Bosch WH17 User manual

High kW Electric Tankless Water Heater
Troubleshooting Guide
Models: WH17, WH27, WH36 AE115, AE125, (FD108 and higher)
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.
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 local 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
Gallon container
Digital multi-meter
Phillips head screwdriver
For Leaks:
Complete Step 1 only and call Bosch Electric Technical
Support 1-866-330-2729. You will need a description
of the leak, its location and may be asked for digital
pictures.
Step 1 – Document installation details
Complete the “Building and Install” questionnaire.
Step 2 – Water supply and water heater settings
Check temperature knob setting:
1. Make sure the temperature knob is set all the way
clockwise as viewed from the bottom of the water heater.
Tip: The temperature knob setting dictates the output
water temperature: try testing the water temperature at
several settings.
Bottom of Heater
Bo
tt
o
m
o
f He
a
te
r
Figure 1
2. Document your findings in the Building and Installation
questionnaire.
Verify flow rate
1. Using a graduated 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.

2
|WH17, WH27, WH36 AE115, AE125, (FD108 and higher)
Data subject to change without notice | Printed in the USA | BTC 722002309 B | 05.2014 Bosch Thermotechnology Corp.
Example:
Water collected (gallons)
X 60 = gallons per minute
Time to fill (seconds)
2. Document your findings in the Building and Installation
questionnaire.
Flow rates below 0.6 gpm on 17kW/36kW units and 0.8 gpm
on 27kW units, will not activate the water heater.
Flow rates above 2.5 gpm on 17kW units may cause
insufficiently warm water.
If flow rates were below 0.6 gpm on 17kW/36kW units and 0.8
gpm on 27kW units, check the inlet water screen:
1. Shut the water supply off to the water heater and open a
hot water faucet to relieve the pressure.
2. Remove the cold water supply line from the bottom of the
water heater.
3. Remove the inlet water screen from the cold water
connection, inspect, and clean it as required.
Heater bottomHeater
b
ottom
Figure 2
4. Install the inlet screen and attach the water line in
accordance with standard plumbing practices. Take care to
not get any pipe sealant inside the unit.
5. Document your findings in the Building and Installation
questionnaire.
Checking for plumbing crossovers:
1. Keep water supply off to the water heater, 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 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 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.
Step 3 – Visual inspection
Visual Inspection
1. Thoroughly inspect the water heater’s internal
components. Note the following:
―Burn or scorch marks on the PCB control on the
lower right side
―Burn or scorch marks on the triac boards on top of each
module. 27/36kW units have 2 traic boards on each
module
―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.
Check the flow transducer (Fig. 3):
1. With the water running at least 1 gpm (gallon/minute),
make sure the flow transducer is turning.
Tip: Use a flashlight to illuminate the flow transducer and
have someone turn the water on and off. Because the flow
transducer spins so fast, it is more likely you will see the
movement as it first begins to spin and as it stops spinning.
2. Document your findings in the Water Heater questionnaire.

WH17, WH27, WH36 AE115, AE125, (FD108 and higher) | 3 |
Data subject to change without notice | Printed in the USA | BTC 722002309 B | 05.2014 Bosch Thermotechnology Corp.
Flow
Transducer
Flo
w
Tr
a
ns
du
cer
Figure 3
Step 4 – Verify the power supply
Check the power supply using an appropriate scale for 240 VAC.
1. Measure the incoming voltage:
―WH17 / AE-115 have two readings (Fig. 4)
―WH27 / WH36 / AE-125 have three readings (Fig. 5)
2. Check the pairing of the incoming power connections by
measuring the voltage of all combinations of L1 to L1 and
L2 to L2. Voltage between any of these pairs (L1-L1 or L2-
L2), indicates an installation error and must be resolved by
an electrician.
2 independent 40 amp double-pole
circuit breakers
17kW terminal block
To Supply 2
To Supply 1
Supply 1 Supply 2
2 in
d
epen
d
ent 40 amp
d
ou
bl
e-po
le
circ
u
it
b
re
a
ker
s
17kW termin
a
l
b
l
o
c
k
T
T
o Supp
l
y 2
T
o Supp
ly
1
S
S
upp
l
y 1
S
S
upp
ly
2
Figure 4
27kW - 3 independent 40 amp double-pole circuit breakers
36kW - 3 independent 60 amp double-pole circuit breakers
27/36kW terminal block
To Supply 1
To Supply 2
To Supply 3
Supply 2
Supply 1 Supply 3
2
7kW - 3 inde
p
endent 40 am
p
double-
p
ole circuit breaker
s
3
6kW - 3 inde
p
endent 60 am
p
double-
p
ole circuit breaker
s
27
/
36kW terminal bloc
k
T
T
o Supp
ly
1
T
T
o Supp
l
y 2
To
T
o Supp
l
y
3
S
S
uppl
y
2
Supp
ly
1
Supp
ly
3
Figure 5
3. Check that voltage is reaching the PCB by measuring the
voltage between the blue and brown power connections
on the board itself (Fig. 6).
Figure 6
PCB power connections
(brown / blue wires)
Heater bottom
4. Shut off the circuit breakers powering the appliance, lock
them, and verify that there is no voltage at the unit.
5. Document your findings in the Water Heater questionnaire.

