Airxcel Marvair MGA3090A User manual

Marvair MGA3090-3210 Wall Mount AC I&O Manual 03/2021 Rev.6
Part #03836
IMPORTANT
This manual may include information for options and features which may not be
included on the unit being installed. Refer to the unit data label or Model Identication
to determine which features and options this unit is equipped with.
INSTALLER: Afx the instructions on the inside of the building adjacent to the thermostat.
END USER: Retain this manual for future reference.
Manufactured By:
Marvair Division of the AIRXCEL®Commercial Group
P.O. Box 400 • Cordele, Georgia 31010 • 156 Seedling Drive • Cordele, Georgia 31015
[phone] • Fax [phone]
www.Marvair.com
The most current version of this manual can be found at www.Marvair.com.
MODELS:
MGA3090A & MGA3120A
(Dual Compressor)
Installation & Operation Manual
10 EER Vertical Wall-Mount Air Conditioners
Installation & Operation Manual
10EER 7.5 & 10 Ton Vertical Wall Mount
Air Conditioners

2
Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
How To Use This Manual
This manual is intended to be a guide to Marvair's line of vertical air conditioners. It contains installation, troubleshoot-
ing, maintenance, warranty, and application information. The information contained in this manual is to be used by the
installer as a guide only. This manual does not supersede or circumvent any applicable national or local codes.
If you are installing the air conditioner rst read Chapter 1 and scan the entire manual before beginning the installation
as described in Chapter 2. Chapter 1 contains general, descriptive information and provides an overview which can speed
up the installation process and simplify troubleshooting.
If a malfunction occurs, follow this troubleshooting sequence:
1. Make sure you understand how the air conditioner works (Chapters 1 & 3).
2. Identify and correct installation errors (Chapter 2).
3. Refer to the troubleshooting information in Chapter 4.
If you are still unable to correct the problem, contact the Factory at 1-229-273-9558 for additional assistance.
Please read the following “Important Safety Precautions” before beginning any work.
Important Safety Precautions
1. USE CARE when LIFTING or TRANSPORTING equipment.
2. TRANSPORT the UNIT UPRIGHT. Laying it down on its side may cause oil to leave the compressor and breakage
or damage to other components.
3. TURN ELECTRICAL POWER OFF AT THE breaker or fuse box BEFORE installing or working on the equipment.
LINE VOLTAGES ARE HAZARDOUS or LETHAL.
4. OBSERVE and COMPLY with ALL applicable PLUMBING, ELECTRICAL, and BUILDING CODES and ordi-
nances.
5. SERVICE may be performed ONLY by QUALIFIED and EXPERIENCED PERSONS.
* Wear safety goggles when servicing the refrigeration circuit
* Beware of hot surfaces on refrigerant circuit components
* Beware of sharp edges on sheet metal components
* Use care when recovering or adding refrigerant
6. Use COMMON SENSE - BE SAFETY-CONSCIOUS
This is the safety alert symbol . When you see this symbol on the air conditioning unit and in the instruction
manuals be alert to the potential for personal injury. Understand the signal word DANGER, WARNING and
CAUTION. These words are used to identify levels of the seriousness of the hazard.
DANGER Failure to comply will result in death or severe personal injury and/or property damage.
WARNING Failure to comply could result in death or severe personal injury and/or property damage.
CAUTION Failure to comply could result in minor personal injury and/or property damage.
IMPORTANT Used to point out helpful info that will result in improved installation, reliability or operation.
© 03/2021 Marvair® Div. Airxcel™, Inc.
SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE.

3Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
WARNING
• If the information in these instructions are not followed exactly, a re may result causing
property damage, personal injury or loss of life.
• Read all instructions carefully prior to beginning the installation. Do not begin
installation if you do not understand any of the instructions.
• Improper installation, adjustment, alteration, service or maintenance can cause property
damage, personal injury or loss of life.
• Installation and service must be performed by a qualied installer or service agency in
accordance with these instructions and in compliance with all codes and requirements
of authorities having jurisdiction.
INSTALLER: Afx the instructions on the inside of the building adjacent to the
thermostat.
END USER: Retain these instructions for future reference.
Table of Contents
Chapter 1 Air Conditioner Description & Specications
1.1 General Description .................................................................................................................................5
1.2 Model Identication .................................................................................................................................6
1.3 Serial Number Date Code ........................................................................................................................6
1.4 Capacities, Weights and Filter Sizes ........................................................................................................6
1.5 General Operation ....................................................................................................................................9
1.6 Programmable Logic Controller (PLC) ...................................................................................................9
1.7 Unit Operation........................................................................................................................................11
1.8 Optional Controls and Packages ............................................................................................................13
1.9 Electrical Operation ...............................................................................................................................13
Chapter 2 Electronic Control Board
2.1 Introduction ............................................................................................................................................15
2.2 Installation and Replacement .................................................................................................................16
2.3 Operation................................................................................................................................................16
2.4 Sequence of Operation ...........................................................................................................................20
Chapter 3 Installation
3.1 Equipment Inspection.............................................................................................................................26
3.2 Installation Requirements.......................................................................................................................26
3.3 Installation Materials .............................................................................................................................27
3.4 Porting and Duct Work ..........................................................................................................................29
3.5 Top Flange Installation ..........................................................................................................................29
3.6 Installing the Lifting Brackets................................................................................................................30
3.7 Mounting the Unit .................................................................................................................................30
3.8 Electrical Connections ...........................................................................................................................31
Chapter 4 Start-Up
4.1 Check-Out of Cooling Cycle .................................................................................................................35
4.2 Check-Out of Heating Cycle..................................................................................................................35

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Table of Contents
Chapter 5 Troubleshooting
5.1 Overview ................................................................................................................................................36
5.2 Failure Symptoms Guide........................................................................................................................37
5.3 Compressor Troubleshooting .................................................................................................................37
Chapter 6 Maintenance
6.1 Scheduled Maintenance .........................................................................................................................39
Chapter 7 Warranty
7.1 Limited Product Warranty ......................................................................................................................40
Chapter 8 Start-Up Check List
8.1 Start-Up Check List................................................................................................................................41
Illustrations
Figure 1. PLC ............................................................................................................................................10
Figure 2. Typical Electrical Schematic......................................................................................................14
Figure 3a. Setting the Speed for the Y1 and Y2 Operation for Indoor Motor ............................................17
Figure 3b. Second Stage Cooling Speed .....................................................................................................17
Figure 4. Output Termination for Indoor Motor Control Signal ...............................................................18
Figure 5. Outdoor Motor Speed Setting ....................................................................................................18
Figure 6. Output Termination for Outdoor Motor Control Signal.............................................................18
Figure 7. Communications SetUp .............................................................................................................19
Figure 8. Top Flange and Lifting Bracket Installation ..............................................................................30
Figure 9. Wall Mounting Detail.................................................................................................................31
Figure 10a. Thermostat Connection Diagram ...............................................................................................34
Figure 10b. CommStat™ 3 Wiring Detail.....................................................................................................34
Tables
Table 1 Efciency and Capacity Ratings at ANSI/AHRI Standard 390..................................................... 7
Table 2 Sensible Capacities - Single Compressor ...................................................................................... 7
Table 3 Sensible Capacities - Dual Compressor......................................................................................... 7
Table 4 Single Compressor Cooling Performance (BTUH) at Various Outdoor Temperatures ................. 8
Table 5 Dual Compressor Cooling Performance (BTUH) at Various Outdoor Temperatures.................... 8
Table 6 Economizer Cooling Performance................................................................................................. 8
Table 7 Return Air Filter Sizes ................................................................................................................... 8
Table 8 Shipping Weights & Dimensions............................................................................................................. 8
Table 9 MODBUS Network Address DIP Swith Positions ...................................................................... 20
Table 10a. MODBUS Discrete Registers ...................................................................................................... 24
Table 10b. MODBUS Registers..................................................................................................................... 25
Table 11 Minimum Clearances................................................................................................................... 27
Table 12 Voltage Limitations ...................................................................................................................... 27

5Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Chapter 1 Description & Specications
1.1 General Description
The Marvair MGA3090A & MGA3120A are a series of vertical wall-mounted air conditioning
systems that provide heating, cooling, and ventilation for telecommunication shelters, and other
applications with high internal heat gains. The series is available in nominal cooling capacities of
90,000-120,000 BTUH. A factory installed economizer, resistance heating elements and reverse ow
congurations are available.
All Marvair MGA wall mount air conditioners feature an exclusive electronic control board or PLC-
based controls.
The control board and PLC consolidate several of the electrical components, improves the air
conditioner’s reliability and has LED’s to indicate operating status and fault conditions to assist the
service technician. A complete description of functions is in Section 1.6
Other standard components include:
• Hot gas by-pass valve to allow operation in cold temperatures
• Thermal expansion valve to improve both efciency and capacity over a wide range of ambient
temperatures
• Phase monitor to prevent operation if the unit is not properly phased
Marvair air conditioners are designed for easy installation and service. Major components are accessible
for service beneath external panels.
All units have internal disconnects. Depending upon state and local code requirements, this feature
may eliminate the need for an external breaker or disconnect.

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
1.2 Model Identication
The model identication number is found on the data sticker. Rating plate located on side panel.
Example M G A 3 0 9 0 A D 1 5 0 C + + + + 1 C A + A 1 1 + + + + + +
Position 1234567891011 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1Unit Designation/Family M = Marvair Wall Mount
2Energy Efciency Ratio (EER) G = 10
3Refrigerant Type A = R-410a
4Compressor Type/
Quantity
1 = Single
3 = Dual Compressors
5Unit Capacity/Nominal
Cooling (BTUH)
090 = 90,000
120 = 120,000
6
7
8System Type A = Air Conditioner
9Power Supply
(Volts-Phase-Hz)
A= 208/230-1-60
C= 208/230-3-60
D= 460-3-60
E= 380-3-50 - 4 Wire
F= 220-1-50
Z= 575-3-60
10 Heat Designation
@ Rated Voltage
KW = Kilowatt
000 = No Heat
050 = 5KW
090 = 9KW
150 = 15KW
180 = 18KW
11
12
13 Ventilation
Conguration
A = Solid Front Door
C = Economizer
D = Motorized Damper w/Pressure Relief
E = Motorized Damper w/Pressure
Relief & Independent Motorized
Damper Control
F = No Free Cooling, 100% Emergency
Ventilation Only w/Independent Control
14 Dehumidication
R = Electric Reheat
T = Electric Reheat w/Humidity Control
+ = None
15 Controls
A = Power Fail Alarm w/Additional Lockouts
C = 24V EMS Relay Kit
D = 24V EMS Relay Kit
w/Factory Installed T-Stat
E = Factory Installed T-Stat
F = ModBus Interface
G = ModBus Interface
w/Factory Installed T-Stat
H = Bac-Net
J = Bac-Net Controls
w/Factory Installed T-Stat
+ = None
16 Operating Condition
A = Evaporator Freeze Sensor (EFS)
C = EFS w/Hot Gas Bypass
M = Extreme Duty w/Hard Start & EFS
N = Hard Start
P = Hard Start w/Low Ambient & CCH
Q = Hard Start w/Low Ambient &
Fan Cycle Control (FCC)
R = Crank Case Heater (CCH)
T = Hard Start w/EFS
U = Hard Start w/Hot Gas Bypass
V = Hard Start w/Low Ambient &
CCH & EFS
W = Low Ambient w/CCH
X = Hot Gas Bypass
Y = Low Ambient w/CCH & FCC
Z = Low Ambient w/CCH & EFS
1 = Low Ambient w/FCC
2 = Low Ambient w/FCC & EFS
3 = CCH w/Hot Gas Bypass
+ = None
17 Indoor Air Quality
Features
D = Dry Bulb Sensor
E = Dry Bulb Sensor w/Dirty Filter
G = Dirty Filter Sensor
+ = None
18 Air Flow 1 = Top Supply/Bottom Return
2 = Center Supply (Reverse)
19 Compressor
Location
C = Center
D = Left Hand
E = Right Hand
20 Filter Option
A = 2” Pleated (MERV 8, AC/HP-C)
C = 2” Charcoal
D = MERV 11 High Filtration Package
E = MERV 13 High Filtration Package
F = Filter Access Through Return Air Grille
W = Aluminum Washable
+ = None
21 Corrosion
Protection
A = Condenser Coil Only
C = Evaporator Coil Only
D = Both Coils Condenser & Evaporator
E = All Coils Cond/Evap/Reheat
F = Coat All
K = Coastal Package
+ = None
$ = Special
22 Engineering
Revision Level A1
23
24 Cabinet Color
1 = Marvair Beige
2 = Gray
3 = Carlsbad Canyon
4 = White
5 = Stainless Steel Exterior
6 = Dark Bronze
7 = .050 Aluminum Stucco
8 = Mesa Tan
9 = Pebble Gray
A = Stainless Steel - Unit
$ = Custom Color (Powder Coat)
25 Sound Attenuation 2 = Compressor Blanket
+ = None
26 Security Option A = Lockable Access Plate/Tamper Proof
+ = None
27 Fastener/Drain Pan
Option
A = Stainless Steel Fasteners
C = Stainless Steel Drain Pan
D = Stainless Steel Fasteners & Drain Pan
E = IP55 Ingress
+ = None
28 Unused + = None
$ = Special
29 Unused + = None
$ = Special
30 Special Variation
+ = None
$ = Special Conguration Not Covered by
Model Nomenclature
1.3 Serial Number Date Code
20 = 2020 01 = January 05 = May 09 = September
21 = 2021 02 = February 06 = June 10= October
22 = 2022 03= March 07 = July 11 = November
23 = 2023 04 = April 08 = August 12 = December
1.4 Capacities, Weights and Filter Sizes
See the Product Data Sheet for complete electrical and performance specications and dimensional
drawings.

7Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Note: Follow local codes and standards when designing duct runs to deliver the required airow.
Minimize noise and excessive pressure drops caused by duct aspect ratio changes, bends, dampers and
outlet grilles in duct runs.
Basic Model Cooling BTUH1EER2Rated Air Flow (CFM)3
MGA3090A 89,000 10 3,500
MGA3120A 118,000 10 4,000
1Cooling rated at 95°F (35°C) outdoor and 80°F DB/67° WB (26.5°C DB/19.5°C WB) return air. 2EER=Energy Efciency Ratio
3CFM=Cubic Feet per Minute
Ratings are with no outside air. Performance will be affected by altitude.
Ratings are at 230 volts for 208/230 volt units (“A” & “C” models) and 460 volts for “D” models. Operation of units at a different voltage
from that of the rating point will affect performance and air ow.
Table 1. Efficiency and Capacity Ratings at ANSI/AHRI Standard 390
MGA3090A (Stage 1)
Return Air DB/WB Sensible Cooling
Capacity
Outside Air Temperature ºF (ºC)
75 (24) 80 (27) 85 (30) 90 (32) 95 (35)
79/66
BTUH
40,953 40,416 39,874 39,339 38,800
75/61 42,039 40,581 39,164 37,706 36,248
80/63 43,700 42,201 40,662 39,164 37,665
85/65 45,320 43,781 42,201 40,662 39,083
MGA3120A (Stage 1)
Return Air DB/WB Sensible Cooling
Capacity
Outside Air Temperature ºF (ºC)
75 (24) 80 (27) 85 (30) 90 (32) 95 (35)
79/66
BTUH
68,061 65,732 63,372 61,043 58,684
75/61 61,999 59,848 57,758 55,608 53,457
80/63 64,448 62,238 59,968 57,758 55,548
85/65 66,837 64,567 62,238 59,968 57,638
Table 2. Sensible Capacities - Single Compressor Operation
MGA3090A (Stage 2)
Return Air
DB/WB
Sensible Cooling
Capacity
Outside Air Temperature ºF (ºC)
75 (24) 80 (27) 85 (30) 90 (32) 95 (35) 100
(38)
105
(41)
110
(43)
115
(46)
120
(49)
125
(52)
130
(54)
79/66*
BTUH
74,445 73,149 71,827 70,547 69,274 67,701 66,718 65,451 64,829 64,256 63,684 63,114
75/61 77,983 76,678 75,416 74,126 72,843 71,567 70,298 69,071 68,443 66,928 65,326 63,724
80/63 87,060 85,763 84,439 83,158 81,884 79,477 76,184 72,802 71,200 69,598 67,996 66,394
85/65 96,006 94,720 92,738 89,356 85,885 82,414 79,032 75,561 73,870 72,268 70,666 69,064
MGA3120 (Stage 2)
Return Air
DB/WB
Sensible Cooling
Capacity
Outside Air Temperature ºF (ºC)
75 (24) 80 (27) 85 (30) 90 (32) 95 (35) 100
(38)
105
(41)
110
(43)
115
(46)
120
(49)
125
(52)
130
(54)
79/66*
BTUH
99,115 97,304 95,482 93,696 91,900 90,117 88,369 86,611 85,748 84,954 84,162 83,373
75/61 103,575 101,753 99,994 98,197 96,412 94,639 92,878 91,178 90,307 89,440 88,462 86,293
80/63 115,351 113,540 111,693 109,907 108,134 106,373 103,166 98,586 96,417 94,247 92,078 89,909
85/65 126,948 125,152 123,321 121,003 116,303 111,602 107,023 102,322 100,032 97,863 95,694 93,524
Table 3. Sensible Capacities - Dual Compressor Operation

