Strebel S-CB Px 120 User manual

STREBEL
S-CB Px 120
Wall hung high efficiency condensing boiler
Installation, Operating & Maintenance Manual
2015-04-21 v3

S-CB Px 120 Manual
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TABLE OF CONTENTS
INTRODUCTION………………………………………………………………………………………………….....5
1.!SAFETY GUIDELINES ............................................................................................................................... 7!
2.!TECHNICAL DATA OF THE STREBEL S-CB Px 120 BOILER................................................................ 9!
2.1.!FUNCTIONAL INTRODUCTION ............................................................................................................9!
2.2.!TECHNICAL SPECIFICATIONS DATASHEET ........................................................................................10!
3.!DIMENSIONS ........................................................................................................................................... 12!
4.!ACCESSORIES AND UNPACKING ........................................................................................................ 13!
4.1.!ACCESSORIES...............................................................................................................................13!
4.2.!UNPACKING ..................................................................................................................................13!
5.!INSTALLATION OF THE S-CB Px 120 ................................................................................................... 14!
5.1.!GENERAL NOTES ...........................................................................................................................14!
5.2.!MOUNTING THE BOILER ..................................................................................................................15!
6.!BOILER CONNECTIONS ......................................................................................................................... 16!
6.1.!GENERAL......................................................................................................................................16!
6.1.1.!Mounting ...........................................................................................................................16!
6.1.2.!Gas connection.................................................................................................................16!
6.2.!CONDENSATE DRAIN CONNECTION..................................................................................................17!
6.3.!FLOW AND RETURN CONNECTIONS..................................................................................................18!
6.4.!THE EXPANSION VESSEL ................................................................................................................18!
6.5.!PRESSURE RELIEF VALVE...............................................................................................................18!
6.6.!BYPASS ........................................................................................................................................18!
6.7.!PUMP FUNCTIONALITY....................................................................................................................18!
6.8.!FROST PROTECTION ......................................................................................................................19!
6.9.!INSTALLING A STRAINER AND/OR DIRT SEPARATOR...........................................................................19!
6.10.!WATER QUALITY ............................................................................................................................20!
6.11.!PLASTIC PIPING IN THE HEATING SYSTEM.........................................................................................20!
6.12.!AUTOMATIC AIR VENT.....................................................................................................................20!
6.13.!AUTOMATIC WATER FILLING SYSTEMS .............................................................................................20!
6.14.!WATER PRESSURE ........................................................................................................................21!
6.15.!CHEMICAL WATER TREATMENT .......................................................................................................21!
6.16.!UNDER FLOOR HEATING .................................................................................................................21!
6.17.!FLUSH THE SYSTEM WITH FRESH WATER .........................................................................................21!
6.18.!INSTALLATION EXAMPLES ...............................................................................................................22!
6.18.1.!Example of a low-resistance heating circuit......................................................................22!
6.18.2.!Example of a normal single boiler heating circuit (preferable)..........................................22!
6.18.3.!Example of a multiple boiler heating circuit ......................................................................23!
7.!HYDRAULIC GRAPH BOILER RESISTANCE (mWC)............................................................................ 24!
7.1.!HIGH EFFICIENCY PUMP .................................................................................................................24!
7.2.!PUMPS:ELECTRICAL POWER LIMITS ................................................................................................25!
8.!FLUE GAS DISCHARGE AND AIR SUPPLY SYSTEM .......................................................................... 26!
8.1.!GENERAL......................................................................................................................................26!
8.2.!AIR SUPPLY...................................................................................................................................26!
8.2.1.!Combustion air quality ......................................................................................................26!
8.2.2.!Air supply through humid areas ........................................................................................26!
8.3.!FLUE TERMINAL .............................................................................................................................26!
8.4.!PIPE HEIGHTS ON A FLAT ROOF.......................................................................................................27!
8.5.!B23P CERTIFIED ...........................................................................................................................27!
8.6.!C63 CERTIFIED .............................................................................................................................28!
8.7.!FLUE GAS OUTLET &COMBUSTION AIR INLET CALCULATION EXAMPLES ..............................................28!
8.7.1.!Maximum straight lengths – five typical examples............................................................29!
8.7.2.!Specific resistance calculation example of configuration B with given lengths.................29!
8.7.3.!Open version (B23, B23P, B33) .......................................................................................30!

