Viessmann Vitocrossal 300 CU3A 26 User manual

Please file in Service Binder
for use by heating contractor
5673 648 - 15 02/2023
Installation Instructions
Vitocrossal 300 CU3A
Models 26, 35, 45, 57, 94, 125, 160, 199
Floor mounted, gas-fired condensing boiler with MatriX gas burner and Lambda Pro control
For operation with natural gas and liquid propane gas
Heating input 19 to 199 MBH
5.6 to 58 kW
VITOCROSSAL 300
Read and save these instructions
for future reference.
Product may not be exactly as shown
IMPORTANT
H

5673 648 - 15
2
Vitocrossal 300 CU3A 26 to 199 Installation
Safety
Safety, Installation and Warranty Requirements
Please ensure that these instructions are read and understood before commencing installation. Failure to comply with
the instructions listed below and details printed in this manual can cause product/property damage, severe personal
injury, and/or loss of life. Ensure all requirements below are understood and fulfilled (including detailed information
found in manual subsections).
WARNING
Installers must follow local regulations with respect to
installation of carbon monoxide detectors. Follow the
Viessmann maintenance schedule of the boiler contained
in this manual.
Product documentation
Read all applicable documentation before commencing
installation. Store documentation near boiler in a
readily accessible location for reference in the future
by service personnel.
uFor a listing of applicable literature,
please see section entitled “Important
Regulatory and Installation Requirements”.
Warranty
Information contained in this and
related product documentation must
be read and followed. Failure to do
so renders the warranty null and void.
Licensed professional heating contractor
The installation, adjustment, service and maintenance
of this equipment must be performed by a licensed
professional heating contractor.
uPlease see section entitled
“Important Regulatory and Installation
Requirements”.
Contaminated air
Air contaminated by chemicals can cause by-products
in the combustion process, which are poisonous to
inhabitants and destructive to
Viessmann equipment.
uFor a listing of chemicals which
cannot be stored in or near the
boiler room, please see subsection
entitled “Mechanical room” in the
“Installation Instructions”.
Advice to owner
Once the installation work is complete, the heating
contractor must familiarize the system operator/
ultimate owner with all equipment, as well as safety
precautions/requirements, shutdown procedure, and
the need for professional service annually before the
heating season begins.
Carbon monoxide
Improper installation, adjustment, service and/or
maintenance can cause flue products to flow into
living space. Flue products contain poisonous carbon
monoxide gas.
uFor information pertaining to the
proper installation, adjustment, service
and maintenance of this equipment to
avoid formation of carbon monoxide,
please see subsection entitled
“Mechanical room” and “Venting requirements”
in the “Venting System Installation Instructions”.
Fresh air
This equipment requires fresh air for
safe operation and must be installed
ensuring provisions for adequate
combustion and ventilation air exist.
uFor information pertaining to the
fresh air requirements of this product,
please see subsection entitled
“Mechanical room” in the “Installation
Instructions”.
Equipment venting
Never operate boiler without an installed venting
system. An improper venting system can cause
carbon monoxide poisoning.
uFor information pertaining to
venting and chimney requirements,
please see section entitled “Venting
Connection”. All products of
combustion must be safely vented
to the outdoors.
Operating and Service Documentation
It is recommended that all product documentation such as
parts lists, operating and service instructions be handed
over to the system user for storage. Documentation is to
be stored near boiler in a readily accessible location for
reference by service personnel.
WARNING
This boiler requires fresh air for safe operation and must
be installed with provisions for adequate combustion
and ventilation air (in accordance with local codes and
regulations of authorities having jurisdiction).
Do not operate this boiler in areas with contaminated
combustion air. High levels of contaminants such as
dust, lint or chemicals can be found at construction
sites, home renovations, in garages, workshops, in dry
cleaning/laundry facilities, near swimming pools and in
manufacturing facilities.
Contaminated combustion air will damage the boiler
and may lead to substantial property damage, severe
personal injury and/or loss of life. Ensure boiler/burner is
inspected and serviced by a qualified heating contractor
at least once a year in accordance with the Service
Instructions of the boiler.

