Viessmann SCU 123 User manual

Thank you for buying this product.
Read this manual carefully to get the best perfomance from this unit.
Please keep this manual carefully.
SCU 123
Solar control unit for the Solar-Divicon-HX
Connection
Operation and function
Commissioning and adjustment
Troubleshooting
Manual
en-US/CA
SCU 123
*49012560* 49012560

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SCU 123
Safety advice:
Please read the following information carefully before in-
stalling and operating the controller. In this way damage to
the solar system caused by wrong installation will be avo-
ided. Please make sure that the mounting is adapted to the
characteristics of the building, that the local regulations are
respected and is conform with the technical rules.
Description of symbols
Signal words describe the danger that may occur, when it
is not avoided.
Warning means that injury, possibly life-threatening injury,
can occur.
Attention means that damage to the appliance can occur.
Information about the product
Proper usage
The solar controller is designed for use in solar thermal
systems using the Solar-Divicon-HX in compliance with the
technical data specified in these instructions.
Improper use excludes all liability claims.
Note
Strong electromagnetic fields can impair the
function of the controller.
ÎMake sure the controller as well as the
system are not exposed to strong electro-
magnetic fields.
WARNING! Warnings are indicated with a
warning triangle!
They contain information on how
to avoid the danger described.
Note
Notes are indicated with an information
symbol.
Subject to technical change. Errors excepted.
Please pay attention to the following safety advice in order
to avoid danger and damage to people and property.
Instructions:
Attention should be paid to
- Valid national and local standards and regulations
- Respective valid standards and directives
Equipment to be installed and used in accordance with the
rules of the National Electrical Code (NEC) or with Cana-
dian Electrical Code (CEC), Part I.
These instructions are exclusively addressed to authorized
skilled personnel.
Only qualified electricians should carry out installation and
maintenance work.
Initial installation should be carried out by qualified per-
sonnel
ÎArrows indicate instruction steps that should be carried
out.
1 General
US
US
C
C
CU 72060171 01
UL 60730-1A:2002
CSA E60730.1:2002
Contents
1 General ................................................................ 2
2 Overview and technical data ............................. 3
2.1 System description ..................................................4
3 Operation and function ..................................... 6
3.1 Push buttons .............................................................6
3.2 System monitoring display .....................................6
3.3 Flashing codes ...........................................................7
4 Commissioning................................................... 8
5 Channel overview ............................................. 10
5.1 Display channels .................................................... 10
5.2 Adjustment channels ............................................ 12
5.3 Electrical connection............................................ 16
5.4 Data communication / Bus ................................. 17
6 Troubleshooting ................................................ 18
6.1 Various: .................................................................... 19
7 Accessories ........................................................ 21

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SCU 123
• System-monitoring-display
• 4 Pt1000 temperature sensors
• 2 semiconductor relays for pump speed control
• Energy metering
• VBus®
• User-friendly operation
Technical data
Housing: plastic, PC-ABS and PMMA
Protection type: IP 20 / EN 60529
Ambient temp.: 32 ... 104 °F [0 ... 40 °C]
Size: 6.8" × 4.3" × 1.9" [172 × 110 × 46 mm]
Mounting: integrated in the Solar-Divicon-HX station
Display: System screen for system visualization, 16-segment
display, 7-segment display, 8 symbols for system status and
operating control lamp
Operation: by 3 push buttons at the front of the housing
Functions: Differential temperature controller for solar
thermal systems with external heat exchanger. Optional
add-on system functions, such as: operating hours counter,
evacuated tube collector function, pump speed control,
energy metering.
Inputs:
for 4 Pt1000 temperature sensors
Outputs: 2 semiconductor relays
Bus: VBus®
Power supply: 100 ... 240 V~
Standby power consumption: < 1 W
Switching capacities:
R1: 1 (1) A 100 ... 240 V~
(semiconductor relay)
R2: 1 (1) A 100 ... 240 V~
(semiconductor relay)
2 Overview and technical data