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9.!ELECTRICAL INSTALLATION ................................................................................................................ 31!
9.1.!GENERAL......................................................................................................................................31!
9.2.!ELECTRICAL CONNECTIONS............................................................................................................31!
9.3.!FUNCTIONS OF THE CONNECTIONS .................................................................................................31!
9.4.!ELECTRICAL SCHEMATICS ..............................................................................................................33!
9.5.!SENSOR VALUES AND CONVERSION TABLES ....................................................................................35!
10.!USER INTERFACE................................................................................................................................... 36!
10.1.!CONTROL PANEL /DISPLAY UNIT .....................................................................................................36!
10.2.!CONTROL PANEL MENU STRUCTURE ...............................................................................................37!
10.3.!DISPLAY DURING OPERATION .........................................................................................................39!
10.4.!MONITOR SCREENS .......................................................................................................................40!
10.5.!SERVICE FUNCTION .......................................................................................................................42!
10.6.!SCHORNSTEINFEGER FUNCTION .....................................................................................................43!
10.7.!PROGRAMMING IN STANDBY MODE..................................................................................................44!
10.8.!SETTING THE TIME &DATE .............................................................................................................44!
10.9.!SET POINTS ..................................................................................................................................45!
10.10.!SETTING THE TIMER PROGRAMS .....................................................................................................46!
10.11.!SETTING THE OUTDOOR SPECIFICATIONS ........................................................................................49!
10.12.!CHECKING THE OPERATING HISTORY ..............................................................................................53!
10.13.!CHECKING THE FAULT HISTORY ......................................................................................................54!
10.14.!SETTING THE MAINTENANCE SPECIFICATIONS ..................................................................................55!
10.15.!SETTING THE USER LOCK ...............................................................................................................58!
10.16.!SETTING THE PARAMETERS WITH THE DISPLAY MENU .......................................................................59!
10.17.!FAULT CODES DISPLAY...................................................................................................................66!
10.17.1.!Lock-out codes .................................................................................................................66!
10.17.2.!Blocking codes..................................................................................................................68!
10.17.3.!Messages .........................................................................................................................69!
11.!CONTROLLING OPTIONS AND SETTINGS........................................................................................... 70!
11.1.!GENERAL......................................................................................................................................70!
11.1.1.!Extra boiler control............................................................................................................70!
11.1.2.!Max cooling time...............................................................................................................70!
11.1.3.!Temperature display on/off ...............................................................................................70!
11.1.4.!Water pressure .................................................................................................................70!
11.1.5.!Gas type selection ............................................................................................................70!
11.1.6.!Soft start option.................................................................................................................71!
11.1.7.!Pump mode (EC technology)............................................................................................71!
11.2.!HEATING.......................................................................................................................................72!
11.2.1.!Controlling behaviour settings ..........................................................................................72!
11.2.2.!Room thermostat on/off ....................................................................................................73!
11.2.3.!Room thermostat OPEN THERM .....................................................................................73!
11.2.4.!Outside temperature related flow control..........................................................................73!
11.2.5.!0-10 Vdc Remote flow temperature set point ...................................................................73!
11.2.6.!0-10 Vdc Remote burner input control..............................................................................74!
11.2.7.!Timer contact function ......................................................................................................74!
11.3.!INDIRECT HOT WATER /CALORIFIER ................................................................................................75!
11.3.1.!Pump and 3-way valve control..........................................................................................75!
11.3.2.!Tank thermostat................................................................................................................75!
11.3.3.!Tank sensor ......................................................................................................................75!
11.3.4.!Low/high flow temperature to tank coil .............................................................................76!
11.3.5.!Heating and hot water switching time ...............................................................................77!
11.3.6.!Heating and hot water switching at sudden temperature drop .........................................77!
11.3.7.!Anti-Legionnaires’ disease function (pasteurisation) ........................................................78!
11.4.!CASCADE CONTROL.......................................................................................................................79!
11.4.1.!Parameter settings for cascaded boilers ..........................................................................79!
11.4.2.!Monitor screens ................................................................................................................81!
11.4.3.!Output control and boiler sequence..................................................................................81!