3
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation Safety
About these Installation Instructions
u
Take note of all symbols and notations intended to draw attention to potential hazards or important product
information. These include “WARNING”, “CAUTION”, and “IMPORTANT”. See below.
IMPORTANT
uWarnings draw your attention to the presence of
potential hazards or important product information.
uCautions draw your attention to the presence of
potential hazards or important product information.
uHelpful hints for installation, operation or maintenance
which pertain to the product.
uThis symbol indicates that additional, pertinent
information is to be found.
uThis symbol indicates that other instructions must
be referenced.
WARNING
Indicates an imminently hazardous situation which,
if not avoided, could result in loss of life, serious injury
or substantial product/property damage.
CAUTION
Indicates an imminently hazardous situation which,
if not avoided, may result in minor injury or product/
property damage.
Safety, Installation and Warranty Requirements
WARNING
Inhaling of fiberglass wool and/or ceramic fiber
materials is a possible cancer hazard. These materials
can also cause respiratory, skin and eye irritation.
Fiberglass wool and ceramic fiber materials
WARNING
Appliance materials of construction, products of
combustion and the fuel contain alumina, silica, heavy
metals, carbon monoxide, nitrogen oxides, aldehydes
and/or other toxic or harmful substances which can
cause serious injury or loss of life and which are
known to the State of California to cause cancer,
birth defects and other reproductive harm. Always
use proper safety clothing, respirators and equipment
when servicing or working nearby the appliance.
The state of California has listed the airborne fibers of
these materials as a possible cancer hazard through
inhalation. When handling these materials, special care
must be applied.
First aid measures
- If eye contact occurs, flush eyes with water to remove
dust. If symptoms persist, seek medical attention.
- If skin contact occurs, wash affected areas gently
with soap and warm water after handling.
Suppliers of fiberglass wool products recommend the
following precautions be taken when handling these
materials:
- Avoid breathing fiberglass dust and contact with
skin and eyes.
- Use NIOSH approved dust/mist respirator.
- Wear long-sleeved, loose fitting clothing, gloves
and eye protection.
- Wash work clothes separately from other clothing.
Rinse washer thoroughly.
- Operations such as sawing, blowing, tear-out and
spraying may generate airborne fiber concentration
requiring additional protection.
Suppliers of ceramic fiber products recommend the
following first aid measures:
- Respiratory tract (nose and throat) irritation:
If respiratory tract irritation develops, move the person
to a dust free location.
-Eye irritation: If eyes become irritated, flush immediately
with large amounts of lukewarm water for at least 15
minutes. Eyelids should be held away from the eyeball
to ensure thorough rinsing. Do not rub eyes.
- Skin irritation: If skin becomes irritated, remove soiled
clothing. Do not rub or scratch exposed skin. Wash area
of contact thoroughly with soap and water. Using
a skin cream or lotion after washing may be helpful.
- Gastrointestinal irritation: If gastrointestinal tract
irritation develops, move the person to a dust free
environment.

Vitocrossal 300 CU3A 26 to 199 Installation
Table of Contents
4
Page
5673 648 - 15
Safety, Installation and Warranty Requirements..............2
Product documentation.............................................2
Warranty................................................................2
Licensed professional heating contractor....................2
Contaminated air....................................................2
Advice to owner......................................................2
Carbon monoxide.....................................................2
Fresh air................................................................2
Equipment venting...................................................2
About these Installation Instructions.............................3
Important Regulatory and Installation Requirements........6
Codes.......................................................................6
Instructing the system user......................................6
Initial start-up..........................................................6
Working on the equipment.......................................6
Technical literature..................................................6
CSD-1 Field Testing of High Limit Switches................7
Applicability..............................................................8
Product Information....................................................8
Mechanical Room.....................................................9
Before Set-up........................................................10
Minimum Clearances.................................................10
Recommended minimum service clearances..............10
Connections............................................................11
Connecting power..................................................11
Connecting heating system.....................................11
Proper piping practice.............................................11
Dimensions...........................................................12
Installation fittings.................................................13
Siting and levelling the boiler...................................14
Gas shut-off valve connection.................................14
Gas connection and piping......................................15
Gas piping pressure test.........................................15
Installing boiler safety devices.................................16
Heating water connections......................................17
Pressure drop (heating water side)...........................18
Installing drain valve...............................................19
Installing boiler fill / expansion tank.........................19
Filling the Heating System..........................................20
Combustion Air Connection........................................21
Removing the Front Panel..........................................21
Condensate Siphon...................................................22
Condensate Connection............................................22
Safety
General Information
Set-up
Boiler Connections