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2.1 System description
Display channels
Channel Description Page
COL Collector temperature 10
TST Tank temperature 10
TPRI Flow temperature - primary loop 10
TSEC Return temperature - secondary loop 10
n1 % Pump speed - primary loop 10
n2 % Pump speed - secondary loop 10
hP1 Operating hours R1 11
hP2 Operating hours R2 11
kWh Heat quantity in kWh 11
MWh Heat quantity in MWh 11
Time Time 11
The SCU 123 controller is used in solar thermal systems
using the Solar-Divicon-HX with an external heat exchanger
which separates the solar loop from the DHW loop.
The controller calculates the temperature difference
between the collector sensor S1 and the upper DHW
store sensor S3. If this temperature difference reaches the
switch-on temperature difference, the primary loop pump
Sensors Description
S1 collector sensor
S2 secondary loop heat exchanger sensor
S3 upper tank sensor
S4 primary loop heat exchanger sensor
Relays Description
R1 primary loop pump
R2 secondary loop pump
is switched on. If the temperature difference falls below the
switch-off difference, the pump is switched off.
If the switch-on temperature difference between the upper
heat exchanger sensor S4 and the upper DHW store sensor
S3 is reached, the secondary loop pump will be switched on.
WARNING! Scald risk!
Risk of injury caused by very high
temperatures!
ÎInstall a DHW mixing valve to
prevent scalding!
S1
S2
R1
(P1)
R2
(P2)
DHW
BW
S4
S3 DHW
CW
DHW mixing valve

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SCU 123
Adjustment channels
Channel Factory setting Change to Description Page
DT1O 12 °Ra Switch-on temperature - primary loop 12
DT2O 12 °Ra Switch-on temperature - secondary loop 12
DTF 8 °Ra Switch-off temperature - secondary loop 12
TAR 140 °F Target temperature 12
TMN 2 min Minimum runtime 12
n1MN 30 % Minimum pump speed R1 12
n2MN 30 % Minimum pump speed R2 12
S MX 140 °F Maximum tank temperature 13
EM 270 °F Collector emergency shutdown temperature 13
OCN OFF Collector minimum limitation option 13
CMN 50 °F Minimum collector temperature 13
OCF OFF Antifreeze function 13
CFRO 39 °F Antifreeze switch-on temperature 13
CFRF 41 °F Antifreeze switch-off temperature 13
O TC OFF Evacuated tube collector function 14
TCST 07:00 Evacuated tube collector function starting time 14
TCEN 19:00 Evacuated tube collector function ending time 14
TCRU 30 s Evacuated tube collector function runtime 14
TCIN 30 min Evacuated tube collector function standstill interval 14
OHQM OFF Energy metering function 15
FMAX 1.6 gal/min Maximum flow rate 15
MEDT 1 Antifreeze type 15
MED% 45 % Antifreeze concentration 15
MAN1 Auto Operating mode R1 15
MAN2 Auto Operating mode R2 15
UNIT °F Temperature unit °C / °F 15
RESET Reset to factory setting 15
W… Version number