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12.!COMMISSIONING THE BOILER ............................................................................................................. 82!
12.1.!FIRST:FLUSHING THE BOILER WITH WATER......................................................................................82!
12.2.!SECOND:FILLING &VENTING THE BOILER AND THE SYSTEM..............................................................82!
12.3.!THIRD:CHECK THE WATER FLOW ....................................................................................................82!
13.!STARTING THE BOILER ......................................................................................................................... 84!
13.1.!GENERAL......................................................................................................................................84!
13.2.!FIRING FOR THE FIRST TIME............................................................................................................84!
14.!ADJUSTING AND SETTING THE BURNER ........................................................................................... 85!
14.1.!SETTING AT THE MAXIMUM LOAD.....................................................................................................85!
14.2.!SETTING AT THE MINIMUM LOAD......................................................................................................86!
14.3.!GAS CONVERSION BUTANE/PROPANE (B/P).....................................................................................88!
15.!PUTTING THE BOILER OUT OF OPERATION....................................................................................... 89!
15.1.!OUT OF OPERATION:ON/OFF FUNCTION ..........................................................................................89!
15.2.!OUT OF OPERATION:POWER OFF....................................................................................................89!
16.!FAULT CODES AND BLOCKING CODES.............................................................................................. 90!
16.1.!FAULT CODES ...............................................................................................................................90!
16.2.!BLOCKING CODES..........................................................................................................................96!
16.3.!MAINTENANCE REMINDER FUNCTION...............................................................................................99!
17.!MAINTENANCE...................................................................................................................................... 100!
17.1.!GENERAL....................................................................................................................................100!
17.2.!ANNUAL INSPECTION &MAINTENANCE...........................................................................................100!
18.!USER INSTRUCTIONS .......................................................................................................................... 103!
19.!INSTALLATION EXAMPLES ................................................................................................................. 104!
20.!S-CB Px 120 SPARE PARTS OVERVIEW AND PART NUMBER LIST............................................... 110!
20.1.!SPARE PARTS 3D DRAWING .........................................................................................................110!
20.2.!S-CB PX 120 SPARE PARTS LIST WITH PART NUMBERS ..................................................................112!
21.!INDEX ..................................................................................................................................................... 114!

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INTRODUCTION
This manual is written for:
!The installer
!The system design engineer
!The service engineer
Please always check that you have the latest version of the manual, the latest version is available on our web site
www.strebel.co.uk . The version number of the manual is shown in the bottom right hand corner of the front cover.
abbreviations
NB
NOTICE
symbols
Warning: important information related to the safety
of persons and/or the appliance
All documentation produced by the manufacturer is subject to copyright law.
1. SAFETY GUIDELINES
Carefully read all the instructions before commencing installation.
Keep these instructions near the boiler for quick reference.
The appliance should be installed by a skilled installer such as GAS SAFE registered person, electrical work car-
ried out by a qualified person, all according to national and regional standards.
Failure to comply with these regulations could deem the warranty invalid. This appliance must be installed in ac-
cordance with the rules that apply and only be used in an adequately ventilated space conforming to standards in
place.
Without written approval of the manufacturer the internals of the boiler may not be changed. When changes are
executed without approval, the boiler certification becomes invalid.
Commissioning, maintenance and repair must be done by a skilled installer/engineer, according to all applicable
standards and regulations.
What should one do when there is the smell of gas:
!Don't use any electrical equipment.
!Don't press any switches.
!Close the gas supply.
!Ventilate the room (open the windows and/or outside boiler room doors).
!Immediately warn the responsible person.
The manufacturer / supplier is not liable for any damage caused by inaccurately following these mount-
ing instructions. Only original parts may be used when carrying out any repair or service work.
This appliance is not intended for use by persons (including children) with reduced physical, sensory or
mental capabilities, or lack of experience and knowledge, unless they have been given supervision or
instruction concerning use of the appliance by a person responsible for their safety. Children should be
supervised to ensure that they do not play with the appliance.