Vitocrossal 300 CU3A 26 to 199 Installation
5
Page
Table of Contents
5673 648 - 15
Control Connections
Installation Examples
Additional Information
Accessing the Control Electrical Connections Box.........24
External Cable Connection.........................................24
Temperature Sensors Overview..................................26
Accessing the X3 Plug..............................................27
Connecting the Outdoor Temperature Sensor..............27
Connecting the Low-loss Header Sensor.....................28
Connecting the DHW Sensor....................................28
Installing the LON Communication Module..................30
Connections............................................................31
External accessory connection.................................31
Routing power/pump accessory cables.....................32
Accessory kit for heating circuit with mixing
valve connections..................................................32
24 VAC Thermostat Connection..............................34
Multi connection....................................................36
Pump connection...................................................37
Pump modulation connection...................................38
Boiler power supply connection................................39
Fitting the Top Panels................................................42
Fitting the Front Panels..............................................42
General...................................................................43
Clearances............................................................43
System Layout 1.....................................................44
System Layout 2.....................................................45
System Layout 3.....................................................46
System Layout 4.....................................................47
System Layout 5.....................................................48
System Layout 6.....................................................49
System Layout 7.....................................................50
System Layout 8.....................................................51
Boiler in Heating and Cooling......................................52
Boiler with Low Water Cut-off....................................53
Venting Connection...................................................53
Technical Data.........................................................54
Electrical Connections..............................................56
Power supply connection of accessories...................56
Lighting and Operating Instructions.............................59

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Vitocrossal 300 CU3A 26 to 199 Installation
General Information
Important Regulatory and Installation Requirements
Codes
The installation of this unit shall be in accordance with
local codes or, in the absence of local codes, use CAN/
CSA-B149.1 or .2 Installation Codes for Gas Burning
Appliances for Canada. For U.S. installations use the
National Fuel Gas Code ANSI Z223.1. Always use latest
editions of codes.
In Canada all electrical wiring is to be done in accordance
with the latest edition of CSA C22.1 Part 1 and/or local
codes. In the U.S. use the National Electrical Code ANSI/
NFPA 70. The heating contractor must also comply with
both the Standard for Controls and Safety Devices for
Automatically Fired Boilers, ANSI/ASME CSD-1, and
the Installation Code for Hydronic Heating Systems,
CSA B214-01, where required by the authority having
jurisdiction.
Instructing the system user
The installer of the system is responsible to ensure the
system operator/ultimate owner is made familiar with the
system functioning, its activation, and its shut-down.
Initial start-up
Initial start-up must be performed by a qualified heating
contractor. Proper completion of the Maintenance Record
by the heating contractor is also required.
Working on the equipment
The installation, adjustment, service, and maintenance
of this boiler must be done by a licensed professional
heating contractor who is qualified and experienced in
the installation, service, and maintenance of hot water
boilers. There are no user serviceable parts on the boiler,
burners, or control.
Ensure main power supply to equipment, the heating
system, and all external controls has been deactivated.
Close main gas supply valve. Take precautions in all
instances to avoid accidental activation of power during
service work.
The following topics must be covered:
Proper system operation sequence.
Explain the equipment.
Demonstrate an emergency shut-down, what to do and
what not.
Explain that there is no substitute for proper maintenance
to help ensure safe operation.
The Maintenance Record is located in the Service
Instructions
Please carefully read this manual prior to attempting start-
up, maintenance or service. Any warranty is null and void
if these instructions are not followed.
For information regarding other Viessmann System
Technology componentry, please reference documentation
of the respective product.
We offer frequent installation and service seminars to
familiarize our partners with our products. Please inquire.
Technical literature
Literature for the Vitocrossal 300 CU3A boiler:
- Technical Data Manual
- Installation Instructions
- Service Instructions
- Operating Instructions and User’s Information Manual
- Instructions of other Viessmann products utilized and
installed
- Installation codes mentioned in this manual
The completeness and functionality of field supplied
electrical controls and components must be verified by
the heating contractor. These include low water cut-
offs, flow switches (if used), staging controls, pumps,
motorized valves, air vents, thermostats, etc.
Leave all literature at the installation site and advise the
system operator/ultimate owner where the literature can
be found. Contact Viessmann for additional copies.
This product comes with several safety instruction labels
attached.
Do not remove!
Contact Viessmann immediately if replacement labels are
required.
Carbon Monoxide Detectors
The installer must verify that at least one carbon
monoxide alarm has been installed within a residential
living space or home following the alarm manufacturer’s
instructions and applicable codes before putting the
appliance into operation.

