Viessmann VITOCROSSAL 300 Instruction manual

Technical Data Manual
VITOCROSSAL 300
5673 647 - 01 04/2015
Model Nos. and pricing: see Price List
For operation with natural gas and liquid propane gas
Floor mounted, gas-fired condensing boiler
Heating input 19 to 199 MBH
5.6 to 58 kW
Product may not be exactly as shown
Vitocrossal 300
CU3A Series 26 to 57
Floor mounted, gas-fired condensing
boiler with modulating MatriX gas burner.
For room air dependent or independent
operation.
H

Vitocrossal 300 CU3A 26 to 57 Technical Data
5673 647 - 01
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Product Information
2
Benefits
The benefits at a glance:
■Best value in its class with new industry-leading
technology and the most standard features.
■Highest Efficiency up to 98% at full modulation and up
to 95% A.F.U.E.
■Lasting performance with industry-leading Viessmann
made SA240 316 Ti stainless steel Inox-Crossal heat
exchanger constructed to CSA B51 and ASME Section IV.
■Low Emission Combustion with Viessmann made
stainless steel MatriX burner. Factory calibrated.
■Fast Installation and Reduced Maintenance with Lambda
Pro, industry-first intelligent combustion management
system: Adjusts automatically to gas type and quality.
No fuel conversion kit required.
■Powerful and User-friendly
Viessmann Vitotronic on-board multi-function outdoor
reset boiler and system control for multi-temperature
space and DHW heating.
Equipped with the industry’s first intelligent combustion
management system and powerful control technology,
the new generation Vitocrossal 300 CU3A gas-fired
Floor mounted condensing boiler delivers unparalleled
performance, reliability and comfort.
■Expanded Application Range with increased capacity up
to 199 MBH (58 kW). Multi-boiler installation up to
1592 MBH (464 kW) with up to 8 boilers.
■Greater Venting Flexibility with increased vent length [up
to 198 ft. (60 m)] and multiple venting options.
- Horizontal or vertical sealed combustion coaxial
venting (field supplied).
- Horizontal, vertical or hybrid sealed combustion
double-pipe venting (field supplied).
- Horizontal or vertical single-wall venting (field
supplied).
■Compact, Esthetic Product Design and zero clearance
to combustibles.
■Extremely Quiet Operation; quieter than most
refrigerators.
■True High-Altitude Operation up to 10,000 ft. (3,000 m)
with simple electronic adjustment.
■Efficiency up to 98% through intensive condensation.
The flue gas temperature is only approximately 9-27ºF
(5-15ºC) above boiler return temperature.

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Product Information
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Cross-Section
Product may not be exactly as shown
Vitocrossal 300 CU3A 26 to 57
Vitocrossal 300 CU3A boiler efficiency dependent
on system heating water return temperatures and
load conditions
Legend
ADigital Vitotronic boiler control unit
BWater-cooled stainless steel combustion chamber
CModulating MatriX gas burner – for extremely clean
combustion
DStainless steel Inox-Crossal heat exchanger
EHighly effective thermal insulation
FFlue gas collector with condensate drain pipe
GCombustion air intake for direct vent (two-pipe)
systems

