Gree GMV-ND28TS/B-T User manual

Owner's Manual
Original Instructions
Commercial Air Conditioners
Multi Variable Air Conditioners
Two-way Cassette Type Indoor Unit
Models:
GMV-ND28TS/B-T
GMV-ND36TS/B-T
GMV-ND45TS/B-T
GMV-ND50TS/B-T
GMV-ND56TS/B-T
GMV-ND63TS/B-T
GMV-ND71TS/B-T
GMV-ND80TS/B-T

To Users
Thank you for selecting Gree product. Please read this instruction manual carefully before installing
and using the product, so as to master and correctly use the product. In order to guide you to correctly
install and use our product and achieve expected operating effect, we hereby instruct as below:
(1) This appliance can be used by children aged from 8 years and above and persons with reduced
physical, sensory or mental capabilities or lack of experience and knowledge if they have been
given supervision or instruction concerning use of the appliance in a safe way and understand
the hazards involved. Children shall not play with the appliance. Cleaning and user maintenance
shall not be made by children without supervision.
(2) In order to ensure reliability of product, the product may consume some power under stand-by
status for maintaining normal communication of system and preheating refrigerant and lubricant.
If the product is not to be used for long, cut off the power supply; please energize and preheat
the unit in advance before reusing it.
(3) Please properly select the model according to actual using environment, otherwise it may impact
the using convenience.
(4) This product can’t be installed at corrosive, inflammable or explosive environment or the place
with special requirements, such as kitchen. Otherwise, it will affect the normal operation or
shorten the service life of the unit, or even cause fire hazard or serious injury. As for above
special places, please adopt special air conditioner with anti-corrosive or anti-explosion function.
(5) If the product needs to be installed, moved or maintained, please contact our designated dealer
or local service center for professional support. Users should not disassemble or maintain the
unit by themselves, otherwise it may cause relative damage, and our company will bear no
responsibilities.
(6) All the illustrations and information in the instruction manual are only for reference. In order to
make the product better, we will continuously conduct improvement and innovation. If there is
adjustment in the product, please subject to actual product.

Exception Clauses
Manufacturer will bear no responsibilities when personal injury or property loss is caused by the
following reasons:
(1) Damage the product due to improper use or misuse of the product.
(2) Alter, change, maintain or use the product with other equipment without abiding by the instruction
manual of manufacturer.
(3) After verification, the defect of product is directly caused by corrosive gas.
(4) After verification, defects are due to improper operation during transportation of product.
(5) Operate, repair, maintain the unit without abiding by instruction manual or related regulations.
(6) After verification, the problem or dispute is caused by the quality specification or performance
of parts and components that produced by other manufacturers.
(7) The damage is caused by natural calamities, bad using environment or force majeure.
User Notice
This appliance can be used by children aged from 8 years and above and persons with reduced
physical, sensory or mental capabilities or lack of experience and knowledge if they have been
given supervision or instruction concerning use of the appliance in a safe way and understand
the hazards involved. Children shall not play with the appliance. Cleaning and user maintenance
shall not be made by children without supervision.
DISPOSAL:
This marking indicates that this product should not be disposed with other
household wastes. To prevent possible harm to the environment or human
health from uncontrolled waste disposal, recycle it
responsibly to promote
the sustainable reuse of material resources. To return your used device,
please use the return and collection systems or contact the retailer where
the product was purchased. They can take this product for environmental
safe recycling.
If the supply cord is damaged, it must be replaced by the manufacturer, its service agent or
similarly qualified persons in order to avoid a hazard.