7
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation General Information
Important Regulatory and Installation Requirements (continued)
CSD-1 Field Testing of High Limit Switches for Vitocrossal 300 CU3A boilers – where required by law.
VIESSMANN IS NOT RESPONSIBLE FOR ANY DAMAGES THAT THE FOLLOWING TEST PROCEDURE MAY RESULT
IN BY OVERHEATING THE SYSTEM.
The fixed high limit, when tripped, produces a fault that will require manual reset of the boiler control.
Fixed High Limit fault simulation:
- Run the boiler to high fire via relays test.
- Throttle the boiler valve to a point where the burner will run with as little flow as possible.
- When the burner shuts down on the electronic limit, close the ball valve to stop the flow.
- The boiler temperature should continue a steady increase to trip the fixed high limit.
- Once the fixed high limit is tripped, slowly open the boiler valve to safely cool the boiler down.

5673 648 - 15
8
Vitocrossal 300 CU3A 26 to 199 Installation
Applicability
General Information
When ordering replacement parts, provide either the
16-digit boiler serial number (on the bar code label)
located underneath boiler front enclosure panel.
Refer to page 21 for instructions on how to remove
front enclosure panel.
Model No. CU3A 26, 94
Serial No. 7553000jjjjjjjjj
Model No. CU3A 35, 125
Serial No. 7553001jjjjjjjjj
Model No. CU3A 45, 160
Serial No. 7553002jjjjjjjjj
Model No. CU3A 57, 199
Serial No. 7553003jjjjjjjjj
Product Information
CAUTION
The boiler serial number must be provided when ordering
replacement parts.
IMPORTANT
Natural gas and liquid propane gas-fired condensing
heating boiler for weather-responsive operation in closed
loop, forced circulation hot water heating systems for
space heating and domestic hot water (DHW) production.
The Vitocrossal CU3A boiler comes factory
set for operation with natural gas. For a fuel conversion
to liquid propane gas no conversion kit is required.
Boiler model must be selected based on an accurate heat
loss calculation of the building. Ensure boiler model is
compatible with connected radiation.
Vitocrossal CU3A boilers are factory-tested and
calibrated. Further gas valve adjustments are not typically
required during field start-up.
Follow the Vitocrossal CU3A Venting System
Installation Instructions to vent this boiler.
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
For conversion to LPG refer to the Service
Instructions.