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Technical Data
Technical Data
4
Boiler Model No. CU3A 26 35 45 57
Natural Gas / Liquid Propane Gas
CSA input
CSA output / DOE heating capacity*1
MBH
(kW)
MBH
(kW)
19-94
(5.6-27.5)
17.7-87
(5.2-25.5)
25-125
(7.3-36.6)
23.3-116
(6.8-34)
43-160
(12.6-47)
40-149
(11.7-43.7)
43-199
(12.6-58)
40-185
(11.7-54.2)
Net AHRI Rating MBH
(kW)
76
(22)
101
(30)
129
(38)
161
(47)
Heat exchanger surface area ft.2
(m2)
16.7
(1.5)
20.7
(1.9)
34.1
(3.2)
34.1
(3.2)
Min. gas supply pressure
Natural gas
Liquid propane gas
“w.c.
“w.c.
4
10
4
10
4
10
4
10
Max. gas supply pressure *3
Natural gas
Liquid propane gas
“w.c.
“w.c.
14
14
14
14
14
14
14
14
A.F.U.E. %95 95 95 95
Weight lbs
(kg)
269
(122)
275
(125)
352
(160)
352
(160)
Boiler water content USG
(L)
13.5
(51)
13.0
(49)
18.8
(71)
18.8
(71)
Max. operating pressure
at 210ºF (99ºC)
psig
(bar)
30
(2)
30
(2)
30
(2)
30
(2)
Boiler water temperature
- Adjustable high limit (AHL) range
space heating (steady state)
DHW production
- Fixed high limit (FHL)
ºF
(ºC)
ºF
(ºC)
ºF
(ºC)
68-194
(20-90)
194
(90)
210
(99)
68-194
(20-90)
194
(90)
210
(99)
68-194
(20-90)
194
(90)
210
(99)
68-194
(20-90)
194
(90)
210
(99)
Boiler connections
Boiler heating supply and return
Pressure relief valve
Boiler drain
Gas valve connection
NPTM
NPTF
NPTM
NPTF
1a”
¾”
1”
¾”
1a”
¾”
1”
¾”
1a”
¾”
1”
¾”
1a”
¾”
1”
¾”
*1 Output based on 140ºF (60ºC), 120ºF (49ºC) system supply/return temperature.
*2 Net AHRI rating based on piping and pick-up allowance of 1.15.
*3 If the gas supply pressure exceeds the maximum gas supply pressure value, a separate gas pressure regulator
must be installed upstream of the heating system.
Note: For high altitude installation at 10,000 ft. the input for model Vitocrossal 300 CU3A will have an altitude
de-ration of 18%.

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Technical Data
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Technical Data
Boiler Model No. CU3A 26 35 45 57
Dimensions
Overall depth
Overall width
Overall height
(with control interface open)
Overall height
inches
(mm)
inches
(mm)
inches
(mm)
inches
(mm)
27
684
26
660
67
1707
61.5
1562
27
684
26
660
67
1707
61.5
1562
31b
801
26
660
67
1707
61.5
1562
31b
801
26
660
67
1707
61.5
1562
Flue gas *4
Temperature (at boiler return
temperature of 86ºF (30ºC)
- at rated full load
- at rated partial load
Temperature (at boiler return
temperature of 140ºF (60ºC)
ºF (ºC)
ºF (ºC)
ºF (ºC)
113 (45)
90 (32)
167 (75)
113 (45)
90 (32)
167 (75)
113 (45)
90 (32)
167 (75)
113 (45)
90 (32)
167 (75)
Max. condensate flow rate *5
for NG and LPG
TS/TR=104/86ºF (40/30ºC) USG/h
(L/h)
0.9
3.43
1.2
4.62
1.6
5.95
2.1
7.92
Condensate connection hose
nozzle
Ø in. ¾¾ ¾ ¾
Boiler flue gas connection *6 Ø
in. (mm) 3 (80) 3 (80) 4 (110) 4 (110)
Combustion air supply connection outer
Ø in. (mm) 3 (80) 3 (80) 3 (80) 3 (80)
Sound Rating (A scale)
- at maximum input
- at minimum input
dB
dB
48
32
55
33
53
33
58
33
Standby losses *7BTU/hr
W/hr
1128
330
1000
292
1120
328
995
291
*4 Measured flue gas temperature with a combustion air temperature of 68°F (20°C).
*5 Based on maximum input rate.
*6 For side wall vent installations (coaxial system):
Do not exceed max. equivalent length specified in the Installation Instructions of the Vitocrossal 300 CU3A
Venting System.
Do not attempt to common-vent Vitocrossal 300 CU3A with any other appliance.
Side wall co-axial vent installation must include Viessmann protective screen!
For details refer to the Installation Instructions for the Vitocrossal 300 CU3A Venting System.
*7 Standby losses measured from the boiler temperature sensor well based on a boiler water temperature of 158°F
(70°C) and a room temperature of 68°F (20°C).
For information regarding other Viessmann System Technology componentry, please reference documentation
of respective product.