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These instructions are written for the installer of Strebel products and contain all necessary information on the
installation and adjustment of S-CB Px 120 boilers. Please read these instructions fully before installation to en-
sure that all work is carried out correctly.
We suggest that you keep a copy of these instructions near the boiler.
These instructions together with those in any supplemental instruction booklet cover the basic principles to en-
sure the satisfactory installation of the boiler, although detail may need slight modification to suit particular local
site conditions.
It is the law that all gas appliances and fittings are installed by a competent person (such as a Gas Safe regis-
tered installer) and in accordance with The Gas Safety (installation and Use) Regulations.
The relevant British standards for installation, codes of practice or rules in force and in accordance with the Manu-
factures’ instructions.
The installation must be carried out in accordance with the following regulations plus relevant codes & standards:
- Due consideration must be given to current Health & Safety Legislation while this product is being in-
stalled.
- Key Approved Documents to the Building Regulations, in the region of the United Kingdom that this prod-
uct is being installed.
- The Local Building Regulations and Local water by-laws, the gas services area and the Local Authority
recommendations.
- The appropriate British & European Standards for the type of installation and fuel used, including but not
exclusively the following standards: -
oBS 5440: Parts 2 (Flues and Ventilation).
oBS 6644: Installation of gas-fired hot water boilers of rated inputs between 70kW & 1.8MW
- The clean air act as defined by the local authority.
- The appropriate documents as produced by the Institution of Gas Engineers and Managers Documents
(IGEM) for the type of installation and fuel used, including but not exclusively the following documents: -
oIGE/UP1
oIGE/UP2
oIGE/UP4
oIGE/UP10
- If the product is being fuelled with LPG then the appropriate UKLPG Codes of Practice (CoP) should be
referred to.
- Wiring to the appliance must be in accordance with the IEE (BS 7671) Wiring Regulations the Health and
Safety Document No 635 “The Electricity at Work Regulations 1989” and any local regulations that apply.
- CP 342: Part 2, 1994. Code of practice for centralised hot water supply – buildings other than individual
dwellings.
- CIBSE, Guides A, B and C.
Adhere to all regulations that are in force at the time of installation or service.

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2. TECHNICAL DATA OF THE Strebel S-CB Px 120 BOILER
2.1. Functional introduction
The S-CB Px boilers are central heating boilers with a maximum high efficiency. Such a performance can be
reached by, amongst other things, using a special heat exchanger made of stainless steel. The heat exchanger
allows the flue gases to cool down below the condensation point, releasing extra heat. This immediately improves
the efficiency considerably.
The S-CB Px 120 boiler is standard set for Natural Gas G20
Gases used must meet the European standard EN 437.
Fuel used should have sulphur rates according to the European standard, a maximum annual peak over a short
period of time of 150 mg/m3and an annual average of 30 mg/m3.
Boiler control includes:
!Cascade control for up to twelve boilers
!Remote operation and heat demand indication from each boiler
!Weather compensation control
!Calorifier control
Connections for:
!Boiler pump
!0-10 VDC remote flow temperature (set point) control
!0-10 VDC remote burner input control
!Outside temperature sensor
!External calorifier pump or diverter valve
Cascade control
When using the integrated cascade control, a maximum of twelve boilers can be controlled in a cascade configu-
ration. By the use of an appropriate external control, this number may be increased at will.
0-10 VDC connection available
The boiler flow temperature or power input can be controlled by an external 0-10 VDC signal. When a number of
boilers are cascaded, the signal should be directed to the master boiler only. A signal of 1,48 Volt will switch on
the boiler(s), less than 1,4 Volt will switch off the boiler(s).
Time program
For both central heating and hot water function of the boiler, time programs with three programmable periods per
day are available. These time programs are set and activated by entering the desired settings directly at the boiler
control panel.

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2.2. Technical specifications datasheet
GENERAL
Product Identification Number
-
CE 0063 BP3254
Classification
-
II2H3B/P
Gas Appliance Type
-
B23, B23P; C13, C33, C43, C53, C63, C83
Type boiler
-
S-CB Px 120
Water content est.
Litre
8,3
Weight (empty)
kg
78
Dimensions (h x w x d)
mm
842 x 476 x 486
Flow/return connection (boiler)
inch
R 1"
Gas connection
inch
R ¾"
Parallel connection
mm
Ø 100-100
CENTRAL HEATING [EN437] Values min-max:
Nominal input (Net)
kW
26,0 - 111
Nominal input (Hs) (G20 G25)
kW
28,9 - 123
Nominal input (Hs) (G31) 1
kW
28,3 - 121
Nominal input (Hs) (G30) 1
kW
34,7 - 120
Nom. output 80/60
kW
24,7 - 106
Nom. output 50/30
kW
27,2 - 116
Nom. output 37/30
kW
28,1 - 120
Efficiency 40/30ºC DIN 4702-8
%
up to 110,6 %
GAS CONSUMPTION [EN437] Values min-max:
amounts
Natural gas G25
m³st/h
3,20 - 13,7
Natural gas G20
m³st/h
2,75 - 11,8
Propane gas G31 1
m³st/h
1,06 - 4,54
Butane gas G30 1(B/P)
m³st/h
0,99 - 3,44
Supply pressure
nom. 2
G25
mbar
25
G20
mbar
20
G31 1
mbar
30/37
G30 1
mbar
50
NOTES
1When using propane or butane, a special air restrictor kit is needed. See accessories list on page
13 and picture page 86.
2Below a table is given in which the min. and max. gas supply pressures are mentioned according
to EN437
p nominal [mbar]
p min [mbar]
p max [mbar]
G25
25
20
30
G20
20
17
25
G31
30
25
35
37
25
45
G30
50
43
57