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The controller is operated via three push buttons below the
display.
Button 1 is used for scrolling forward through the indication
menu or to increase the adjustment values. Button 2 is used
for scrolling backward and reducing values. Button 3 is used
for selecting channels and confirming adjustments.
During normal operation, only the display channels are
shown.
ÎScroll through the display channels by pressing buttons
1 and 2
Accessing the adjustment channels:
ÎScroll down in the display menu and press button 1 for
approx. two seconds after you have reached the last
display item.
When an adjustment value is shown on the display,
is indicated to the right of the channel name.
ÎPress button 3 in order to access the adjustment mode
starts flashing.
ÎAdjust the value using buttons 1 and 2
ÎBriefly press button 3, permanently appears,the
adjusted value will be saved.
The system monitoring display consists of three blocks:
channel display, tool bar and system screen (active
system layout).
The channel display consists of two lines. The upper line is an
alpha-numeric 16-segment display (text display) for displaying
channel names and menu items. In the lower 7-segment
display, the channel values and the adjustment parameters
are displayed. Temperatures are either indicated in °F or
°C, whereas temperature differences are indicated in K or
°Ra respectively.
The additional symbols of the tool bar indicate the current
system status.
3 Operation and function
3.1 Push buttons
3.2 System monitoring display
Status standard flashing
An adjsutmen value is being changed -
SET mode
relay 1 active
relay 2 active
maximum tank temperature exceeded
tank emergency shutdown active +
collector emergency shutdown active
collector minimum limitation active
antifreeze function activated
antifreeze function active /
manual operation relay 1 ON +
manual operation relay 2 ON +
manual operation relay 1 / 2 OFF
sensor defective
channel display
system screen with symbols and tool bar
1
3
2
backward forward
SET
(selection / adjustment mode)
operating control lamp

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green: everything OK
red/green flashing: initialization phase / manual opera-
tion
red flashing: sensor fault (sensor symbol is
flashing quickly)
3.3 Flashing codes
LED flashing codes
System screen flashing codes • Pumps are flashing when the corresponding relay is switched
on
• Sensor symbols are flashing if the corresponding sensor
display channel is selected
• Sensors are flashing quickly in the case of a sensor fault
• Burner symbol is flashing if the backup heating is active

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4 Commissioning ÎEstablish the power supply
During a short initialization phase, the operating control
lamp flashes red and green.
When the controller is commissioned for the first time or
after a reset, it will run a commissioning menu. The com-
missioning menu leads the user through the most important
adjustment channels needed for operating the system.
Operating the commissioning menu:
ÎEnter the channel by pressing button 3
ÎThe symbol flashes.
ÎAdjust the value by pressing buttons 1 and 2
ÎSave the adjustment by pressing button 3 again
ÎThe symbol stops flashing.
ÎPress button 1 or 2 to switch to the next or previous
channel
1
23
(OK)
The three push buttons of the SCU 123 controller
The commissioning menu consists of the following channels:
1. Unit
ÎAdjust the unit in which temperatures and temperature
differences shall be displayed
2. Time
ÎAdjust the current time for the real time clock
The hours and minutes have to be adjusted separately, first
the hours, then the minutes. Time format is 24 hours, e. g.
15:00h = 3 pm.
3. Maximum tank temperature
ÎAdjust the maximum tank temperature
4. Switch-on difference of primary loop
ÎAdjust the switch-on difference
5. Switch-on difference of secondary loop
ÎAdjust the switch-on difference
6. Switch-off difference
ÎAdjust the switch-off difference

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8. Target temperature
ÎAdjust the target temperature
Now the controller is ready for operation with typical set-
tings to suit the selected system layout.
The settings made in the commissioning menu can be chan-
ged later on in the corresponding adjustment channels. Ad-
ditional functions and options can of course be individually
adjusted as well (see chap. 5.2).
Completing the commissioning menu
After the last channel of the commissioning menu has been
adjusted and confirmed, the controller asks for confirmation
of the adjustments.
ÎTo confirm the adjustments made in the commissioning
menu, press button 3

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Note:
The displayed values and adjustment channels
depend on the options and functions that have
been selected. Only values and adjustment chan-
nels available for the individual settings selected
will appear in the menu.
5 Channel overview
Indication of the collector temperature Indicates the current collector temperature.
COL:
Collector temperature
Display range:
-40 ... +500 °F [-40 ... +260 °C]
Indication of the tank temperature Indicates the current upper tank temperature.
TST:
Tank temperature
Display range:
-40 ... +500 °F
[-40 ... +260 °C]
Indication of the flow temperature - primary loop Indicates the current flow temperature of the primary loop
in the Solar-Divicon-HX station.
TPRI:
Flow temp. primary loop
Display range:
-40 ... +500 °F [-40 ... +260 °C]
Indication of the flow temperature - secondary loop Indicates the current flow temperature of the secondary
loop in the Solar-Divicon-HX station.
TSEC:
Flow temp. secondary loop
Display range:
-40 ... +500 °F [-40 ... +260 °C]
Indication of current pump speed Indicates the current pump speed of the corresponding
pump.
• n1 % : current pump speed pump 1 - primary loop
• n2 % : current pump speed pump 2 - seconday loop
n1 %, n2 %
Current pump speed
Display range: 30 ... 100 %
5.1 Display channels