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Dimensional Information
Models 300 CU3A 26 to 57
6
Model 26 to 35 Model 45 and 57
a in. (mm) 27 (684) a in. (mm) 31b(801)
b in. (mm) 17 (430) b in. (mm) 21b(545)
c in. (mm) 23b(595) c in. (mm) 28 (712)
d in. (mm) 4¾(120) d in. (mm) 5b(140)
Note: All height dimensions of the boiler have a tolerance of +.6 in. (+15 mm) due to the factory installed adjustable feet.
Legend
A Overall boiler height (boiler control open)
B Overall boiler height (boiler control closed)
C Safety header connection
D Boiler supply
E Gas connection
F Boiler return
G Boiler drain
H Vent connection
I Condensate drain connection
J Combustion air connection

Vitocrossal 300 CU3A 26 to 57 Technical Data
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5673 647 - 01
Minimum Clearances
7
Recommended Minimum Service Clearances
Note: The Vitocrossal 300 CU3A series boiler conforms
to the zero inches vent clearance to combustibles
testing requirements dictated by the boiler
Harmonized Standard ANSI Z21.13. CSA/4.9
and therefore is listed for zero clearance to
combustibles when vented with a single-wall UL/
ULC certified special venting system. The zero
inches vent clearance to combustibles for the
Vitocrossal 300 CU3A boiler supercedes the
clearance to combustibles listing that appears
on the special venting system label.
The back or side clearance shall be increased if
piping installation is required.
Clearances
Top Front Rear Left Right Vent pipe
0 0 AL CL 0 0 0 0
See the Vitocrossal 300 CU3A Venting
System Installation Instructions for details.
Recommended minimum service clearances
For typical Vitocrossal CU3A series boiler installations,
Viessmann recommends installing the boiler with the
clearances shown in the illustration on the left.
AL = Alcove
CL = Closet
The Vitocrossal CU3A series boiler is approved for
closet and alcove installation with the clearances to
combustibles listed above.

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Waterside Flow
Pressure Drop (heating water side)
Legend
ABoiler models CU3A 26 to 35 kW
BBoiler models CU3A 45 to 57 kW
The Vitocrossal 300 is only suitable for fully pumped hot
water heating systems.
Boiler models CU3A
∆T = 20ºF (10 K) ∆T = 30ºF (15 K) ∆T = 40ºF (20 K)
Flow rate
USGPM (m3/h)
Flow rate
USGPM (m3/h)
Flow rate
USGPM (m3/h)
26 9.9 (2.2) 6.6 (1.5) 4.9 (1.1)
35 13.3 (3.0) 8.9 (2.0) 6.6 (1.5)
45 17.0 (3.9) 11.4 (2.6) 8.5 (2.0)
57 22.7 (5.2) 15.0 (3.4) 11.4 (2.6)
Pressure drop
Flow rate

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Boiler Control
On-Board Vitotronic 200, HO1B Control
Modular structure
The control unit is integrated into the boiler. The control
unit comprises a standard unit, electronics modules and
a programming unit.
Standard unit:
■ON/OFF switch
■Optolink laptop interface
■Operating and fault indicators
■Reset button
■Fuses
Programming unit:
■Easy operation through:
– Plain text display with graphic ability
– Large font and black & white depiction for good
contrast
– Context-sensitive help text
– Removable programming unit; can be mounted
on the wall with separate accessory
■With digital time switch
■Control keys for:
– Navigation
– Confirmation
– Help and additional information
– Menu
■Setting the:
– Room temperature
– Reduced room temperature
– DHW temperature
– Heating program
– Time programs for central heating, DHW heating
and DHW recirculation
– Economy mode
– Party mode
– Holiday program
– Heating curves
– Codes
– Actuator tests
– Test mode
■Displaying the:
– Boiler water temperature
– DHW temperature
– Operating details
– Diagnostic details
– Fault messages