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FLUE SYSTEM min-max:
CO2flue gas 3
G25/G20
%
8,5 -9,0
G31 1
%
9,6 - 10,3
G30 (B/P) 1
%
9,6 - 10,4
NOx class [EN483 / EN15420]
-
5
NOx emission at 0% O2,year emission
ppm
22,2
mg/kWh
39,9
Temp. flue gas at combustion air temp.
20°C
°C
≈85-95
Available pressure for the flue system 4
Pa
160
INSTALLATION
Max. flow temperature
°C
90
Hydraulic resistance of the boiler at ΔT =
20 K
Head mWC
3,8
Hydraulic resistance of the boiler at ΔT =
25 K
Head mWC
2,4
Pressure CH-system min./max. 5
bar
1-4
ELECTRIC
Power supply
V / Hz
230 / 50
Power consumption
W
150
Protection class
-
IPX4D
NOTES
3CO2of the unit measured/set without the boiler door in place
4 Maximum combined resistance of flue gas and air supply piping at high fire
5 With a pressure switch instead of the standard sensor, pressure may go up to 6,0 bar

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3. DIMENSIONS
Combustion air inlet connection
(hole with sealing ring and pipe support bracket)
Flue gas outlet
Condensate discharge.
Flexible hose Ø25/21 x 800 mm
Gas supply connection
Circulation flow connection
Circulation return connection
(Circulation pump side)
461
237
118
311
91
Ø
10
0
495
468
842
65
R 1" (2x) R¾"
65
76
119
72 235 77
Ø100
176 828 135

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4. ACCESSORIES AND UNPACKING
4.1. Accessories
Depending on the selected controlling behaviour for the central heating system and/or the optional use of a calo-
rifier, the following items can be supplied with the boiler. Ask your supplier for the specifications.
Item
Part no.
Outside (air) temperature sensor: 12kOhm@25°C (to be mounted to the boiler connections)
E04.016.585
External flow temperature sensor for behind the low loss header: 10kOhm@25°C (to be
mounted to the boiler connections)
E04.016.304
Calorifier temperature sensor: 10kOhm@25°C (to be mounted to the boiler connections)
E04.016.303
Conversion set for parallel to concentric flue-air terminal (100-100 →100/150)
E61.001.186
Software + interface cable for programming the boiler with a computer (laptop)
S04.016.586
Propane / butane air restrictor kit
S05.000.149
4.2. Unpacking
The S-CB Px 120 boiler will be supplied with the following documents and accessories:
•One “Unpacking instructions” document.
•One “Mounting, user and service instructions” manual.
•One suspension bracket.
•Siphon with condensate discharge hose.
•One spare fuse and two burner door spare nuts, attached to the front of the gas valve.
After delivery, always check the boiler package to see if it is complete and without any defects. Report any imper-
fections immediately to your supplier.