Contents
1 Safety Notices (Please be sure to abide) ......................................................................1
2 Product Introduction ......................................................................................................4
2.1 Names of Key Components..................................................................................................... 4
2.2 Three-view drawing of product ................................................................................................ 4
2.3 Rated Working Condition......................................................................................................... 5
3 Preparations for Installation ..........................................................................................5
3.1 Standard Fittings ..................................................................................................................... 5
3.2 Installation Position Selection.................................................................................................. 6
3.3 Requirements of Communication Wire Selection .................................................................... 7
3.4 Wiring Requirements ............................................................................................................... 8
4 Installation Instructions .................................................................................................8
4.1 Indoor Unit Installation............................................................................................................. 8
4.2 Refrigerant Pipe Connection ................................................................................................. 10
4.3 Drainage Pipe Installation and Drainage System Testing.......................................................11
4.4 Panel Installation ................................................................................................................... 14
4.5 Installation of Wired Controller .............................................................................................. 16
5 Wiring Work...................................................................................................................16
5.1 Connection of Wire and Patch Board Terminal...................................................................... 17
5.2 Power Cord Connection ........................................................................................................ 18
5.3 Connection of Communication Wire between Indoor Unit and Outdoor Unit (or indoor unit) 18
5.4 Connection of Communication Wire for Wired Controller...................................................... 19
5.5 Wiring Instructions of Wired Controller and Indoor Units Network ........................................ 20
6 Function list ..................................................................................................................21
7 Routine Maintenance....................................................................................................21
7.1 Cleaning of Filter ................................................................................................................... 21
7.3 Maintenance after the Seasonal Use .................................................................................... 22
8 Table of Error Codes for Indoor Unit...........................................................................22
9 Troubleshooting............................................................................................................23

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
1
1 Safety Notices (Please be sure to abide)
WARNING:If not abide strictly, it may cause severe damage to the unit or the people.
NOTE:If not abide strictly, it may cause slight or medium damage to the unit or the people.
This sign indicates that the operation must be prohibited. Improper operation may cause
severe damage or death to people.
This sign indicates that the items must be observed. Improper operation may cause damage
to people or property.
WARNING!
This product can’t be installed at corrosive, inflammable or explosive environment or the place
with special requirements, such as kitchen. Otherwise, it will
affect the normal operation or
shorten the service life of the unit, or even cause fire hazard or serious injury. As for above
special places, please adopt special air conditioner with anti
-corrosive or anti-explosion
function.
This Multi
VRF System shall
only be connected to an appliance suitable for the same
refrigerant.
All
units in Manual Cover is a partial unit, complying with partial unit of IEC
60335
-2-40:2018, and must only be connected to other units that have been confirmed as
complying to corres
ponding partial unit requirements of this international standard.
The electrical interface shall comply with electrical security requirement, voltage shall be 220
-
240
V~50Hz or 208-230V~60Hz,current shall be referred to the table of “Dimension of power
cord
and capacity of air switch” in Section 3.4, and safety class of construction is I.
Please install the unit
according to instructions in
this manual. Read this
manual carefully before
starting up or checking the
machine.
Installation should be
performed by dealer or
qualified technicians. Do
not install the product by
yourself. Improper
installation may result in
water leakage, electric
shock or fire hazard.
Make sure the local power
supply is in accordance
with units before
installation, and check the
power supply carefully.
This air conditioner must
be properly grounded
through the receptacle to
avoid electric shock. The
grounding wire shouldn't
be connected with gas
pipe, water pipe, lightning
arrester or telephone line.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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Please use specialized
accessories or parts to
carry out installation, or
water leakage, electric
shock, fire hazard may
resulted.
R410A refrigerant can
produce poisonous gas
once it meets fire, so
please ventilate the room
immediately if refrigerant
leaks out during
installation.
Diameter of power cord
must be large enough.
Damaged power cord and
connecting wire must be
replaced by specialized
electric cable.
After the power cord is
connected, please install
the cover of electric box to
avoid danger.
Nitrogen must be charged
according to technical
requirements.
Short circuit is forbidden.
Do not cancel the pressure
switch in case the unit may
be damaged.
For units with wired
controller, do not connect
power supply until the
wired controller is well
installed. Otherwise, the
wired controller cannot be
used.
When the installation is
finished, please check and
make sure the drain pipe,
pipeline and electric wire
are all well connected in
order to avoid water
leakage, refrigerant
leakage, electric shock or
fire hazard.
Do not extend fingers or
objects into air outlet or air
return grille.
If you use gas heater or
petroleum heater in the
same room, please open
the door or window to
maintain good air
circulation in case the
room may lack of oxygen.
Never start or stop the air
conditioner by inserting or
removing the power cord.
Do not turn off the air
conditioner until it runs for
at least 5 minutes.
Otherwise, oil-return of the
compressor will be
affected.
Children are not allowed to
operate the air conditioner.
Do not operate the air
conditioner with wet
hands.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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Please turn the unit off and
unplug your air conditioner
before cleaning.
Otherwise, it may cause
electric shock or personal
injury.
Do not spray water on the
air conditioner or it will
cause malfunction or
electric shock.
Do not expose the air
conditioner directly to
water or place it in a damp
or corrosive environment.
Connect power supply 8
hours before operation. Do
not disconnect power if
you want to stop the unit in
a short period of time, e.g.
in one night (This is for
protecting the
compressor).
Volatile liquid like thinner
or gasoline will damage
the appearance of air
conditioner. Please use
soft dry cloth or wet cloth
with mild detergent to
clean the outer case of air
conditioner.
During Cooling mode,
indoor temperature should
not be set too low. Keep
the difference between
indoor temp and outdoor
temp within 5℃.
If abnormal condition
occurs (e.g. unpleasant
smell), please turn off the
unit at once and
disconnect power supply.
Then contact GREE
service center. If the air
conditioner continues to
operate despite of
abnormal condition, the
unit may be damaged and
it may cause electric shock
or fire hazard).
Do not repair the air
conditioner by yourself.
Improper repair will cause
electric shock or fire
hazard. Please contact
GREE service center and
have it repaired by
professional technicians.
GREE will not assume responsibility of personal injury or equipment damage caused by improper
installation and commission, unnecessary service and incapable of following the rules and instructions
listed in this manual.
DISPOSAL: Do not dispose this product as unsorted municipal waste. Collection of such waste
separately for special treatment is necessary.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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2 Product Introduction
2.1 Names of Key Components
Fig.2.1
2.2 Three-view drawing of product
2.2.1 Three-view drawing of main body
Fig.2.2.1
Unit: mm
Model
A
B
C
D
E
F
GMV-ND28~80TS/B-T
835
790
630
495
280
261