Vitocrossal 300 CU3A 26 to 57 Technical Data
5673 647 - 01
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Frost protection
■The frost protection function will be started when the
outside temperature drops below approx. 34°F (1°C).
With the frost protection function, the heating circuit
pump will be switched ON and the boiler water is
maintained at a lower temperature of approx. 68°F
(20°C).
The DHW tank will be heated to approx. 68°F (20°C).
■The frost protection function will be stopped when the
outside temperature rises above approx. 37°F (3°C)
(default settings).
Summer Operation
Heating program “w”
The burner is only activated upon a call for domestic hot
water from the DHW storage tank (controlled by DHW
tank temperature sensor).
Design and Function
Boiler Control
Control Characteristics
PI characteristics with modulating output.
Time Switch
Digital time switch.
■Individual and 7-day program
■Automatic daylight savings time changeover
■Automatic function for DHW heating and DHW
recirculation pump
■Time, day and standard switching times for space
heating, DHW heating and the DHW recirculation
pump are factory-set
■Switching times are individually programmable,
i.e. up to four switching periods per day
Shortest switching interval: 10 minutes
Power backup: 14 days.
Functions
■Weather-compensated control of the boiler water
and/or supply temperature
■Control of one heating circuit without mixer and two
heating circuits with mixer (with accessory mixing
valve extension modules)
■Demand-dependent heating circuit pump and burner
off control
■Adjustment of a variable heating limit
■Anti-seizing pump protection
■Integral diagnostic system
■Maintenance display
■DHW temperature control with priority
■Auxiliary function for DHW heating (short-term
heating to a higher temperature)
■0-10V External signal input
Setting the Operating Programs
The heating system frost protection (see
frost protection function) applies to all heating programs.
You can select the following heating programs with the
program keys:
■Heating and DHW
■DHW only
■Standby mode
External heating program changeover.

Vitocrossal 300 CU3A 26 to 57 Technical Data
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5673 647 - 01
Boiler Control
Design and Functions (continued)
Heating Curve Adjustment (slope and shift)
The control unit regulates the boiler water temperature
(= supply temperature of heating circuit without mixing
valve) and the supply temperature of the heating circuit
with mixing valve (in conjunction with the accessory kit
for a heating circuit with mixing valve) according to the
outdoor temperature. The boiler water temperature is
automatically raised by 0 to 72 F / 0 to 40 K higher than
the currently required set supply temperature
(in the factory default setting the differential temperature
is 14.4 F / 8 K). See Service Instructions for coding
address “9F” in coding level 2.
The supply temperature that is required to achieve a
given room temperature depends on the heating system
and the thermal insulation of the building that is being
heated.
The adjustment of the two heating curves is used to
match the boiler water temperature and the supply
temperature to these conditions. The boiler water
temperature is limited upwards by the fixed high limit
and the temperature set for the electronic high limit.
The supply temperature cannot rise above the boiler
water temperature.
Technical Data
Rated supply voltage: 120VAC
Rated frequency: 60 Hz
Rated current: Less than 12 A
Max. ambient temp.
- at operation: 32 to 104ºF (0 to 40ºC)
Installation in living spaces or
boiler rooms (standard ambient
conditions)
- when storing
or transporting: -4 to +158ºF (-20 to +70ºC)
Max. operating temp.
setting (space heating): 194ºF (90ºC)
DHW production: 194ºF (90ºC)
Setting fixed high limit: 210ºF (99ºC)
(not adjustable)
Adjustment range of
DHW tank set-point
temperature: 50 to 149ºF (10 to 65ºC)
Heating curve settings
- Heating curve slope: 0.2 to 3.5
- Heating curve shift: -12 to +33ºC
-13 to 40 K
Legend
ALow temperature heating system,
e.g. radiant floor heating
BMedium temperature heating system,
e.g. cast iron radiation, staple-up radiant floor heating
CHigh temperature heating system,
e.g. fintube radiation, fan coils
Boiler water or supply temperature
Room
set-point
temperature Outdoor temperature