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5. INSTALLATION OF THE S-CB Px 120
5.1. General notes
At both sides of the boiler at least 50 mm of clearance should be applied to walls or wall units, 350 mm above the
top side of the boiler and enough distance from the bottom side of the boiler, so that the pump can be mounted
here.
The installation area/room must have the following provisions:
•230 V - 50 Hz power source socket with earth connection.
•Open connection to the sewer system for draining condensing water.
•A sound-deadening wall.
•The wall used for mounting the boiler must be able to hold the weight of the boiler.
Other considerations related to the boiler location.
•The ventilation of the boiler room must meet all applicable standards and regulations, regardless of the se-
lected supply of fresh air to the boiler location.
•Both the air supply and the flue gas tubes must be connected to the outside wall and/or the outside roof.
•The installation area must be dry and frost-free.
•The boiler has a built-in fan that will generate noise, depending on the total heat demand. The boiler location
should minimise any disturbance this might cause. Preferably mount the boiler on a brick wall.
•There must be sufficient lighting available in the boiler room to work safely on the boiler.
•When a boiler is positioned at the highest point of the installation, the supply and return pipes must first pro-
trude 0.5 m above the top of the boiler, before these pipes go to the installation side. In other words, the water
level must always be 0.5 meter above the top of the boiler and an automatic air vent must be installed in the
supply or return pipe. A low-water level protection should also be installed at the installation side.
•Notice the positioning of electrical components in relation to the temperature sensitivity.
•Make sure there is an open connection with the sewer to drain the condensate. This connection should be
lower than the condensate drain level of the boiler:
←Apply an open connection to the sewer
The boiler must be positioned and installed by a skilled installer in accordance with all applicable standards and
regulations. Commissioning the boiler must be done by a skilled service/commissioning engineer, who is trained
for this type of boiler.

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5.2. Mounting the boiler
Before mounting and installing the boiler the following connections should be considered:
•Flue gas system and the flue gas pipe connections
•Air supply system and connections
•Flow and return pipe connection and pump position
•Condensate and pressure relief valve drainage
•Power supply (preferably the power connection positioned above the boiler)
All lines/piping must be mounted free of tension. The weight of all the installation components
should be supported separately from the boiler so there will be no standing force on the connec-
tions.
→Pay attention to this while choosing the mounting position of the boiler.
Determine the position of the flow and return pipes by using the included suspension bracket or a suspension
frame (when supplied).
While marking the holes, ensure that the suspension bracket or frame is perpendicular and the boiler does not
lean forward. If necessary adjust the position with the adjusting bolts at the lower rear side of the back panel (see
drawing).
When the adjusting bolts aren’t sufficient, fill the gap behind the bolts to get the boiler in position. The exact boiler
position lies between the boiler hanging level and hanging slightly backwards. The boiler should not lean forward
in the mounted position.
Boiler suspension
Detail boiler suspension
3. Lock boiler with locking plate
and two bolts
2. Suspend boiler with suspension
bracket on mounting bracket
4. Level boiler using
adjusting bolts
1. Attach mounting bracket to wall
with inclined side facing upwards

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6. BOILER CONNECTIONS
6.1. General
1 - Flow CH
2 - Return CH
3 - Siphon cleaning point
4 - Condensate drain
5 - Gas
6.1.1. MOUNTING
When mounting the connections, always make
sure that no extra tension, forces and/or tor-
ques are applied to the connecting pipes
and/or the boiler and the boiler suspension.
Keep the connections fixed in place, as shown
in the picture, by using two wrenches.
6.1.2. GAS CONNECTION
Always install a manual shutter valve in the gas supply line, directly underneath the boiler.
NOTICE: This valve is NOT delivered with the boiler.
2
1
5
4
3

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6.2. Condensate drain connection
The condensate drain has a 19mm flexible discharge hose. The siphon must always be filled with water. As a
safety measure, the siphon has been provided with a floating ball, which closes the outlet in case of water ab-
sence, preventing large flue gas leakage.
Use only condensate resistant materials for the external condensate drainage system. Blockage of this drain
might damage the boiler. The drain connection is correct when the condensate can be seen flowing away, e.g.
using a funnel. Any damage that might occur, when the drain is not installed correctly, is not covered by the war-
ranty.
There should be an open connection of the condensate hose into the sewage system, see also the picture on
page 14. Pressure fluctuations in the sewer system may not affect the pressure in the condensate drain hose,
supplied with the siphon:
←Open connection to sewer
The siphon must be checked at least once a year. Disassemble the siphon and clean every part of it.
Check the functioning of the siphon by filling it with water. Then blow into the top condensate inlet and
gently increase the pressure. At some point water starts coming out of the siphon outlet. During this the
floating ball should gradually drop into its seat. By doing this, the ball then should close the outlet (al-
most) completely.
When mounting the siphon, before commissioning the boiler and/or after maintenance, the siphon must
ALWAYS be completely filled with water.
This is a safety measure: the water in the siphon keeps the flue gases from leaking out of the
heat exchanger via the condensate drain.