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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2.2.2 Three-view drawing of panel
Fig.2.2.2
Unit: mm
Model
A
B
C
D
E
F
TE03
813
300
46
28
1100
710
2.3 Rated Working Condition
—
Indoor Side Condition Outdoor Side Condition
Dry Bulb Temp℃Wet Bulb Temp℃Dry Bulb Temp℃Wet Bulb Temp℃
Rated Cooling 27 19 35 24
Rated Heating 20 15 7 6
3 Preparations for Installation
NOTE:this picture is for reference only, please refer to the actual product, the unit of dimension is
mm.
3.1 Standard Fittings
Please use the supplied standard fittings listed below as instructed.
No. Name Appearance Q'ty Usage
1 Drainage hose assembly 1 To connect the drainage pipe
2 Insulation 2 To insulate the gas/ liquid pipe
3 Special Nut
2 GMV-ND28~50TS/B-T
1 GMV-ND56~80TS/B-T

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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No. Name Appearance Q'ty Usage
4 Corrugated pipe 1 GMV-ND56~80TS/B-T
5 Remote controller 1+2 To control the indoor unit
6 Washer fixing plate 4 Prevent the washer from falling off
3.2 Installation Position Selection
(1) The location should be able to withstand the weight of unit.
(2) The water can be drained conveniently from drainage pipe.
(3) There should be no obstruction near air inlet and air outlet.
(4) Follow the installation distance required in the Fig.3.2.1 below to ensure sufficient space for
maintenance.
(5) The installation location should be far from heat sources, flammable or explosive gas, or smog
spread in the air.
(6) The appliance shall not be installed in the laundry.
(7) The indoor unit, outdoor unit, power cord and connection electricity wire should be at least 1m
from television and radio in order to prevent interference and noise (Even though 1m distance
is ensured, there may be noise if the electric wave is too strong) .
Unit: mm
Fig.3.2.1
NOTE:
①The unit shall be installed in accordance with national standards or local regulations.
②Only qualified personnel can carry out installation work, please contact with local dealer before
installation.
③Make sure all the installation work completed before energizing.
④When installed in the narrow room, corresponding measures must be taken to prevent the
concentration of refrigerant from exceeding the limit value when the refrigerant is leaking,
which might lead to anoxia or asphyxia.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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3.3 Requirements of Communication Wire Selection
NOTE:If air conditioner used under strong electronic-magnetic interference circumstance, STP
(shielded twisted pair) communication cable must be adopted.
3.3.1 Selection of communication wire between indoor unit and wired controller
Fig.3.3.1
Wire type
Total length of
communication wire
between indoor unit and
wired controller (m)
Wire diameter
(mm²) Wire standard Remark
Light/Ordinary
polyvinyl chloride
sheathed cord
(60227 IEC 52
/60227 IEC 53).
L≤250 2×0.75~2×1.25 IEC 60227-
5:2007
Total length of communication
cable can't exceed 250m.
The cord shall be Circular cord
(the cores shall be twisted
together).
If unit is installed in places with
intense magnetic field or strong
interference, it is necessary to
use shielded wire.
3.3.2 Selection of Communication Wire between Indoor Unit and Indoor Unit (or outdoor
unit)
Fig.3.3.2