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Sensors
Boiler Temperature Sensor (supplied)
The boiler temperature sensor is connected at the control
unit for weather-responsive operation and is built into the
boiler.
Specification sensor type: Viessmann NTC 10K ohms
at 77ºF (25ºC)
Max. ambient temp.
- at operation: 32 to 266ºF (0 to 130ºC)
- when storing
or transporting: - 4 to+158ºF (-20 to+70ºC)
Outdoor Temperature Sensor (supplied)
Sensor location:
- North or northwest wall of building
- 6.6 to 8.2 ft. (2 to 2.5 m) above ground or in case
of a multi-storey building approximately halfway up
the second floor
Electrical connection:
- 2-wire cable, max. cable length 115 ft. (35 m)
with a wire size of min. AWG 16 copper
- Cable to the outdoor sensor must not be laid near
line voltage wiring (120/240V)
Max. ambient temp.
- at operation: 32 to 194ºF (0 to 90ºC)
- when storing
or transporting: - 4 to+158ºF (-20 to+70ºC)
DHW Tank Temperature Sensor (supplied)
- integrated DHW temperature sensor
Accessories
Immersion Temperature Sensor (accessory)
To capture the low loss temperature
(if the LLH is used)
Specification
Lead length fully wired: 12 ft. (3.75 m),
Permissible ambient temp.
- during operation: 32 to 194°F (0 to 90°C)
- when storing
or transporting: - 4 to+158°F (-20 to+70°C)
Optional Accessories
Supply Temperature Sensor
(strap-on sensor, included with mixing valve actuator
accessory kit),
- when storing
or transporting: -4 to+158°F (-20 to+70ºC)
Installed with a strapping band.
Cable length 19.7 ft. (6 m), ready to plug in.
Max. ambient temp.
- at operation: 32 to 212ºF (0 to 100ºC)
- when storing
or transporting: -4 to+149ºF (-20 to+70ºC)
KM BUS Expansion Module
To connect 2 to 9 devices (mixing valve actuator,
Vitotrol, input module, etc.) to the single KM BUS
connection of the boiler.
Specification
Lead length 10 ft. (3.0 m), fully wired
Permissible ambient temp.
- during operation 32 to 104°F (0 to 40°C)
- during storage and
transport: - 4 to+149°F (-20 to+65°C)

Vitocrossal 300 CU3A 26 to 57 Technical Data
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5673 647 - 01
Accessories
Optional Accessories (continued)
Mixing Valve Actuator Kit
The mixing valve actuator is mounted directly on the
Viessmann ¾ to 2½” mixing valve.
The mixing valve actuator is a motor-driven control unit.
The rotational direction is reversible.
Rated voltage: 120VAC
Rated frequency: 60 Hz
Power consumption: 4W max. ambient temperature
- at operation: 32º F to 104ºF (0º C to 40ºC)
application in living areas and installation sites
(normal ambient conditions)
-storage & shipping: -4º F to 149º F (-20ºC to 65ºC)
Room Temperature Sensor,
Separate room temperature sensor as supplement to the
Vitotrol 200A and 300A; to be used if the Vitotrol 200A
or 300A cannot be installed inside the main living room
or in a suitable position where the unit can capture and
adjust the temperature.
Installation in the main living room on an internal wall
opposite radiators. Never install inside shelf units,
recesses, immediately by a door or heat source
(e.g. direct sunlight, fireplace, TV set, etc.).
Connect the room temperature sensor to the Vitotrol
200A or 300A.
Electrical connection:
- 2-wire cable with a wire size of min. AWG 18 copper
- The cable length between the control unit, remote
control unit and room temperature sensor must not
exceed 98 ft. (30 m)
Max. ambient temp.
- at operation: 32 to 104ºF (0 to 40ºC)
- when storing
or transporting: -4 to+149ºF (-20 to +65ºC)
Mixing Valve Extension Module
Rated voltage: 120VAC
Rated frequency: 60 Hz
Rated current: 2 A
Power consumption: 4 W
Max. ambient temp.
- at operation: 32 to 104ºF (0 to 40ºC)
- when storing
or transporting: -4 to+149ºF (-20 to+65ºC)
Relay output for
heating circuit pump: 2 A, 120VAC
Actuator torque: 3 Nm
Time of 90º: 120 seconds
LON Communication Module
Electronic PCB for data exchange with the Vitotronic
200-H, Vitocom 200 and for connecting to a higher level
building management system.
LON Connecting Cable
(for data exchange between control units)
Cable length 23 ft. (7 m), fully wired
LON Terminal End Resistor
To terminate the LON BUS at the first and the last control
unit.

