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)

old H

old

nstallation

ront view

)

nstallation

ront view

)

Figure 1

|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

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.

|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

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

|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: 1-866-330-2729

Fax: 1-603-965-7581

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.

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