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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Wire type
Total length of
communication wire between
indoor unit and indoor
unit(outdoor unit) (m)
Wire
diameter
(mm²)
Wire standard Remark
Light/Ordinary
polyvinyl chloride
sheathed cord
(60227 IEC 52
/60227 IEC 53)
L≤1000 ≥2×0.75 IEC 60227-
5:2007
If the wire diameter is enlarged
to 2 × 1 mm², the total
communication cable length can
reach 1500m.
The cord shall be Circular cord
(the cores shall be twisted
together).
If unit is installed in places with
intense magnetic field or strong
interference, it is necessary to
use shielded wire.
3.4 Wiring Requirements
Dimension of power cord and capacity of air switch.
Model Power Supply Air Switch
Capacity (A)
Minimum Sectional
Area of Grounding
Wire(mm²)
Minimum Sectional
Area of Power Cord
(mm²)
GMV-ND28~80TS/B-T 220-240V~50Hz &
208-230V~60Hz
6 1 1
NOTE:
①The circuit breaker and power cord specification in above sheet are based on max power (max
current) of the unit.
②The power cord specification in above sheet is based on ambient temperature of 40℃.
③The circuit breaker specification in above sheet is based on ambient temperature of 40℃.
④If the working condition is different, please adjust it according to the specification sheet of circuit
breaker.
4 Installation Instructions
4.1 Indoor Unit Installation
4.1.1 Ceiling Opening Dimension and Suspension Bolt Position
Unit: mm
Fig.4.1.1