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Vitocrossal 300 CU3A 26 to 57 Technical Data
Accessories
Optional Accessories (continued)
Connection:
- 2-core lead 18 AWG, length max. 164 ft. (50 m)
(even if connecting
several remote control units)
- Never route this lead immediately next to 120/208/460 V
cables
- LV plug as standard delivery must be installed in the
main living room (lead room).
Specification:
Power supply via KM BUS
Power consumption: 0.2 W
Protection class: III
Max. ambient temp.
- at operation: 32 to 104ºF (0 to 40ºC)
- when storing
or transporting: -4 to+149ºF (-20 to+65ºC)
Set room temp. range 37 to 99ºF (3 to 37ºC)
The set room temperature for reduced mode is adjusted
at the control unit.
Vitotrol 200A
One Vitotrol 200A can be used for every heating circuit
in a heating system. The Vitotrol 200A can regulate one
heating circuit and up to two remote controls may be
connected to the control unit.
KM BUS Participant
The Vitotrol 200A remote control regulates the required
set room temperature in standard mode, from any room
in the house.
- Party and economy mode can be enabled via keys
- Only for heating circuit with mixing valve
Fitted room temperature sensor for room temperature
hook-up
WS function:
Installation anywhere in the building.
RS function:
Note: Never activate the RS function for underfloor
heating circuits (inertia). In heating systems with
a heating circuit without mixing valve and heating
circuits with mixing valve, the RS function must
only affect the heating circuit with mixing valve.
Room temperature hook-up:
Installation in the main living room on an internal wall
opposite radiators. Never install inside shelf units,
recesses, immediately by a door or heat source (e.g.
direct sunlight, fireplace, TV set, etc.).
The integral room temperature sensor captures the actual
room temperature and effects any necessary corrections
of the supply temperature as well as a rapid heat-up
at the start of the heating operation (if appropriately
programmed).

15
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Vitocrossal 300 CU3A 26 to 57 Technical Data
Vitotrol 300A
One Vitotrol 300A can be used for every heating circuit
in a heating system. The Vitotrol 300A can regulate up
to three heating circuits. Up to two remote controls may
be connected to the control unit.
KM BUS Participant
The Vitotrol 300A remote control regulates the required
set room temperature for one heating circuit in standard
and reduced mode, the heating program and the switching
times for central heating, DHW heating and the DHW
recirculation pump.
Displaying:
- Room temperature
- Outside temperature
- Heating program
- Operating condition
- Solar yield as graphic display
Settings:
- Set room temperatures for standard mode
(day temperature) and reduced mode (night temperature)
via the standard display
- Heating program, switching times for heating circuits,
DHW heating and DHW recirculation pump plus further
settings via plain text menu on the display
- Party and economy mode can be enabled via the menu
- Only for heating circuit with mixer:
Fitted room temperature sensor for room temperature
hook-up
Note: For room temperature hook-up, the Vitotrol 300A
must be installed in the main living room (lead room).
RS function:
Installation in the main living room on an internal wall
opposite radiators. Never install inside shelving units,
in recesses, or immediately by a door or heat source
(e.g. direct sunlight, fireplace, TV set, etc.). The fitted
room temperature sensor captures the actual room
temperature and effects any necessary correction of
the flow temperature.
WS function:
Installation anywhere in the building.
Connection:
- 2-core lead 18 AWG, length max. 164 ft. (50 m)
(even if connecting several remote control units)
- Never route this lead immediately next to 120/208/460V
cables
- LV plug as standard delivery must be installed in the main
living room (lead room).
Specification:
Power supply via KM BUS
Power consumption: 0.2 W
Protection class: III
Max. ambient temp.
- at operation: 32 to 104ºF (0 to 40ºC)
- when storing
or transporting: -4 to+149ºF (-20 to+65ºC)
Set room temp. range 37 to 99ºF (3 to 37ºC)
The set room temperature for reduced mode is adjusted
at the control unit.
Optional Accessories (continued)
Accessories