9
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation
Mechanical Room
General Information
During the early stages of designing a new home, we recommend that proper consideration be given to constructing a
separate mechanical room dedicated to the gas- or oil-fired heating equipment and domestic hot water storage tank(s).
The boiler must be located in a heated indoor area, near a floor drain, and as close as possible to a wall. Whenever
possible, install the boiler near an outside wall so that it is easy to duct the venting system to the boiler.
Ensure that the boiler location does not interfere with the proper circulation of combustion and ventilation air of other
fuel burning equipment within the mechanical room (if applicable).
The maximum room temperature of the mechanical room where the boiler is located must not exceed 104ºF (40ºC).
HEnsure ambient temperatures are higher than 32°F
(0°C) and lower than 104°F (40°C).
HPrevent the air from becoming contaminated by
halogenated hydrocarbons (e.g. as contained in paint
solvents or cleaning fluids) and excessive dust (e.g.
through grinding or polishing work). Combustion air
for the heating process, and ventilation of the boiler
room must be free of corrosive contaminants. To that
end, any boiler must be installed in an area that has no
chemical exposure. The list to the right indicates the
main, currently known sources.
HAvoid continuously high levels of humidity (e.g. through
frequent drying of laundry).
HNever close existing ventilation openings.
Sources of combustion and ventilation air contaminants
Areas likely to contain contaminants:
HNew building construction
HSwimming pools
HRemodelling areas, hobby rooms
HGarages with workshops
HFurniture refinishing areas
HDry cleaning/laundry areas and establishments
HAuto body shops
HRefrigeration repair shops
HMetal fabrication plants
HPlastic manufacturing plants
HPhoto processing plants
HBeauty salons
Products containing contaminants:
HChlorine-type bleaches, detergents and cleaning
solvents found in household laundry rooms
HPaint and varnish removers
HHydrochloric acid, muriatic acid
HChlorine-based swimming pool chemicals
HSpray cans containing chlorofluorocarbons
HChlorinated waxes and cleaners
HCements and glues
HRefrigerant leaks
HCalcium chloride used for thawing
HSodium chloride used for water softening salt
HPermanent wave solutions
HAdhesives used to fasten building products and other
similar items
HAntistatic fabric softeners used in clothes dryers
WARNING
If you notice fire coming from the appliance, call the fire
department immediately! Do not attempt to extinguish
the fire unless qualified to do so.
WARNING
Fire causes a risk of burns and explosion!
HShut down the boiler
HClose fuel shut-off valves
HUse a tested fire extinguisher, class ABC.
IMPORTANT
The service life of the boiler’s exposed metallic surfaces,
such as the casing and fan housing, is directly influenced
by proximity to damp and salty marine environments. In
such areas, higher concentration levels of chlorides from
sea spray, coupled with relative humidity, can lead to
degradation of the exposed metallic surfaces mentioned
above. Therefore, it is imperative that boilers installed
in such environments not be installed using direct vent
systems which draw outdoor air for combustion. Such
boilers must be installed using room air dependent vent
systems; i.e. using room air for combustion. The indoor
air will have a much lower relative humidity and, hence,
potential corrosion will be minimized.
Installation area conditions
WARNING
Incorrect ambient conditions can lead to damage to the
heating system and put safe operation at risk.

5673 648 - 15
10
Vitocrossal 300 CU3A 26 to 199 Installation
Set-up
Before Set-up
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
(latest edition) 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.
Before placing boiler in its installation location, ensure all
necessary accessories are installed.
Minimum Clearances
CAUTION
The boiler must be installed in such a way that gas
ignition system components are protected from water
(spraying, splashing, etc.) during boiler operation and
service.
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
service clearances shown in the illustration.
The Vitocrossal CU3A series boilers are approved for closet and alcove installation with the clearances to combustibles listed.
Boiler model CU3A 26, 94 35, 125 45, 160 57, 199
Top 0 0 0 0
Sides (left and right) 0 0 0 0
Vent pipe 0 0 0 0
Front (alcove or closet) 0 0 0 0
Rear 0 0 0 0
Floor combustible combustible combustible combustible
Minimum clearances to combustibles
1*
*1 Clearance between boilers may be reduced to zero
in multi-boiler installations.
Note: The burner, boiler control, condensate trap,
venting and heat exchanger are still fully
accessible from the front and rear of the boiler.