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SCU 123
Operating hours counter The operating hours counter accumulates the operating
hours of the respective relay (h P1, h P2). Full hours are
displayed.
The accumulated operating hours can be set back to zero.
As soon as one operating hours channel is selected, the
symbol is displayed.
ÎIn order to access the RESET-mode of the counter, press
button 3 for approx. two seconds.
The display symbol will flash and the operating hours
will be set to 0.
ÎConfirm the reset with button 3 in order to finish the
reset.
In order to interrupt the RESET-process, do not press a
button for about five seconds. The display returns to the
display mode.
h P1, h P2 :
Operating hours counter
Display channel
kWh/MWh:
Heat quantity in kWh / MWh
Display channel
Indicates the energy gained in heat quantity – only available
if energy metering (OHQM) is activated.
The flow rate as well as the reference sensors S2 (heat
sink) and S4 (heat source) are used for calculating the heat
quantity supplied. It is shown in kWh in the channel kWh
and in MWh in the channel MWh. The overall heat quantity
results from the sum of both values.
The accumulated heat quantity can be set back to 0. As soon
as one of the display channels of the heat quantity is selected,
the symbol is permanently shown on the display.
ÎPress button 3 for about two seconds in order to access
the RESET mode of the counter.
The display symbol will flash and the heat quantity value
will be set to 0.
ÎIn order to finish this process, press button 3 to confirm.
In order to interrupt the RESET-process, do not press a
button for about five seconds. The display returns to the
display mode.
Heat quantity in kWh/MWh
TIME Indicates the current time.
ÎPress button 3 for two seconds to adjust the hours
ÎSet the hours by pressing buttons 1 and 2
ÎPress button 3 again to adjust the minutes
ÎSet the minutes by pressing buttons 1 and 2
ÎPress button 3 in order to save the adjustments

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5.2 Adjustment channels
∆T-regulation
DT1O / DT2O :
Switch-on temperature diff.
Adjustment range: 2.0 ... 40.0°Ra
[1.0 ... 20.0 K]
in steps of 1 °Ra [0.5 K]
Factory setting:
12.0°Ra [6.0 K]
DTF:
Switch-off temperature diff.
Adjustment range:
1.0 ... 39.0°Ra [0.5 ... 19.5 K]
in steps of 1 °Ra [0.5 K]
Factory setting: 8.0°Ra [4.0 K]
The controller works as a standard differential controller. It
calculates the temperature difference between the collec-
tor sensor S1 and the upper DHW store sensor S3. If this
temperature difference reaches the switch-on temperature
difference DT1O, the primary loop pump is switched on. If
the temperature difference falls below the switch-off diffe-
rence DTF, the pump is switched off.
If the switch-on temperature difference between the upper
heat exchanger sensor S4 and the upper DHW store sensor
S3 is reached (DT2O), the secondary loop pump will be
switched-on.
Solar loading will be carried out for the adjustable minimum
runtime TMN during which the switch-off condition will be
ignored in order to stabilize the system.
If the temperature difference between the upper heat ex-
changer sensor S4 and the lower heat exchanger sensor S2
falls below the switch-off temperature difference DTF both
loops will be switched off.
Note:
The switch-on temperature difference must be
at least 1 °Ra [0.5 K] higher than the switch-off
tempe rature difference.
A relative minimum pump speed can be allocated to the
outputs R1 and R2 via the adjustment channels n1MN and
n2MN
n1MN, n2MN:
Adjustment range:
30 ... 100
in steps of 5 %
Factory setting: 30
Minimum pump speed
TMN:
Minimum runtime
Adjustment range:
1 ... 10 min
in steps of 1 min
Factory setting: 2 min
TAR
Target temp.
Adjustment range:
70 ... 210 °F [20 ... 99 °C]
in steps of 2 °F [1 K]
Factory setting:140 °F [60 °C]
The pump speed is controlled dependant on the target
temperature (see above).
The primary and the secondary loop pumps will run at mini-
mum speed until the target temperature at S4 is reached. If
the target temperature is exceeded by 5 K, the speed of the
primary loop pump will increase by one step. This state will
be hold for 2 min even if the target temperature decreases.
If, during this time, the target temperature (S4) increases by
more than 5 K, the pump speed of the secondary pump will
as well increase by 1 step (10 %).
This state will be hold for 2 min each time the target tempe-
rature increases unless the temperature increases by 5 K. If
the temperature decreases, first the secondary pump speed
and then the primary pump speed will decrease by 1 step.
Pump speed control

