected (gallons)
X 60 = gallons per minute
Time to fill (seconds)
Flow rates below 0.6 gpm will not activate the water heater.
Flow rates above 2.0 gpm may cause insufficiently warm
water.
2. Document your findings in the Building and Installation
questionnaire on page 5.
Checking for plumbing crossovers:
1. Keep water supply off to the water heater using the
contractor provided isolation valves, isolating the hot water
side of the system.
Do not turn off the water supply to the whole house; only
to the water heater. If you do not have an isolation valve on
the water heater, you cannot perform this test.
2. Open ALL hot water taps connected to the water heater
and set all fixtures to hot only.
3. Allow some time (approx. 5 minutes) for water to stop
running and pipes to drain. If pipes are drained you should
be able to place a hand over end of faucet and feel no
pressure.
4. If any water continues to flow, you have found a crossover
and one of the following conditions could exist and must
be corrected:
―The most likely cause of this condition is that a mixing
valve has an internal leak allowing cold water to mix with
the hot. While this may not be causing the symptoms
at your water heater, it will certainly affect the ability
of the water heater to reach its maximum stated output
and can cause activation issues and temperature
fluctuations.
―While unlikely, a cold water pipe could be connected to
a hot water pipe. If you have had some plumbing work
done recently and the symptoms coincide with the work
done, you may want to contact the person that did the
work.
5. Close all fixtures and reopen water supply to the water
heater.
6. Document your findings in the Building and Installation
questionnaire on page 5.
Step 3 – Physical inspection
Visual Inspection
1. Thoroughly inspect the water heater’s internal
components. Note the following:
―Burn or scorch marks on the PCB (Printed Circuit Board)
―Any signs of melted or damaged wires
―Cracks in plastic components
―Alignment of components
―Screw tightness
2. Document your findings in the Water Heater questionnaire
on page 6.

Troubleshooting Guide | AE12, RP12T, US12 | 3
Data subject to change without notice | Printed in the USA | BTC 721002308 A | 09.2013 Bosch Thermotechnology Corp.
Checking the Thermal Cut Out:
1. Locate the thermal cut out switch (see Fig. 3).
Figure 3

Reset button on Thermal Cut Out
2. Press the button located in the center of the cut out
switch. If you feel it click (similar to a ball point pen), then
the thermal cut out was tripped.
NOTICE:
The thermal cut out trips because of another underlying
problem, such as scale, low flow with warm inlet water,
high temperature setting or preheated water. It is
important to find the issue before returning the heater to
service.
3. Document your findings in the Water Heater questionnaire
on page 6.
Step 4 – Checking continuity and resistance
Ensure the circuit breaker powering the appliance is still shut
off.
Check the thermal cut out switch for continuity.
1. Check for continuity between connections labeled A-B and
also C-D (see Fig. 4).
2. Document your findings in the Water Heater questionnaire
on page 6.
Figure 4
A
CD
B
Check the flow switch for continuity.
1. With no water flowing through the unit, check continuity
between the brown wire and the yellow wire on the circuit
board using 100 ohms scale (see Fig. 5). A stuck or dama-
ged flow switch will show a closed circuit without water
flow causing the TCO to trip.
2. Now with water flowing through the unit, check continuity
between the brown wire and the yellow wire on the circuit
board using 100 ohms scale (see Fig. 5).
Figure 5
3. Document your findings in the Water Heater questionnaire
on page 6.

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|Troubleshooting Guide | AE12, RP12T, US12
Data subject to change without notice | Printed in the USA | BTC 721002308 A | 09.2013 Bosch Thermotechnology Corp.
Check the resistance of the elements.
1. Ensure that the power to the unit is turned off.
2. Set your multimeter to an appropriate setting to be able to
test for resistance in the 10 ohms range.
3. Place the red probe on the steel bar as shown in Figure 6
below and keep it there.
Figure 6

4. Place the black probe down inside the two white plug
connectors shown in Figure 7, one after the other, and
write down the resistance readings of each. Ensure that
you make good contact with the probes to improve the
accuracy of your test result.
5. Document your findings in the Water Heater questionnaire,
page 6.
Figure 7
2
1
Check to see if either element is grounded to the water tubing.
1. Ensure power supply to the unit is off.
2. Set your multimeter to Mohms (2000K, 2M, or 20M).
3. Place one multimeter probe on the common steel bar that
joins the two element terminals together (Fig. 6) and touch
the other probe to the copper water tubing to see if you
have any continuity or resistance there.
4. Document your findings in the Water Heater questionnaire,
page 6.
Step 5 – Verify the power supply
The power will need to be turned on for this test, please use
caution. Check the power supply using an appropriate scale.
1. Measure the incoming voltage across L1 and L2 terminals
(see Fig. 8) to be sure the unit is receiving 240 VAC.
Figure 8
L1 and L2 Incoming power terminals
2. Record your results in the Water Heater questionnaire on
page 6.

Troubleshooting Guide | AE12, RP12T, US12 | 5
Data subject to change without notice | Printed in the USA | BTC 721002308 A | 09.2013 Bosch Thermotechnology Corp.
Questionnaires
Building and Installation:
STEP 1: Bosch Customer # (if known)
Owner’s Name?
Owner’s address?
Owner’s phone number?
Model and serial number? Model: Serial Number:
Where did you purchase this water heater?
Name of installer and phone number? Installer: Phone:
Date of installation? Date of Installation:____/____/____
Where in the building is this water heater installed?
Water supply and water pressure?
Municipal Well Other:_____________________
Water pressure:_______PSI
What is the water supply material to and from the heater?
Copper Plastic Stainless Other:___________
Flex
If plastic, does plastic piping connect directly to unit?
Yes No
When facing water heater, which side of the water heater
does the cold water pipe connect to? Left Right Top Bottom
Circuit breaker size and type Single pole Double pole
Amperage:________
What is the gauge of the wiring supplied to the unit? _______ AWG
STEP 2: Temperature Control Knob Setting: Low Middle High
Fixture(s) used for troubleshooting this water heater? Sink Shower Tub All Fixtures
Flow rate of fixture(s) used for troubleshooting? _______GPM
Plumbing crossover test results - crossover present? Yes No

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|Troubleshooting Guide | AE12, RP12T, US12
Data subject to change without notice | Printed in the USA | BTC 721002308 A | 09.2013 Bosch Thermotechnology Corp.
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: [phone]
Fax: [phone]
www.bosch-climate.us
Water Heater:
STEP 3: Are there any burned or melted wires in the unit? Yes No
Are there any signs of water leakage? Yes No
Is the thermal cut out tripped? Yes No
If the thermal cut out was tripped, have you reset it? Yes No
Are there any signs of water leakage? Yes No
STEP 4: Is there continuity across the flow switch when water is
running through the unit Yes No
Resistance of each element Element 1: ________ Ω
Element 2: ________ Ω
Are Either Of The Two Elements Grounded Yes No
Thermal Cut Out Continuity:
A-B Terminals: Yes No
C-D Terminals: Yes No
STEP 5: Voltage Supply: Voltage across L1 - L2: ________VAC
After completing this questionnaire, please have your
technician call us while still at the unit at 1-866-330-2729
for diagnosis and resolution or if it is more convenient,
please email the completed questionnaire to
ldy[email protected] and we will reply within one
business day.