11
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation Boiler Connections
Connections
Note: The Vitocrossal is only suitable for fully pumped
hot water heating systems. Never install 4-way
mixing valves, overflow valves or other flow/return
bypass devices. Never connect a heating return
to the safety return.
Legend
ASafety connection
(pressure relief valve and air vent valve) 1ain. NPT
BBoiler supply 1ain. NPT
CBoiler return 1ain. NPT
DCondensate drain Ø ¾ in. (19 mm) barbed fitting
ESafety return and drain (diaphragm expansion vessel)
1 in. NPT
F Gas connection ¾ in. NPT
G Boiler flue termination
H Combustion air intake
CAUTION
To prevent equipment damage, install all pipework
free of load and torque stresses.
Proper piping practice
Support piping by proper suspension method. Piping
must not rest on or be supported by boiler. Use moderate
amount of dope.
Use an approved pipe sealant or teflon tape when
connecting the following installation fittings.
Connecting power
The Vitocrossal 300 CU3A boiler has external accessories
connection modules installed behind the front cover,
which requires a 120VAC power supply from a wall
receptacle. Refer to the wiring diagram (see page 57).
2 imperfect threads
Leave 2 threads bare
Connecting heating system
Flush heating system thoroughly prior to connecting boiler
rear view

5673 648 - 15
12
Vitocrossal 300 CU3A 26 to 199 Installation
Connections (continued)
Boiler Connections
Model 26, 35, 94, 125 Model 45, 57, 160, 199
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
Dimensions

13
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation Boiler Connections
Connections (continued)
Installation fittings
Safety header
APressure relief valve ¾ in., 30 psig
B Nipple ¾ in. x 1bin.
C Street elbow ¾ in.
D Air vent
E Bushing ¾ in. to ein.
F Tee, ¾ in.
G Nipple ¾ in. x 3bin.
H Reducing coupling 1ain. x ¾ in.
Supply fitting*1
I Reducing tee 1ain. x 1ain. x bin.
J Thermowell
K Temperature and pressure gauge
Gas fitting
L Gas ball valve ¾ in.
Drain fitting
M Reducing tee 1 in. x 1 in. x ¾ in.
N Sediment faucet ¾ in.
Venting
O CPVC vent adaptor 80 mm to 3 in.
(used with CPVC/ABS air intake pipe connections)
*1 Install anywhere in supply piping near boiler

5673 648 - 15
14
Vitocrossal 300 CU3A 26 to 199 Installation
Manual gas shutoff
Closed
Open
Gas shut-off valve connection
1. Make gas connections in accordance with codes
CAN/CSA B149.1 and .2 or National Fuel Gas Code
ANSI Z223.1/NFPA 54, as well as local codes.
2. Connect gas shutoff valve to the boiler gas
connection.
3. Once connected close gas shutoff valve on boiler.
Connections (continued)
Boiler Connections
Siting and levelling the boiler
Damage to the flue gas connection can lead to flue gas
escaping. Never lift or move the boiler by the flue gas
connection.
Note: The adjustable feet are supplied preinstalled.
1. Level the boiler horizontally using the adjustable feet.
Note: Special foundations are not required.
IMPORTANT
It is essential to use appropriate lifting
devices certified of lifting a minimum
400 lbs. (180 kg).