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Maximum tank temperature Once the adjusted maximum temperature is exceeded,
the solar pump is switched off and further loading of the
tank is prevented to reduce scald risk or system damage. A
fixed hysteresis of 4 °Ra [2 K] is set for the maximum tank
temperature.
When the temperature at the corresponding sensor exceeds
the adjusted maximum tank temperature, the symbol is
shown on the display.
S MX:
Maximum tank temp.
Adjustment range:
40 ... 200 °F [4 ... 95 °C]
in steps of 2 °Ra [1 K]
Factory setting:140 °F [60 °C]
Collector temperature limitation
Emergency shutdown of the collector
If the adjusted collector emergency shutdown temperature
EM is exceeded, the controller switches off the solar pump
(R1) in order to protect the system against overheating
(collector emergency shutdown). A hysteresis of 20 °Ra
[10 K] is set for the collector temperature limitation. Whi-
le the collector is in emergency shutdown, (flashing) is
shown on the display.
EM:
Collector temperature limitation
Adjustment range:
170 ... 390 °F
[80 ... 200 °C]
in steps of 2 °Ra [1 K]
Factory setting: 270 °F [130 °C]
CMN:
Minimum collector temp.
Adjustment range:
50 ... 190 °F [10 ... 90 °C]
in steps of 1 °Ra [0.5 K]
Factory setting:
50 °F [10 °C]
Collector minimum limitation option
OCN:
Collector minimum limitation
Adjustment range: OFF / ON
Factory setting: OFF
If the collector minimum limitation option is activated, the
pump (R1) is only switched on if the adjustable collector
minimum temperature is exceeded. The minimum tempera-
ture prevents the pump from being switched on too often
at low collector temperatures. A fixed hysteresis of 10 °Ra
[5 °K] is set for this function
If the collector minimum limitation is active, (flashing) is
shown on the display.
Antifreeze option
OCF:
Antifreeze function
Adjustment range: OFF / ON
Factory setting: OFF
The antifreeze function activates both pumps at 100% when
the temperature falls below the adjusted temperature
CFRO. This will protect the fluid against freezing or coagu-
lating. If CFRF is exceeded, the pumps will be switched off.
When the antifreeze function is activated, is shown on
the display. If the antifreeze function is active, is flashing.
Note:
Since this function uses the limited heat quantity
of the tank, the antifreeze function should be
used in regions with few days of temperatures
around the freezing point.
The antifreeze function will be suppressed if
the tank temperature falls below 40 °F [5 °C]
in order to protect the tank from frost damage.
CFRO:
Antifreeze temperature on
Adjustment range:
-40.0 ... +48.0 °F
[-40.0 ... +9.0 °C]
in steps of 2 °Ra [0.5 K]
Factory setting: 39.0 °F [4.0 °C]
CFRF:
Antifreeze temperature off
Adjustment range:
-38.0 ... +50.0 °F
[-39.0 ... +10.0 °C]
in steps of 2 °Ra [0.5 K]
Factory setting: 41.0 °F [5.0 °C]