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Basic Model Outdoor Temperature
75ºF / 24ºC 80ºF / 26.5ºC 85ºF / 29ºC 90ºF / 32ºC 195ºF / 35ºC
MGA3090A 46,980 45,360 43,740 42,120 40,500
MGA3120A 69,286 66,896 64,507 62,118 59,729
1Based upon ANSI/AHRI std. 390 return air conditions of 80°F DB/67° WB (26.5°C DB/19.5°C WB) at various outdoor temperatures.
Table 4. Single Compressor Cooling Performance (BTUH) at Various Outdoor Temperatures
Basic Model Outdoor Temperature
75ºF / 24ºC 80ºF / 26.5ºC 85ºF / 29ºC 90ºF / 32ºC 195ºF / 35ºC
MGA3090A 103,240 99,680 96,120 92,560 89,000
MGA3120A 139,804 134,984 130,163 125,342 120,521
1Based upon ANSI/AHRI std. 390 return air conditions of 80°F DB/67° WB (26.5°C DB/19.5°C WB) at various outdoor temperatures.
Table 5. Dual Compressor Cooling Performance (BTUH) at Various Outdoor Temperatures
Basic Model Fan Watts Fan CFM EER @ 55 F OAT Outside Air Temperature ºF (ºC)
50 (10) 55 (13) 60 (16) 65 (18) 70 (21)
MGA3090A 730 3,500 125 110,128 91,140 72,153 53,165 34,178
MGA3120A 967 4,000 108 125,860 104,160 82,460 60,760 39,060
Based upon indoor setpoint temperature of 79 (26).
Table 6. Economizer Cooling Performance
Model Description INCHES MILLIMETERS PART NO. FILTERS PER UNIT MERV RATING
MGA3090A Exterior Access Return Air Filter 25" x 16" x 2" 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15" x 20" x 2" 381 x 508 x 51 92365 3 8
MGA3090A
Reverse Flow
Exterior Access Return Air Filter 25” x 16” x 2” 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15” x 20” x 2” 381 x 508 x 51 92365 3 8
MGA3090A
Reverse Flow
w/Economizer
Exterior Access Return Air Filter 25” x 16” x 2” 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15” x 20” x 2” 381 x 508 x 51 92365 3 8
Economizer Pre-lter 9.25” x 37” x .375” 235 x 940 x 10 92127 1 N/A
MGA3120A
Exterior Access Return Air Filter 25” x 16” x 2” 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15” x 20” x 2” 381 x 508 x 51 92365 3 8
For Optional Fresh Air Hood, #K/04657 11” x 22” x 1” 279 x 559 x 25 80119 2 N/A
MGA3120A
Reverse Flow
Exterior Access Return Air Filter 25” x 16” x 2” 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15” x 20” x 2” 381 x 508 x 51 92365 3 8
MGA3120A
w/Economizer
Exterior Access Return Air Filter 25” x 16” x 2” 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15” x 20” x 2” 381 x 508 x 51 92365 3 8
Fresh Air Hood Pre-lters 26” x 12” x 1” 660 x 305 x 25 92526 2 N/A
MGA3120A
Reverse Flow
w/Economizer
Exterior Access Return Air Filter 25” x 16” x 2” 635 x 406 x 51 80137 3 8
Interior Access Return Air Filter 15” x 20” x 2” 381 x 508 x 51 92365 3 8
Economizer Pre-lter 9.25” x 37” x .375” 235 x 940 x 10 92127 1 N/A
Table 7. Return Air Filter Sizes
Model Unit Weight Shipping Weight Shipping Dimensions Height Width Depth
LBS KG LBS KG Inches MM Inches MM Inches MM
MGA3090A 1,085 492 1,160 527 98 2,489 56 1,422 48 1,219
MGA3090A Reverse Flow 1,085 492 1,160 527 98 2,489 56 1,422 48 1,219
MGA3090A Reverse Flow w/Economizer 1,131 513 1,210 550 98 2,489 56 1,422 48 1,219
MGA3120A 1,195 542 1,277 579 98 2,489 56 1,422 48 1,219
MGA3120A Reverse Flow 1,195 542 1,277 579 98 2,489 56 1,422 48 1,219
MGA3120A w/Economizer 1,245 565 1,331 604 98 2,489 56 1,422 48 1,219
MGA3120A Reverse Flow w/Economizer 1,245 565 1,331 604 98 2,489 56 1,422 48 1,219
Table 8. Shipping Weights & Dimensions

9Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
1.5 General Operation
Hot Gas By-Pass
Normally used in specialty applications (i.e Magnetic Resonance Imaging (MRI) buildings) to prevent
magnetic voltage disturbance caused by cycling. This technology is applied in this product to extend
the operation envelope for the compressor to 20º F (-6.6ºC). Combined with a condenser low ambient
Fan Cycle feature, compressor operation can be extended to 0º F (-17.8ºC). During Hot Gas operation
mode, system performance will be reduced. If product operation is in mild outdoor ambient conditions,
the installed shut-off ball valve may be closed, thus disabling the Hot Gas By-Pass feature.
Refrigerant Cycle (Cooling Mode)
The air conditioners use R-410A refrigerant in a conventional vapor-compression refrigeration cycle to
transfer heat from air in an enclosed space to the outside. A motorized impeller assembly blows indoor
air across the evaporator. Cold liquid refrigerant passing through the evaporator is boiled into gas by
heat removed from the air. The warmed refrigerant gas enters the compressor where its temperature
and pressure are increased. The hot refrigerant gas condenses to liquid as heat is transferred to outdoor
air drawn across the condenser by the condenser fan. Liquid refrigerant is metered with a thermal
expansion valve (TXV) into the evaporator to repeat the cycle.
MGA3090A/3120A (Dual Compressor) Units: These units are factory wired for maximum cooling
utilizing both compressors. If 2 stage compressor operation is desired, the factory installed jumper
between terminals 1 and 2 of the low voltage terminal strip must be removed. The 1st stage cooling
input is terminal 1 of the low voltage terminal strip and the 2nd stage cooling input is terminal 2 of the
low voltage terminal strip. The thermostat must be programmed for 2 stage cooling operation when 2
stage compressor operation is desired.
Heating Mode
A wall-mounted thermostat controls the heating cycle of models which incorporate resistance heating
elements. On a call for heat, the thermostat closes the heat relay to energize the indoor fan and the
resistance elements.
1.6 Programmable Logic Controller (MGA3120)
MGA3120 models include a factory installed PLC to control the operation of the HVAC system (MGA3090
models are standard with the Marvair printed circuit board, see Chapter 2). LEDs on the PLC show
operational status and provide assistance with diagnosis if troubleshooting is ever required. Various
control functions are eld selectable and programmable. The PLC is also capable of communicating to
other PLCs to allow run time leveling and does not require additional equipment to be installed. The
PLC provides improved reliability because of the reduction of components. The components utilized
are more durable and the control box wiring has been simplied. Pertinent statistical data about the life
of the refrigeration system can be accessed through the PLC.
The PLC is factory installed and tested, requires no adjustments or changes when the air conditioning
system is installed.
Location
The PLC is located in the unit control panel. Remove the HVAC unit front access panel and open the
panel door to the control cabinet.