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6.3. Flow and return connections
In the picture, an example is shown of the installation
of the flow and return connections in combination with
several functional and/or safety components. The pic-
ture does not necessarily contain ALL safety compo-
nents that may or must be applied →always have the
installation carried out by a skilled installer according to
all applicable standards and regulations.
It is advised to install two service valves underneath
the boiler, so the boiler can be isolated from the heat-
ing system, when needed. The valves must be posi-
tioned in such a way that no safety components like
pressure relief valves and expansion vessels can be
isolated from the boiler.
The boiler pump should always be mounted in the re-
turn pipe of the boiler.
When using a system pump, this pump should always
be mounted in the return pipe of the heating system.
Never use chlorine-based fluxes for soldering any
pipes of the water system.
NOTICE: Accessories in the picture are NOT supplied
with the boiler.
6.4. The expansion vessel
A properly selected expansion vessel must be mounted according to all applicable standards an regulation.
6.5. Pressure relief valve
The boiler has no internal pressure relief valve. This should be installed in the flow pipe of the heating system and
close to the boiler. When having cascaded boilers, each boiler should have its own pressure relief valve. It is ad-
vised to use a T-piece for this.
Advise is always to install service valves, so the boiler can be isolated from the heating system, when needed.
Make sure that the pressure relief valve is mounted between the boiler and the service valves.
The pressure relief valve must always be installed in such a way that it cannot be isolated from the boiler by a
valve. The specifications and size of the relief valve should be determined by the installer and must comply with
all applicable regulations and boiler capacity.
6.6. Bypass
The boiler has no internal bypass. When many thermostatic valves are being used, the system should have a
bypass to allow an adequate flow when all thermostatic valves are closed. Instead of a bypass also a low-loss
header can be used for this function.
The boiler flow will also be influenced when a pipe of the heating system is frozen/blocked. Make sure all hea-ting
pipes are free from the risk of frost. If there is the risk of freezing of the heating system, all the pipe sections must
be insulated and/or protected with the help of a tracing.
6.7. Pump functionality
High efficiency pump (default settings)
The pump is controlled by a PWM signal from the burner controller. Its set point is based on a delta T of 20°C.
This means, that when the burner is on full load, delta T = 20°C, and when the burner modulates down, the pump
also modulates down keeping delta T 20°C (provided it is still in range of the lowest limit of the pump).
•The external pump must be mounted in the return pipe of the boiler.
•The boiler pump must be controlled by the S-CB Px 20 boiler control. If, for any reason, an exter-
nal pump control is applied without written approval of Strebel Ltd, the complete warranty on the
S-CB Px 120 boiler and all supplied parts will become invalid.
flow return
BOILER
NOT
supplied
with boiler
NOT
supplied
with boiler
boiler bleed
valve for
service
pressure
relief
valve
boiler
service valves
expansion
vessel