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SCU 123
Evacuated tube collector function
O TC:
Evacuated tube collector
function
Adjustment range: OFF / ON
Factory setting: OFF
TCEN:
Evacuated tubecollector function
ending time
Adjustment range:
00:30 ... 23:30
in steps of 00:15
Factory setting: 19:00
TCST:
Evacuated tube collector func-
tion
starting time
Adjustment range:
00:00 ... 23:00
in steps of 00:15
Factory setting: 07:00
TCRU:
Evacuated tube collector function
runtime
Adjustment range: 30 ... 500 s
in steps of 5 s
Factory setting: 30 s
TCIN:
Evacuated tube collector function
standstill interval
Adjustment range: 5 ... 30 min
in steps of 1 min
Factory setting: 30 min
This function helps overcome the disadvantages caused by
the non-ideal sensor position with some tube collectors.
This function operates within an adjusted time frame (be-
ginning at TCST and ending at TCEN). It activates the
collector loop pump for an adjustable runtime (TCRU)
between adjustable standstill intervals (TCIN) in order
to compensate for the delayed temperature measurement.
If the runtime TCRU is set to more than ten seconds, the
pump will be run at 100 % for the first ten seconds of the
runtime. For the remaining runtime, the pump will be run
at the adjusted minimum speed n1MN.
If the collector sensor is defective or the collector is blo-
cked, this function is suppressed or switched off.
The primary loop heat exchanger temperature limitation is
set to 212°F [100°C] to protect the system against over-
heating.
Heat exchanger temperature limitation
Heat exchanger antifreeze function The antifreeze function of the heat exchanger activates R2
(100%) if the heat exchanger temperature falls below 40°F
[5°C].

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Energy metering
OHQM:
Energy metering
Adjustment range: OFF / ON
Factory setting: OFF
If OHQM is activated, the heat quantity gained can be calcu-
lated and displayed. Energy metering is possible if a flowme-
ter is used. To enable energy metering, proceed as follows:
ÎRead the flow rate (gal/min) from the flowmeter at ma-
ximum pump speed and adjust it in the FMAX channel
ÎAdjust the heat transfer fluid and the concentration of
the antifreeze in the channels MEDT and MED%.
FMAX:
Flow rate in gal/min
Adjustment range:
0.0 ... 5.0 gal/min
in steps of 0.2 gal/min
Factory setting: 1.6 gal/min
MEDT:
Heat transfer fluid
Adjustment range: 0 ... 3
Factory setting: 1
MED%:
Antifreeze ratio
in Vol-% (MED% is hidden
when MEDT 0 or 3 is used.)
Adjustment range: 20 ... 70
in steps of 1 %
Factory setting: 45
Heat transfer fluid:
0 : Water
1 : Propylene glycol
2 : Ethylene glycol
3 : Tyfocor® LS / G-LS
For control and service work, the operating mode of the
controller can be manually adjusted. For this purpose, select
the adjustment value MAN1, MAN2 in which the following
adjustments can be made:
Operating mode
• MAN1 / MAN2
Operating mode
ON : relay on (flashing) + + /
Auto : relay in automatic operation
OFF : relay off (flashing) +
n1(2)LO: minimum speed
MAN1:
Operating mode R1
Adjustment range:
ON / Auto / OFF / n1LO
Factory setting: Auto
Note:
Always adjust the operating mode back to “Auto”
when the control and service work is completed.
Normal operation is not possible in manual mode.
MAN2:
Operating mode R2
Adjustment range:
ON / Auto / OFF / n2LO
Factory setting: Auto
Temperature unit
UNIT:
Temperature unit
Selection: °F, °C
Factory setting: °F
In this adjustment channel the temperature unit can be
chosen.
The unit can be switched between °C and °F during operation.
Reset
RESE:
If this item is selected, the controller will indicate the com-
missioning assistant and all prevous adjustment will be set
back to the factory settings.
Note:
When using Tyfocor® HTL, set MEDT to 1 and
MED% to 50.