10
Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Figure 1. Programmable Logic Controller
PLC Inputs & Outputs
e PLC has inputs located along the top right of the controller and outputs along the bottom right of
the controller. An input is a signal to the PLC from either the thermostat, sensors in the air conditioner,
or a customer supplied input, e.g., EMS (Energy Management System). An output is a signal from the
PLC to the air conditioner, to the thermostat or to the customer.
PLC Inputs
e PLC inputs are powered by 24 VDC. e inputs are:
• Y – Compressor
• Y2 – Dual Compressor
• HUM – Humidier (uses optional electric heat)
• HPS – High Refrigerant Pressure Switch. The HPS is ON during normal operation. No light indicates
an open switch. See lockout indicator “A” under Outputs. HPS B available for dual compressor
systems.
• LPS – Low Refrigerant Pressure Switch. The LPS is ON during normal operation. No light indicates
an open switch. See lockout indicator “A” under Outputs. LPS B available for dual compressor
systems.
• EMS – Energy Management System. A shutdown input from an external source. When EMS input
is de-energized, the unit compressor and fans will also de-energize.
• PMF - Phase monitor fault to monitor phase operation for 3 phase systems, light is on when 3 phase
power is powered and connected correctly
PLC Outputs
The PLC outputs are connected to 24VDC and supply 24VDC loads within the control panel. These
outputs are:
• OFM – Outdoor Fan Motor Starter
• IBM – Indoor Blower Motor Starter
• CC – Compressor Starter
• CC2 – Compressor Starter B (Dual Systems)

11 Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
• LOI – Lock Out Indicator. A blinking LED indicates that a pressure switch has opened or a motor
fault has occured. Refer to section 1.8 for additional information concerning this indicator.
• FR – Fault Relay. An isolated form C relay contact is provided for the customer’s use to monitor
for fault conditions with the HPS or LPS switches.
On the left side of the PLC unit, there are three LED’s that indicate the operational status of the PLC.
1. STOP/RUN
a. Solid yellow indicates STOP mode
b. Solid green indicates RUN mode
c. Flashing (alternating green and yellow) indicates that the CPU is in STARTUP mode
2. ERROR
a. Flashing red indicates an error, such as an internal error in the CPU, an error with the memory
card, or a conguration error (mismatched modules)
b. Solid red indicates defective hardware
c. All LEDs ash if the defect is detected in the rmware
3. MAINT – (Maintenance) ashes whenever you insert a memory card. The CPU then changes to
STOP mode. After the CPU has changed to STOP mode, perform one of the following functions to
initiate the evaluation of the memory card:
a. Change the CPU to RUN mode
b. Perform a memory reset (MRES)
c. Power-cycle the CPU
The PLC has indicator LED’s that show the status of all thermostat inputs and outputs. For example, if
the “G” LED is on, this means that voltage is present from the “G” terminal on the thermostat.
1.7 Unit Operation
Modes of Operation
• Normal Start-Up – On a call for cooling, and with the high pressure switch closed, the cooling
system (compressor, indoor blower motor and outdoor fan motor) will be energized. (Note: See
the Delay on Make feature). e cooling system will remain energized during the three minute low
pressure switch bypass cycle. If the low pressure is closed, the cooling system will continue to operate
aer the three-minute bypass. If the low pressure switch is open aer the three-minute bypass, the
cooling system will be de-energized.
• Lockout Mode – If either the high or low pressure switch opens on the same call for cooling, the
PLC system enters into and indicates the lockout mode. In the lockout mode, the compressor is
turned o, the alarm output is energized and the status LED’s will blink to indicate which fault has
occurred. If there is a call for air ow, the indoor blower will remain energized. When the lockout
condition has cleared, the unit will reset if the demand of the thermostat is removed or when power
is reset. e lockout circuit has a 3-second delay to prevent premature activation and is factory wired
for normally open contacts. e user can select either normally closed or normally open remote
alarm dry contacts.

