Technical Service Bulletin
Low kW Electric Tankless Water Heater
Troubleshooting Guide
Models: AE3.4, AE7.2, AE9.5, RP1P, RP2P, RP3P, RP7P, RP9P, US3, US6, US7, US9
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.
Step 1 – Document installation details

DANGER: ELECTRIC SHOCK

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.
Record information in Step 1 of the Building and Install
questionnaire on page 5.
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 shown
in Fig. 1.
Figure 1
Installation #1 (front view)
Introduction
Hot
Cold
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
Installation #2 (front view)
Hot
Cold
Hot
Cold
2 | Troubleshooting Guide | AE3.4, AE7.2, AE9.5, RP1P, RP2P, RP3P, RP7P, RP9P, US3, US6, US7, US9

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).

Remove heater cover and check power setting by making sure
that the Power Selection Screw (see Fig. 2) is set all the way to
HI or LO and screwed down tight.

Verify flow rate
1.
Using a measured container (a ½ or 1 gallon container
works well), time how long it takes to fill it with water from
a hot water only faucet or shower. 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)
Figure 2
Time to fill (seconds)
X 60 = gallons per minute
See Table 1 below for required flow rates
2.

Document your findings in the Building and Installation
questionnaire, page 5.
Checking for plumbing crossovers:
1.
Shut 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.
Power selection screw
NOTICE:
If the Power Selector Screw is set to LO, only one of the
two heating elements will operate and the output will be
reduced by 50%.
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.
Expected Rise in Water Temperature
Model
Volts
Amps
Table 1
Element
Resistance
High
Setting
(kW)
Low
Setting
(kW)
Temperature Rise (°F) at Flow Rate (GPM)
0.5
GPM
0.75
GPM
1.0
GPM
1.5
GPM
2.0
GPM
2.5 GPM
US3, AE3.4, RP3P
120
27
2x8Ω
3.4 - 3.0
―
41 °F
28 °F
20 °F
N/A
N/A
N/A
RP2P
277
11
1 x 25 Ω
―
3.0
41 °F
28 °F
21 °F
14 °F
N/A
N/A
RP2P
277
22
2 x 25 Ω
6.0
―
81 °F
55 °F
42 °F
27 °F
N/A
N/A
US7, AE7.2, RP7P
240
15
1 x 16 Ω
―
3.6 - 3.05
N/A
32 °F
24 °F
16 °F
12 °F
N/A
US7, AE7.2, RP7P
240
30
2 x 16 Ω
7.2 - 6.1
―
N/A
63 °F
48 °F
32 °F
24 °F
N/A
US9, AE9.5, RP1P
208
18
1 x 12 Ω
―
3.5
N/A
32 °F
24 °F
16 °F
12 °F
N/A
US9, AE9.5, RP1P
208
35
2 x 12 Ω
7.1
―
N/A
64 °F
48 °F
32 °F
24 °F
N/A
US9, AE9.5, RP1P
240
20
1 x 12 Ω
―
4.75
N/A
42 °F
32 °F
21 °F
16 °F
N/A
US9, AE9.5, RP1P
240
40
2 x 12 Ω
9.75
―
N/A
84 °F
64 °F
42 °F
32 °F
N/A
RP9P
277
17.5
2 x 15.8 Ω
―
4.75
N/A
42 °F
32 °F
21 °F
16 °F
N/A
RP9P
277
35
1 x 15.8 Ω
9.75
―
N/A
84 °F
64 °F
42 °F
32 °F
N/A
Data subject to change without notice | Printed in the USA | BTC 721002307 A | 09.2013
Bosch Thermotechnology Corp.
Troubleshooting Guide | AE3.4, AE7.2, AE9.5, RP1P, RP2P, RP3P, RP7P, RP9P, US3, US6, US7, US9 | 3 |
4.
If any water continues to flow, you have found a crossover
and one of the following conditions could exist and must
be corrected:
Figure 3
― 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 may affect the ability of the water
heater to reach temperature and can cause activation
issues along with temperature fluctuations.
Direction of movement
― 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, page 5.
Flow switch disc

Check the Thermal Cut Out:
1.
Locate the thermal cut out switch (see Fig. 3).
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.
Step 3 – Physical inspection