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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Unit: mm
Model
Indoor unit(A)
Suspension bolt pitch(B) Ceiling opening(C) Decoration panel(D)
GMV-ND28~80TS/B-T 790 834 990 1100
IMPORTANT:The drilling work must be carried out by qualified personnel.
4.1.2 Suspend the Indoor Unit
(1) Drill bolt holes and install bolts.
1) Stick the paper pattern on the installation position; drill 4 holes according to the hole site on
the cardboard as shown in Fig.4.1.2; diameter of drilling hole is according to the diameter of
expansion bolt and the depth is 60-70mm, as shown in Fig.4.1.3.
Unit: mm
Fig.4.1.2Fig.4.1.3
2) Insert the M10 expansion bolt into the hole and then knock the nail into the bolt, as shown in
Fig.4.1.4.
NOTE:The length of bolt depends on the installation height of the unit, bolts are field supplied.
Fig.4.1.4
(2) Install the indoor unit temporarily.
Assemble suspension bolt on the expansion bolt, attach the hanger bracket to the suspension
bolt. Be sure to fix it securely by using a nut and washer from upper and lower sides of the
hanger bracket. The washer fixing plate will prevent the washer from falling.
(3) The usage of paper pattern.
Refer to paper pattern of installation for ceiling opening dimension. The center of ceiling opening
is indicated on the paper pattern. Fix the paper pattern to the unit with 4 screws and fix the
corners of the waterspout at the drainage pipe by screws.
(4) Adjust the unit to the right position.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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(5) Check the level of the unit.
The indoor unit is equipped with build-in water pump and float switch, verify the levelness of 4
directions by level gauge or vinyl tube (filled with water) respectively.
(6) Remove the washer locating plate and then tighten the nut on it.
(7) Remove the paper pattern.
Fig.4.1.5
4.2 Refrigerant Pipe Connection
(1) Aim the flaring port of copper pipe at the center of screwed joint and then tighten the flaring nut
with hand as shown in Fig.4.2.
(2) Use a torque wrench to tighten up the flaring nut until the wrench gives out a click sound.
Fig.4.2
Torque for tightening nut
Pipe diameter (mm)
Torque (N·m)
Φ6.35
15~30
Φ9.52
35~40
Φ12.7
45~50
Φ15.9
60~65
Φ19.05
70~75
(3) The pipe should not be bent too much or it may crack. Use a pipe bender when bending the
pipe.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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(4) Wrap the connection pipe and joint with sponge and then tie them firmly with tape.
(5) For the model which the diameter of gas pipe is ≥15.9mm, we recommend choosing the
bellows to avoid any problems.
4.3 Drainage Pipe Installation and Drainage System Testing
4.3.1 Notice for Installation of Drainage Pipe
(1) It is not allowed to connect the condensate drain pipe into waste pipe or other pipelines which
are likely to produce corrosive or peculiar smell to prevent the smell from entering indoors or
corrupt the unit.
(2) It is not allowed to connect the condensate drain pipe into rain pipe to prevent rain water from
pouring in and cause property loss or personal injury.
(3) Condensate drain pipe should be connected into special drain system for air conditioner.
(4) The drainage pipe should be short and the gradient downwards should be at least 1%~2% in
order to drain condensation water smoothly.
(5) The diameter of drainage hose should be bigger or equal to the diameter of drainage pipe joint.
(6) Install drainage pipe according to the following Fig.4.3.1 and arrange insulation to the drainage
pipe. Improper installation may lead to water leakage and damp the furniture and other things in
the room.
(7) You can buy normal hard PVC pipe used as the drainage pipe. During connection, insert the
end of PVC pipe into the drainage hole and then tighten it with drainage hole and wire binder.
Never connect the drainage hole and drainage hose with glue.
(8) When the drainage pipelines are used for several units, the position of pipeline should be about
100mm lower than the drainage port of each unit. In this case, thicker pipes should be applied.
Fig.4.3.1
4.3.2 Installation of Drainage Pipe
(1) Drainage pipe should have the same diameter or larger diameter than the connection pipes
(PVC pipe, outside diameter 25mm, thickness≥1.5mm).
(2) Keep drainage pipe short and sloping downwards at a gradient of at least 1% for preventing
forming air bubbles.
(3) If the gradient of drainage pipe could not meet the installation requirements, raising pipe should
be applied.
(4) Insert the drainage hose into drain socket and then tighten the metal clamp securely.
(5) Warp the sealing pad over drainage hose and metal clamp for heat insulation.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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(6) Make sure to perform insulation work for all drainage hoses in the room in order to prevent any
possible water dropping due to dew condensation.
(7) Apply the suitable diameter for converging drainage pipe according to the operating capacity of
the unit, as show in Fig.4.3.2.
Fig.4.3.2
(8) The horizontal pipe can’t be connected to vertical pipe in the same level; please select the
connection way as shown in following.
No.1: Three-way connection of drainage pipe joints (Fig.4.3.3).
No.2: Connection of downspout elbow (Fig.4.3.4).
No.3: Inserting horizontal pipe connection (Fig.4.3.5).
Fig.4.3.3 Fig.4.3.4Fig.4.3.5
(9) Drainage pipes should have a downward slope of at least 1%~2%, in order to prevent pipes
from sagging; install hanger bracket at intervals of 1000~1500mm.
Unit: mm
Fig.4.3.6
(10) The installation height of raising pipe for drainage should be lower than B. The gradient from
raising pipe towards drainage direction should be at least 1%~2%. If the raising pipe is vertical
with the unit, the raising height should be less than 1030mm, as shown in Fig.4.3.7.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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Unit: mm
Fig.4.3.7
model A(mm) B(mm)
GMV-ND28~80TS/B-T
120
1080
(11) During the installation, distance from soft drain pipe to the gasket is C mm when the bolt is
tightened. It is not allowed to apply PVC or other related glue in the joints of two ends of drain
pipe.
Fig.4.3.8
Model C(mm)
GMV-ND28~80TS/B-T
10±3
4.3.3 Test of Drainage System
(1) Please test drainage system after electric work is finished.
Inject approximately 1L purified water to drain pan from air vent, ensure that not to splash the
water over the electrical components (e.g. water pump etc.).
1) In case of commissioning finished, please energize the IDUs and switch to cooling or dry
mode, meanwhile, the water pump operates, you can check the draining through the drain
socket.
2) If communication wire is not connected, communication malfunction “C0” will occur after 60s
of energizing. In this case, the water pump operates automatically. Check if the water pump
drains normally through drainage port. The water pump will stop automatically after running
for 10mins.