S-CB Px 120 Manual
19
The delta T monitoring parameters have been set so, that a malfunctioning of the pump or an extreme resistance
in the hydraulic system will be detected by the burner controller. When the limits of these parameters are exceed-
ed, the display message “dT Block” will be displayed. This message will disappear when the delta T is within limits
again, if not the boiler will go into a lock out after 45 seconds and the display will blink with the message “Flow-
Return dTfault” (see also § 16.1. , page 90 ff).
ON/OFF pump
If an on/off pump is used, it must be connected to the P3 control (23-24-25).
When this pump has multiple speed settings, make sure it is set at the highest setting and do not change this
setting. The boiler has an internal pump switch that has a programmable delay before it turns off (also for hot
water supply this delay is programmable).
The pump starts running in case of a heat demand. When heat demand stops, the pump will continue to run for a
programmed period and after that it will stop. The pump and (when installed) the three-way valve or hot water
pump for the calorifier can be activated every 24 hours (for a programmable period). This 24-hour cycle starts as
soon as the power supply of the boiler is connected.
6.8. Frost protection
The boiler has a built-in frost protection, which is automatically activating the CH pump when the boiler return
temperature drops below the 5°C. When the boiler return temperature drops below the 3°C, the burner is also
ignited. The pump and/or burner will shut down as soon as the return temperature has reached the 10°C. The
mentioned temperatures are related to the temperatures measured by the RETURN sensor of the boiler. This
frost protection function will not fire up the boiler in case of a “general blocking” of the burner demand.
NOTICE: This frost protection is only to reduce the risk of frost damage to the boiler, not the whole system.
Be aware that flow restrictions in the system, caused by frozen pipes, will also prevent the water flow, needed for
the boiler frost protection function to work. Because it concerns a programmable setting, boiler damage caused by
frost/freezing will affect warranty.
6.9. Installing a strainer and/or dirt separator
Always install a strainer (water filter)
and/or a dirt separator in the return pipe
of the boiler; in such a way that the wa-
ter going to the boiler is free of any de-
bris/particles. When using a water filter
always check a week after installation to
determine the strainer cleaning interval.
Advice is to mount valves before and
after the strainer, including an air bleed
valve, so the strainer can be isolated
from the heating circuit for service oper-
ations. Clean water is very important,
blocked and/or polluted heat exchang-
ers, including failures and/or damages
caused by this blockage are not covered
by the warranty.
SYSTEM WITH DIRT SEPARATOR
SYSTEM WITH STRAINER
SYSTEM WITH STRAINER AND DIRT SEPARATOR
DIRT SEPARATOR
DIRT SE-
PARATOR
WATER RE-
TURN FROM
SYSTEM
WATER
RETURN
FROM
SYSTEM
WATER
RETURN
FROM
SYSTEM
WATER FLOW
TO BOILER(S)
WATER FLOW
TO BOILER(S)
WATER
FLOW TO
BOILER(S)
DIRT
BLEED
VALVE
DIRT
BLEED
VALVE
AIR
BLEED
VALVE
AIR
BLEED
VALVE
VALVE
VALVE
VALVE
VALVE
STRAINER
(WATER FILTER)
STRAINER
(WATER FILTER)

S-CB Px 120 Manual
20
6.10. Water quality
The pH value of the water must be within the following limits: 7,5 < pH < 9,5. This pH value is reached with the
steady conditions. These steady conditions will occur, when after filling the heating system (pH around 7) with
fresh water, the water will lose its air because of the air bleeding operation and heating up (dead water condi-
tions).
Water hardness must be within the following limits:
3,5° Clark (50 ppm CaCO3) < total hardness < 10,5° Clark (150 ppm CaCO3)
When the water might contain aluminium particles, this should be of a maximum of 8.5 ppm. If there is any risk of
contamination of the water by any kind of debris/chemicals in the period after installing, a plate heat exchanger
should be used to separate the boiler circuit from the heating circuit (see drawing below).
It is advised to prevent the possible air intake and water leakage of the central heating system. Fresh oxygenated
water might damage the heat exchanger of the boiler and should therefore be prevented. Usual spots where air is
most likely to seep in are: suction gaskets, pumps, air valve working as a venting pipe, O-rings / gaskets in stuf-
fing box, under floor heating pipes.
6.11. Plastic piping in the heating system
When plastic pipes are used in the central heating system, these should be separated from the boiler system by
using a plate heat exchanger. Diffusion (of plastic) can cause air to enter the heating system. Be aware that plas-
tic piping is often used in under floor heating systems. When no measures have been taken to prevent the en-
trance of air into the boiler system, the warranty of the boiler and any boiler part may be deemed invalid.
6.12. Automatic air vent
This automatic air vent is only used for bleeding the air from the heat exchanger of the boiler, while filling the sys-
tem. Externally measures must be taken to bleed air from the heating system, using air separators in combination
with automatic air vents.
DE-AERATION PROGRAM. When the unit is fired for the first time the unit starts a de-aeration program. One
cycle is 5 seconds pump running and 5 seconds pump off. A complete de-aeration program consists out of
THREE cycles. The de-aeration program can be interrupted/stopped by briefly pressing the service button.
6.13. Automatic water filling systems
When using an automatic water refill system some precautions should be taken (fresh water is bringing fresh oxy-
gen into the system), like installing a water meter to measure and evaluate the total water volume that is added to
the system. Some form of logging should take place, so that continuously filling of the system with large amounts
of oxygen rich fresh water, indicating leakage, is detected in time.
boiler
boiler
boiler
boiler
heating
zone
plate heat
exchanger
plate
heat exchanger
heating
zone