4
|WH17, WH27, WH36 AE115, AE125, (FD108 and higher)
Data subject to change without notice | Printed in the USA | BTC 722002309 B | 05.2014 Bosch Thermotechnology Corp.
Step 5 – Troubleshoot the element assemblies
Verify that circuit breakers are off and there is no voltage at the
unit.
Measure resistance of element assemblies (Fig. 7):
—For 17kW models:
1. Using a 200 ohm range on a digital multi-meter, test all the
elements on left heating module.
―Outer to Outer (Brown wire with the black insulation
to left side of the TCO)
―Inner to Inner (Black wire with the black insulation to
left side of the TCO)
2. Using a 200 ohm range on a digital multi-meter, test all the
elements on left heating module.
―Outer to Outer (Brown wire with the black insulation
to left side of the TCO)
―Inner to Inner (Blue wire with the black insulation to
left side of the TCO)
—For 27kW and 36kW models:
1. Using a 200 ohm range on a digital multi-meter, test all the
elements on left heating module.
―Outer to Outer (Brown wire with the black insulation
to the bottom left side of the TCO)
―Inner to Inner (Blue wire with the black insulation to
the bottom left side of the TCO)
―Middle to Middle (Gray wire with the black insulation to
top left side of the TCO)
Outer
Middle
Outer
Middle
Inner
Inner
Rear of Heater
Front of Heater
Outer
Middle
Outer
Middle
Inner
Inner
Rear of Heater
Front of Heater
Center Module Top View
(36kW model only)
RIGHT MODULE Top View
6720647022-02.1V
Outer
Middle
Outer
Middle
er
Rear of Heater
Front of Heater
LEFT MODULE Top View
NOTE: on 17kW models, there is no middle element
O
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e
r
M
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C
ent
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Modu
l
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T
p
V
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ew
(
36kW model onl
y)
RI
GH
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M
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iew
6
72
06
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0
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0
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T
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N
OTE: on 17kW models
,
there
i
s no m
i
ddle element
Figure 6
2. Using a 200 ohm range on a digital multi-meter, test all the
elements on right heating module.
―Outer to Outer (Brown wire with the black insulation
to the bottom left side of the TCO)
―Inner to Inner (Black wire with the black insulation to
the bottom left side of the TCO)
―Middle to Middle (Gray wire with the black insulation to
top left side of the TCO)
—36kW model only:
1. Using a 200 ohm range on a digital multi-meter, test all the
elements on center heating module.
―Outer to Outer (Brown wire with the black insulation
to the bottom left side of the TCO)
―Inner to Inner (White wire with the black insulation to
the bottom left side of the TCO)
―Middle to Middle (Gray wire with the black insulation to
top left side of the TCO)
—All Models:
1. Using the 20 Mohm (Mega ohm) range on a digital multi-
meter, test each element tip to ground.
2. Record all readings along with where you encountered
them.
3. Using the 200 ohm range on a digital multi-meter, test the
resistances across the terminals of the TCOs (Thermal Cut
Outs).
Record your results in the Water Heater questionnaire.

WH17, WH27, WH36 AE115, AE125, (FD108 and higher) | 5 |
Data subject to change without notice | Printed in the USA | BTC 722002309 B | 05.2014 Bosch Thermotechnology Corp.
Questionnaires
Building and Installation:
STEP 1: 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? Municiple Well Water pressure:_______PSI
What is the water supply material?
Copper Plastic
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? Left side Right side
Circuit breaker size and quantity Single pole Double pole
Amperage:________ Quantity:________
STEP 2: To what position is the temperature knob set? 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
Inlet filter screen clean? Yes No
Plumbing crossover test results - crossover present? Yes No
Water Heater:
STEP 3: Are there any burn or scorch marks on the PCB? Yes No
Are there any cracked or melted wires in the unit? Yes No
Were each of the wires into the control PCB secure? Yes No
Are the blue and brown wires into the terminal blocks
secure? Yes No
Are there any damaged plastic components in the unit? Yes No
Is the flow transducer spinning? Yes No
STEP 4: Feed voltage:
Pair 1 - L1 to L2: _______VAC
Pair 2 - L1 to L2: _______VAC
Pair 3 - L1 to L2: _______VAC (27kW only)
Feed voltage polarity:
Pair 1 - L1 to L1: _______VAC L2 to L2: _______VAC
Pair 2 - L1 to L1: _______VAC L2 to L2: _______VAC
Pair 3 - L1 to L1: _______VAC L2 to L2: _______VAC(27/36kW only)
Voltage to PCB: ________VAC

6
|WH17, WH27, WH36 AE115, AE125, (FD108 and higher)
Data subject to change without notice | Printed in the USA | BTC 722002309 B | 05.2014 Bosch Thermotechnology Corp.
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: [phone]
Fax: [phone]
www.bosch-climate.us
Water Heater continued:
STEP 5: Right hand module element resistance
in ohms:
Outer to Outer:______Ω
Middle to Middle:______Ω(27/36kW only)
Inner to Inner ______Ω
Left hand module element resistance in ohms:
Outer to Outer:______Ω
Middle to Middle:______Ω(27/36kW only)
Inner to Inner ______Ω
Center module element resistance
in ohms (36kW model only):
Outer to Outer:______Ω
Middle to Middle:______Ω
Inner to Inner ______Ω
Are there any grounded elements? If so, which ones? Yes No
Thermal Cut Out readings: Right hand
module Top terminals________ΩBottom terminals________Ω
Thermal Cut Out readings: Left hand
module Top terminals________ΩBottom terminals________Ω
Thermal Cut Out readings: Center
module Top terminals________ΩBottom terminals________Ω
Notes:
After completing this questionnaire, please have your
technician call us while still at the unit at 1-800-798-8161
for diagnosis and resolution. If it is more convenient for
you, please email the completed questionnaire to
ldy.asa@ us.bosch.com and we will reply within one
business day.