15
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation Boiler Connections
Connections (continued)
Gas piping pressure test
When performing the gas piping pressure test, ensure the
following requirements are met.
1. Isolate the boiler from the gas supply piping system
using the individual manual shutoff valve during
pressure tests equal to or less than ½ psig (14 “w.c.)
2. The boiler and its individual shutoff valve must be
disconnected from the gas supply piping system
during any pressure testing of that system at test
pressures in excess of ½ psig (14 “w.c.)
3. Perform leak test.
Use approved liquid spray solution for bubble test.
Ensure that no liquid is sprayed on any electrical
components, wires or connectors. Do not allow
leak detection fluid to contact gas valve regulator
or regulator vent opening.
4. Correct any and all deficiencies.
5. Remove air from gas line.
IMPORTANT
½ psig = 14 “w.c.
WARNING
Never check for gas leaks with an open flame.
WARNING
Exposing boiler gas pressure regulator and gas valve to
extreme pressures renders warranty null and void.
Legend
AGas connection 7¾ in. (NPT)
B Accessible manual gas shutoff valve (supplied)
C Ground joint union
DDrip leg
1. Refer to current CAN/CSA B149.1 and .2 or National
Fuel Gas Code ANSI Z223.1/NFPA 54, as well as
local codes for gas piping requirements and sizing.
Pipe size to the boiler must be determined based on:
- pipe length
- number of fittings
- type of gas
- maximum input requirements of all gas appliances
in the residence.
CAUTION
Ensure that gas piping is large enough for all appliances in
the residence. No noticeable gas pressure drop in the gas
line must occur when any unit (or combination of units)
lights or runs.
IMPORTANT
Design piping layout in such a way that piping does not
interfere with serviceable components.
IMPORTANT
Max. gas supply pressure: 14 “w.c.
WARNING
The gas supply piping must be leak tested before placing
the boiler in operation.
2. Before connecting boiler to gas line, install ground
joint union, capped drip leg and a manual equipment
shutoff valve as shown. Valves must be listed by a
nationally recognized testing agency. Make boiler gas
connection as shown.
3. Perform gas piping pressure test as described in the
following subsection.
4. Identify shutoff valves as such with a tab and
familiarize operator / ultimate owner of boiler with
these valves.
See Vitocrossal 300, CU3A Service Instructions
for applicable system coding changes.
Gas connection and piping

5673 648 - 15
16
Vitocrossal 300 CU3A 26 to 199 Installation
Connections (continued)
Boiler Connections
Legend
AReducing coupling, 1ain. x ¾ in.
BLong nipple, ¾ in. x 4 in.
CTee, ¾ in.
DBushing, ¾ in. x ein.
EAir vent
FStreet elbow, ¾ in.
GShort nipple, ¾ in. x 1bin.
HPressure relief valve, ¾ in. 30 psig
IStreet elbow, ¾ in. (field supplied)
Installing boiler safety devices
1. Apply sufficient amount of pipe sealant to both ends
of all pipe fittings B,D,Fand G and install onto
coupling A and tees C.
2. Install pressure relief valve Hand air vent E and
tighten.
3. Install discharge pipe on pressure relief valve in such
a way that...
- the end of the pipe is not threaded.
- the pressure relief discharge pipe extends to a floor
drain and ends approximately 6 in. (150 mm) above
the drain.
Ensure that...
- there is no shutoff valve installed in the discharge pipe.
- discharge pipe diameter is not reduced.
- discharge is not piped to outdoors.
Minimum connection diameters:
Pressure relief valve......................................¾ in.
Discharge pipe.............................................¾ in.
Piping to precharged expansion tank..(field supplied)
IMPORTANT
Install the (approved) factory supplied pressure relief
valve.
Removal of air from the system must occur via use of air
vent(s) in the system supply. To ensure the boiler can be
purged of all air, ensure supply/return water lines do not
contain restrictive piping where air could be trapped.
WARNING
Do not install an isolation valve between boiler and
pressure relief valve.
The discharge pipe for the pressure relief valve must
be oriented to prevent scalding of attendants.
Pipe pressure relief valve discharge pipe close to floor
drain. Never pipe discharge pipe to the outdoors.
Standard safety header configuration
Optional safety header configuration