5673 647 - 01
16
Accessories
Accessories for Vitocrossal 300 CU3A
Low-Loss Header
When used in conjunction with the Vitocrossal 300
CU3A boiler, the low-loss header acts as hydraulic break,
decoupling boiler and system circuits from each other.
Viessmann recommends the use of a low-loss header
in cases where the system head and flow rates are
unknown.
The temperature sensor connection [TS] typically located
at the top of the low-loss header ensures low return
temperatures to the Vitocrossal 300 CU3A boiler at all
times, increasing operational efficiency.
Use only a Viessmann temperature sensor (accessory).
Do not use any other manufacturer’s temperature sensor.
In addition, the low-loss header helps eliminate air and
debris [D] from the heating system.
See illustrations for design and principle of operation.
Fuel Conversion Label Kit (NG>LPG)
(included in boiler technical literature set)
The Vitocrossal 300 CU3A boiler comes factory set for
operation with natural gas. All Vitocrossal 300 CU3A
models can be field converted to operate with liquid
propane gas (as well as back to natural gas as required).
The kit includes instructions and labels for field conversion.
Legend
AB Air Bleed
BR Boiler Return
BS Boiler Supply
BY Bypass (with laminar flow)
D Debris and/or air
DV Drain Valve
SC Sensor Cable
SR System Return
SS System Supply
TS Viessmann Temperature Sensor
SW Sensor Well
IMPORTANT
Low-loss header design (Typical)
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Accessories
Accessories for Vitocrossal 300 CU3A
Low-Loss Header (continued)
When installing a low-loss header, the system mixed
supply temperature (T3) must be calculated as follows:
Vprimary < Vsecondary
T1 > T3
T2 = T4
Qprimary = Qsecondary
T1 167° F (75° C)
Vsecondary=Vprimary+Vbypass
IMPORTANT
Model No. Max. system flow rate
Type 80/60 17.6 USGPM (4 m3/h)
Type 120/80 35.2 USGPM (8 m3/h)
Type 160/80 *1 44 USGPM (10 m3/h)
Type 200/120 *180 USGPM (18 m3/h)
*1 Floor-mounted version (typically used for
multiple-boiler installations).
Principle of Operation
Legend
T1 Boiler supply temperature
T2 Boiler return temperature
T3 System supply temperature
T4 System return temperature
Vprimary Boiler circuit flow rate
Vsecondary Heating circuit flow rate
Vbypass Bypass flow rate
Qprimary Heat supplied by boiler
Qsecondary Heat consumed by system
Vitocrossal 300 CU3A 26 to 57 Technical Data