Visual Inspection
1.
2.
Figure 4
Thoroughly inspect the water heater’s internal
components. Note the following:
― Any signs of melted or damaged wires
― Alignment of components
― Screw tightness
― Any signs of water leakage
Document your findings in the Water Heater questionnaire,
page 5.
Thermal Cutout Switch

Check the Flow Switch:
1.
Begin with no water running through the unit. Ensure
power is still off.
2.
Open a faucet to allow maximum water to flow through the
unit.
3.
If possible, use isolation valve on cold supply to close, then
open water supply to the unit. This will allow you to open
and close water flow while being in front of the heater to
observe. If an isolation valve is not installed, a second
person will be needed to open and close the faucet, while
another person is at the heater to observe the flow switch.
4.
Close, then open water flow through the unit. While
doing this, observe whether or not the flow switch
activates. When adequate water flows through the unit
the water pressure will push the flow switch disc against
microswitches (see Fig. 3). You should see that movement
and also hear a click.
Data subject to change without notice | Printed in the USA | BTC 721002307 A | 09.2013
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,
page 6.
Bosch Thermotechnology Corp.
4 | Troubleshooting Guide | AE3.4, AE7.2, AE9.5, RP1P, RP2P, RP3P, RP7P, RP9P, US3, US6, US7, US9
Step 4 – Checking continuity and resistance

Step 5 – Verify the power supply
Check the resistance of the elements (see Fig. 5) to be
sure they meet the requirements as listed in Table 1 of this
document.

Turn on circuit breaker supplying appliance and check the
power supply using an appropriate scale.
1.
Figure 5
Measure the incoming voltage across L1 and L2 terminals
(see Fig. 6) to be sure the unit is receiving the correct
voltage for the particular model# according to Table 1 on
page 2 of this document.
Copper water tubing
Figure 6
Bar
Two exposed element terminals
1.
2.
Ensure power supply to the unit is off.
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 and touch the
other probe to the copper water tubing to see if you have
any continuity there.
4.
Document your findings in the Water Heater questionnaire,
page 6.
5.
Set multimeter to 100 ohms scale. While keeping one
probe on the bar, use the other probe to touch each of the
two exposed element terminals in turn and write down
resistance readings of each.
6.
Document your findings in the Water Heater questionnaire,
page 6.
Data subject to change without notice | Printed in the USA | BTC 721002307 A | 09.2013
L1 and L2 Incoming
power terminals
2.
Record your results in the Water Heater questionnaire,
page 6.
Bosch Thermotechnology Corp.
Troubleshooting Guide | AE3.4, AE7.2, AE9.5, RP1P, RP2P, RP3P, RP7P, RP9P, US3, US6, US7, US9 | 5 |
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:
Name of installer and phone number?
Installer:
Phone:
Date of installation?
Date of Installation:____/____/____
Where did you purchase this water heater?
Where in the building is this water heater installed?
Water supply and water pressure?
Municipal Well
Copper
What is the water supply material to and from the heater?
Plastic Stainless
Flex
Left
Circuit breaker size and type
Single pole
Amperage:________
What is the gauge of the wiring supplied to the unit?
_______ AWG
Right
Top
Bottom
Double pole
Low
High
Fixture(s) used for troubleshooting this water heater?
Sink
Shower
Flow rate of fixture(s) used for troubleshooting?
_______GPM
Plumbing crossover test results - crossover present?
Yes
Data subject to change without notice | Printed in the USA | BTC 721002307 A | 09.2013
Other:___________
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?
STEP 2: Temperature Power Selector Screw setting:
Other:_____________________
Water pressure:_______PSI
Tub
All Fixtures
No
Bosch Thermotechnology Corp.
6 | Troubleshooting Guide | AE3.4, AE7.2, AE9.5, RP1P, RP2P, RP3P, RP7P, RP9P, US3, US6, US7, US9
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
Does the flow switch operate with water flow?
Yes
No
STEP 4 : Resistance of each element
Is there continuity between the bar and the copper water
tubing?
STEP 5: Voltage Supply:
Element 1: ________ Ω
Element 2: ________ Ω
Yes
No
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.asa@us.bosch.com and we will reply within one
business day.
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: 1-866-330-2729
Fax: 1-603-965-7581
www.bosch-climate.us
Data subject to change without notice | Printed in the USA | BTC 721002307 A | 09.2013
Bosch Thermotechnology Corp.