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5.3 Electrical connection
ATTENTION! ESD damage!
Electrostatic discharge can lead to da-
mage to electronic components!
ÎTake care to discharge properly
before touching the inside of the
device. To do so, touch a grounded
surface such as a radiator or tap!
The controller is equipped with two semiconductor relays,
to which loads such as pumps, valves etc. can be connected:
Relay 1
18 = conductor R1
17 = neutral conductor N
13 = ground conductor
Relay 2
16 = conductor R2
15 = neutral conductor N
14 = ground conductor
The power supply is at the terminals:
19 = neutral conductor N
20 = conductor L
12 = ground terminal
The temperature sensors (S1 up to S4) are connected
to the following terminals with either polarity:
1 / 2 = Sensor 1 (collector sensor)
3 / 4 = Sensor 2 (secondary loop heat exchanger sensor)
5 / 6 = Sensor 3 (upper tank sensor)
7 / 8 = Sensor 4 (primary loop heat exchanger sensor)
The controller is mostly prewired in the factory. Sensors
S1 (collector temperature) and S3 (upper tank) must be
connected on site. Use appropriate 1812 or 1612 low vol-
tage sensor wire, and connect to terminals as shown below.
The following points only serve as information.
WARNING! Electric shock!
Opening the housing will expose live
parts!
ÎSwitch off power supply and discon-
nect the device from power supply
before opening the housing!
The station is connected to the power supply (100 ... 240 V~)
with the already mounted connection cable. Works on cur-
rent-carrying parts of the installation may only be carried out
by a licensed specialist firm. Relevant regulations and norms
must be observed.
Make sure that the hydronic parts of the installation are
appropriately grounded.
Note:
The power supply connection must be con-
nected to the common ground of the building, to
which the pipework of the solar thermal system
is connected!
Note:
When the display does not work, check the po-
wer supply of the controller. If the power supply
is not interrupted, the fuse in the controller is
defective.

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The controller is equipped with a VBus® for data transfer
with and energy supply to external modules. The connec-
tion is carried out at the terminals marked “VBus” (either
polarity). One or more VBus® modules can be connected
via this data bus, such as
• GA3 large display, SD3 smart display
• DFA comfort remote display
• AM1 alarm module
5.4 Data communication / Bus

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6 Troubleshooting
Operating control lamp
Warning symbols
Operating control lamp flashes red. On the display the
symbols and appear.
VBus® terminals
power supply
terminals
fuse
load terminals
sensor terminals ground
terminals
In the case of an error, a message is shown on the display
of the controller:
Sensor defect. An error code instead of
a temperature is displayed in the sensor
display channel.
- 88.8888.8
Cable broken.
Check cable.
Short loop. Check
cable.
Disconnected Pt1000 temperature sen-
sors can be checked with an ohmmeter. In
the following table, the resistance values
with the corresponding temperatures are
shown.
Operating control lamp off.
Check the power supply. Is it disconnected?
no
The fuse of the controller
could be blown. It can be
replaced after the front
cover has been removed
(spare fuse is enclosed in
the accessory bag).
yes
Check the supply line and
reconnect it.
°C °F Ω °C °F Ω
-10 14 961 55 131 1213
-5 23 980 60 140 1232
0 32 1000 65 149 1252
5 41 1019 70 158 1271
10 50 1039 75 167 1290
15 59 1058 80 176 1309
20 68 1078 85 185 1328
25 77 1097 90 194 1347
30 86 1117 95 203 1366
35 95 1136 100 212 1385
40 104 1155 105 221 1404
45 113 1175 110 230 1423
50 122 1194 115 239 1442
Resistance values of the Pt1000-sensors