Vitocrossal 300 CU3A 26 to 57 Technical Data
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18
Standard Equipment/Management System
Standard Equipment
Combustion Management System
The Vitocrossal 300 CU3A gas-fired condensing combi-
boiler with Inox-Crossal heat exchanger, modulating
MatriX burner for liquid propane gas and natural gas
(available on all models), comes standard with:
■boiler control unit with outdoor
temperature sensor, power/pump module:
- installation fittings with 30 psig pressure relief valve
- Temperature and pressure gauge field installation
on supply pipe.
- The boiler comes prewired and fully piped
internally for field connections.
- Side wall vent installations must include
Viessmann protective screen!
- Enclosure finish: black steel, powder-coated
Viessmann silver
- The Vitocrossal 300 CU3A comes ready for use
with natural gas and can be fuel converted to liquid
propane gas in the field.
The combustion management system utilizes the physical
correlation between the level of the ionization current
and the air factor λ. For all gas qualities, the maximum
ionization current results with air factor λ.
The ionization signal is evaluated by the combustion
management system, and the air factor is adjusted to
between λ=1.24 and 1.44. This range provides for an
optimum combustion quality. Thereafter, the electronic
gas valve regulates the required gas volume based on the
prevailing gas quality.
To check the combustion quality, the CO2content or the
O2content of the flue gas is measured. The actual values
enable the prevailing air factor to be determined. The
relationship between the CO2or O2content and air factor
λis illustrated in the table below.
To achieve an optimum combustion control, the system
regularly performs an automatic self-calibration; also after
a power failure (shutdown). For this, the combustion is
briefly regulated to max. ionization current (equals air factor
λ=1). The automatic calibration is performed shortly after
the burner starts and lasts approximately 5 seconds. During
the calibration, higher than normal CO2emissions may
occur briefly.
The Vitocrossal 300 CU3A boilers come equipped with Lambda Pro, the industry’s first intelligent combustion
management system. The boiler adjusts automatically to any gas type and quality without the need for a fuel
conversion kit.
Air factor λ- CO2/O2content
Air factor λO2content (%) CO2content (%) for natural gas CO2content (%) for liquid propane gas
1.20 3.8 9.6 11.3
1.24 4.4 9.2 10.9
1.27 4.9 9.0 10.6
1.30 5.3 8.7 10.3
1.34 5.7 8.5 10.0
1.37 6.1 8.3 9.8
1.40 6.5 8.1 9.6
1.44 6.9 7.8 9.3
1.48 7.3 7.6 9.0

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Installation
Installation Examples
Please note that in the following piping layout examples
all pumps external to the boiler are field supplied.
IMPORTANT
The examples on the following pages depict possible
piping layouts of the Vitocrossal 300 CU3A boiler
equipped with Viessmann System Technology.
Please note that the following examples are simplified
conceptual drawings only!
Piping and necessary componentry must be field verified.
A low water cut-off (LWCO) must be installed where
required by local codes.
Proper installation and functionality in the field is the
responsibility of the heating contractor.
Hydraulic Connection
System design
Viessmann condensing boilers can generally be installed
in any pumped hot water heating system (closed system).
Minimum system pressure 14 psi.
The boiler water temperature is limited to 194ºF (90ºC).
To minimise distribution losses, we recommend that
you size the heat distribution system to a max. supply
temperature of 158ºF (70ºC).
For apartments with less than 860 ft2(80 m2) living
space or for low energy houses with low heat demand
we recommend, due to the immediate capturing of the
room-influencing factors, the utilisation of the Vitocrossal
300 CU3A with a constant temperature control unit in
conjunction with the Vitotrol 200A / 300A.
To reduce burner cycling in low energy houses with a
correspondingly low heat demand, we recommend the
use of a low loss header.

Vitocrossal 300 CU3A 26 to 57 Technical Data
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Installation Examples
System Layout 1
20
Vitocrossal 300 CU3A with a direct-connected heating circuit
Installation of ...
■radiator heating circuit (high-temp. circuit)
■DHW production
Legend
AVitocrossal 300 CU3A boiler with
Vitotronic 200, KW6B outdoor reset control
BOutdoor temperature sensor !
CVitotrol remote (optional)
DHeating circuit
EHeating circuit pump 20
FDHW pump sA
GDHW storage tank
HDHW tank temperature sensor %
IExpansion tank
JDHW recirculation pump 28/20 *
KTemperature and pressure relief valve
LPressure relief valve
* Function based on coding of address 33