17
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation
Heating water connections
1. Thoroughly flush heating system (particularly before
connecting the boiler to an existing system).
2. Connect boiler to the heating system.
IMPORTANT
Apply sufficient amount of thread sealant when making
the connections.
Connections (continued)
Boiler Connections
CAUTION
To prevent equipment damage, install all pipework
free of load and torque stresses.
Typical connection
with supplied;
- reducing tee 1ain.
x 1ain. x bin.
- thermal well
- tridicator (temp and
pressure gauge)
All other piping is
field supplied.
Vertical connection
with supplied;
- reducing tee 1ain.
x 1ain. x bin.
- thermal well
- tridicator (temp and
pressure gauge)
All other piping is
field supplied.
Horizontal (left)
connection with
supplied;
- reducing tee 1ain.
x 1ain. x bin.
- thermal well
- tridicator (temp and
pressure gauge)
All other piping is
field supplied.
Horizontal (right)
connection with
supplied;
- reducing tee 1ain.
x 1ain. x bin.
- thermal well
- tridicator (temp and
pressure gauge)
All other piping is
field supplied.
Min. operating pressure............................ 1 bar (14 psig)
Max. allowable working pressure (MAWP)..2 bar (30 psig)
WARNING
Exposing the boiler to pressures and temperatures in
excess of those listed will result in damages, and will
render warranty null and void.

5673 648 - 15
18
Vitocrossal 300 CU3A 26 to 199 Installation
Connections (continued)
Legend
ABoiler models CU3A 26, 35, 94, 125
BBoiler models CU3A 45, 57, 160, 199
The Vitocrossal 300 is only suitable for fully pumped hot
water heating systems.
Boiler models CU3A
ΔT = 20ºF (10 K) ΔT = 30ºF (15 K) ΔT = 40ºF (20 K)
Flow rate
USGPM (m3/h)
Flow rate
USGPM (m3/h)
Flow rate
USGPM (m3/h)
26, 94 9.9 (2.2) 6.6 (1.5) 4.9 (1.1)
35, 125 13.3 (3.0) 8.9 (2.0) 6.6 (1.5)
45, 160 17.0 (3.9) 11.4 (2.6) 8.5 (2.0)
57, 199 22.7 (5.2) 15.0 (3.4) 11.4 (2.6)
Pressure drop
Flow rate
Boiler Connections
Pressure drop (heating water side)
Note: This boiler does not require a flow switch.

19
5673 648 -15
Vitocrossal 300 CU3A 26 to 199 Installation
Connections (continued)
Boiler Connections
CAUTION
To prevent equipment damage, install all pipework
free of load and torque stresses.
Legend
AReducing tee, 1 in. x 1 in. x ¾ in.
BSediment faucet, ¾ in.
Installing drain valve
Installing boiler fill / expansion tank
IMPORTANT
Apply sufficient amount of thread sealant when making
the connections.
Install drain valve assembly as shown in the diagram.
Legend
AAutomatic fill
BPrecharged expansion tank

5673 648 - 15
20
Vitocrossal 300 CU3A 26 to 199 Installation
Boiler Connections
Filling the Heating System
1. Check the pre-charge pressure of the diaphragm
expansion vessel when the system is still cold.
Note: If the pre-charge pressure of the diaphragm
expansion vessel is lower than the static pressure
of the system, top up with nitrogen until the
precharge pressure 1.5 to 3 psi (0.1 to 0.2 bar)
is higher than the static system pressure.
The static pressure corresponds to the static head.
2. Close the gas shutoff valve.
3. Open any installed shutoff valves.
4. Fill the heating system with water and vent it, until
the fill pressure 1.5 to 3 psi (0.1 to 0.2 bar) is higher
than the pre-charge pressure of the diaphragm
expansion vessel using the boiler drain/fill valve A.
Max. operating pressure:.............30 psi (2 bar)
Min. operating pressure:..............14 psi (1 bar)
Pressure relief valve:...................30 psi (2 bar)
5. Mark the set pressure at temperature and pressure
gauge B.
6. Record the fill volume, water hardness and pH value.
7. Return the shutoff valves to their operating position.
8. Close boiler drain/fill valve A.
CAUTION
Unsuitable fill water increases the level of deposits and
corrosion, and may lead to damage to the equipment.
HThoroughly flush the entire heating system prior to
filling with water.
HOnly use water of potable quality.
HThe water will have to be softened if it is harder than
8.77 grains / 150 ppm total dissolved solids.
HInhibitors or antifreeze additives suitable for heating
systems may be added manually.