© Viessmann 10187 SCU 223.monus.indd
19 |
SCU 123
6.1 Various:
Pump starts for a short moment, switches off, switches
on again, etc.
Temperature difference
at the controller too
small?
no yes
Wrong position of collec-
tor sensors?
yes
Change ∆Ton and ∆Toff
correspondingly. Problem
solved?
Mount the collector sen-
sor at solar flow (warmest
collector output); use sen-
sor well of the respective
collector.
Pump starts up very late The temperature difference between tank and collector
increases enormously during operation; the collector cir-
cuit cannot dissipate the heat.
Collector circuit pump
defective?
no yes
Heat exchanger calcified?
yes
Check / replace it
Decalcify it
no
Heat exchanger blocked?
yesno Clean it
Heat exchanger too
small?
yes Replace with correctly
sized one.
no
Plausibility control of
the option tube coll-
ector special function
Change ∆Ton and ∆Toff
correspondingly.
Switch-on temperature
difference ∆Ton to large?
no yes
Non-ideal position of the coll-
ector sensor (e.g. flatscrew
sensor instead of sensor in
sensor wells)?
no
Pump is overheated, but no heat transfer from the
collector to the tank, flow and return have the same tem-
perature; perhaps also air / gas bubbles in the lines.
Air in the system; in crease
the system pressure to at
least static primary pres-
sure plus 7. 25 psi (0.5 bar);
if necessary continue to
increase pressure; switch
the pump off and on for a
short time.
Air in the system?
no yes
Is the collector circuit
blocked at the dirt trap?
yes
Clean the dirt trap
Activate tube collector
function if necessary.
yes
o.k.
yes
o.k.

© Viessmann 10187 SCU 223.monus.indd
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SCU 123
Tanks cool down at night
Collector circuit pump
runs during the night?
no yes
Check controller:
Manual operation active?
Tube collector function
active? Tank cooling
or antifreeze function
active?
Collector temperature is
at night higher than the
outdoor temperature
no yes
Check the check valve in
the flow and the return
pipe with regard to the
functional efficiency.
Sufficient tank insulation?
yes no Increase insulation.
Insulation close enough
to the tank?
yes no Replace insulation or in-
crease it.
Are the tank connections
insulated?
yes no Insulate the connections.
Warm water outflow
upwards?
no yes
Change connection and
let the water flow hori-
zontally or through a si-
phon (downwards); less
tank losses now ?
Does the warm water
circulation run for a very
long time?
no yes
Use the circulation pump
with timer and switch-off
thermostat (energy effici-
ent circulation).
The solar circuit pump does not work, although the coll-
ector is considerably warmer than the tank.
Is the control lamp (LED)
illuminated?
yes no
Does the pump start up
in manual operation?
yes
There is no current; check
fuses / replace them and
check power supply.
The adjusted temperature
difference for starting the
pump is to high; choose a
value which makes more
sense.
no
Is the pump current
enabled by the control-
ler?
yes
Is the pump stuck?
Turn the pump shaft using
a screwdriver; now pas-
sable?
Pump is defective - re-
place it
Are the controller fuses
o.k. ?
Controller might be de-
fective - replace it or
contact the distributor.
no
yes
no
no yes
Replace fuses.
Circulation pump and
blocking valve should be
switched off for one night;
less tank losses?
yes no
Check whether the
pumps of the backup he-
ating circuit run at night;
check whether the non-
return valve is defective;
problem solved ?
no
no yes
o.k.
Control the non-return
valve in warm water cir-
culation - o.k.
yes no
Further pumps which are
connected to the solar
tank must also be che-
cked.
The thermosiphoning in
the circulation line is too
strong; insert a stronger
valve in the non-return
valve or an electrical
2-port valve behind the
circulation pump; the
2-port valve is open
when the pump is activa-
ted, otherwise it is closed;
connect pump and 2-port
valve electrically in paral-
lel; activate the circulation
again
Clean or replace it
a
a b
b
yes
o.k.