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
14
(2) During the test, please carefully check the drainage joint, make sure no any leakage occur.
(3) It’s strongly recommend to do the drain test before ceiling decoration.
Unit: mm
Fig.4.3.9
4.4 Panel Installation
4.4.1 Notices for Installation
(1) Improper decorative panel installation could cause the following problems.
Fig.4.4.1
(2) Ensure that it’s clearance-free between decoration panel and ceiling board after installation, if
not, please adjust the body position.
Fig.4.4.2
(3) Connect the decoration panel terminals (Female) to body terminals (male) as shown in Fig.4.4.3.
Fig.4.4.3

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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4.4.2 Panel Installation
(1) Adjust the panel direction. Let the swing motor on the panel keep the same side with connection
pipe.
Fig.4.4.4
(2) Adjust the position of panel to make the panel holes fix at the holes on the unit.
(3) Fix the 4 screws at the two side of panel and then fix the 2 screws in the middle of panel.
(4) Tighten the two “J”-shape hooks in the accessories on the panel with screws, please refer to
Fig.4.4.6.
(5) Tighten the two “L”-shape metal sheet in the accessories on the grille with screws, please refer
to Fig.4.4.7
(6) Install it to the air inlet grille and buckle it.
(7) Install the air inlet grille and then lock the clasp.
(8) Tighten the screw until the thickness of sealing material between panel and indoor unit is about
5~8mm.
Unit: mm
Fig.4.4.5
Fig.4.4.6

Multi Variable Air Conditioners Two-way Cassette Type Indoor Unit
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Fig.4.4.7
4.4.3 Disassembly
Unlock the buckle of two or four panels, the side screws will be exposed after the grille is opened
partly, then dismantle the two screws at the side.
Fig.4.4.8
4.5 Installation of Wired Controller
Wired controller is optional accessory. If wired controller is needed, please contact your local dealer
and install the wired controller according to the instruction manual.
NOTE:Do perform the commissioning operation before first use; automatic addressing or other
settings, please refer to the manual of ODU.
5 Wiring Work
NOTE:
(1) Units must be grounded securely, or it may cause electric shock.
(2) Please carefully read the nameplate and the wiring diagram before carry out the wiring work,
incorrect wiring could cause malfunction or even damage the unit.
(3) The capacity of power supply must be sufficient and the sectional area of wires in the room
should be above 2.5mm2.
(4) The unit should be powered by independent circuit and specific socket.
(5) The wiring should be in accordance with related regulations in order to ensure the units operate
reliably.
(6) Install circuit breaker for branch circuit according to related regulations and electrical standards.
(7) All wiring must use pressure terminal or single wire. Multi-twisted wire that connects directly to
the wiring board may cause fire hazard.
(8) Keep cable away from refrigerant piping, compressor and fan motor.
(9) Do not alter the inner wires of air conditioner. Manufacturer does not assume
responsibility for damage or abnormal operation due to this reason.
(10) If the unit is installed in places with strong electromagnetic interference, it's recommended to
use twin-twisted shield wire. During wire connection, please pay attention that the metal shield
layer of the twin-twisted wire must be grounded (outer case) in order to prevent the unit from
electromagnetic interference.