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
PLC External LED Indicator Lights
Type LED Indicator Color Description
Power
RUN/STOP
Green PLC is in run mode
Yel-
low
PLC is in stop mode
ERROR Red PLC has encountered an error
MAINT Yel-
low
Maintanence is required
Inputs
DI a .0 Green High Pressure Switch A
DI a .1 Green Low Pressure Switch A
DI a .2 Green High Pressure Switch B
DI a .3 Green Low Pressure Switch B
DI a .4 Green Emergency Management System
DI b .1 Green Phase Monitor
DI b .2 Green Call for rst stage cooling from thermostat
DI b .3 Green Call for second stage cooling from thermostat
DI b .4 Green Call for heating from thermostat
DI b .5 Green Call for indoor blower from thermostat
Outputs
DQ a .0 Green Output to call for compressor A
DQ a .1 Green Output to call for for the indoor blower
DQ a .2 Green Output to call for the outdoor fan
DQ a .4 Green Output to call for compressor B
DQ a .5 Green Output to call for the heater
DQ a .6 Green Flashing lockout indicator (See below for ash
code)
DQ a .7 Green Fault relay
PLC Flash Code for Lockout Indicator
Number of Flashes Description of Fault
1High pressure switch A fault
2Low pressure switch A fault
3High pressure switch B fault
4Low pressure switch B fault
5Phase fault
10 Heater current overload fault
11 Emergencey Management System
fault
• Delay on Make – On initial power up or on resumption of power, the air conditioner will wait 3
minutes from a call for cooling before allowing the contactor to energize.

13 Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
1.8 Optional Controls & Packages
Protective Coating Packages
Two corrosion protection packages are offered- one for the condenser section (the Coastal Environmental
package) and the other for the entire unit (the Coat-All Package).
The condenser protection package includes:
• Corrosion resistant fasteners
• Sealed or partially sealed condenser fan motor
• Two layer epoxy/urethane applied to all exposed internal copper and metal in the condenser section
• A protective coating on the condenser coil
The Coat-all package includes all of the above but also includes a protective coating on the evaporator
coil and the two layer epoxy/urethane on all exterior and interior components and sheet metal. (Note:
the internal sheet metal which is insulated and the internal control box are not coated).
Dirty Filter Indicator
A diaphragm type of indicator measures the air pressure on either side of the lter and when the pressure
drops below the set point, a red LED is illuminated. The set point is adjustable.
Protective Coil Coatings
Either the condenser or evaporator coil can be coated. For harsh conditions, e.g., power plants, paper
mills or sites were the unit will be exposed to salt water, the condenser coil should be coated. Note:
Cooling capacity may be reduced by up to 5% on units with coated coils.
Cabinet Color and Material
The air conditioners are available in two standard cabinet colors -the standard grey with beige as an
option. The standard cabinet’s sides, top and front panels are constructed of 16 gauge painted steel.
Contact your Marvair representative for color chips, custom colors and 316 stainless steel cabinets.
Dual Compressors With Lead/Lag Operation
Freeze Sensor On Indoor Coil
Prevents frost on the indoor coil caused by a loss of air ow or restrictive duct work.
Filter Access From Return Air Grille
Factory or eld installed lter bracket allows changing and access to the lters from the return air grille.
See model ID, special option code “I”.
Reverse Air Flow Conguration
Location of Supply and Return Air Openings are reversed.
1.9 Electrical Operation
The compressor and condenser fan are energized with a contactor controlled by a 24 VAC pilot signal.
Some compressors incorporate an internal PTC crankcase heater that functions as long as primary power
is available. The heater drives liquid refrigerant from the crankcase and prevents loss of lubrication
caused by oil dilution. Power must be applied to the unit for 24 hours before starting the compressor.
The condenser (outside fan) motor is energized by the same contactor.
The indoor evaporator fan motor is controlled by the fan purge on the electronic control board.

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
RELAY OUTPUTS
X12
DQ.a
1L .0 .1 .2 .3 .4 2L .5 .6 .7 .0 .1
DQ b
CPU 1215C
DC/DC/RLY
L+ M
X12
24VDC 24VDC
L+ M1M .0 .1 .2 .3 .4 .5 .6 .7 .0 .1 .2 .3 .4 .5 2M 01
Dl a Dl b
24VDC INPUTS
3M 01
X11
AQ
ANALOG
OUTPUTS INPUTS
ANALOG
Al
PLC
Figure 2. Typical Electrical Schematic

15 Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Chapter 2 Electronic Control Board
2.1 Introduction
WARNING
Failure to observe the instructions contained in this document may result in personal
injury and/or property damage and may void the warranty. Read this manual before
installing, replacing or using this product.
The MGA3090 is equipped with Marvair’s proprietary Printed Circuit Board (PCB), which sets the
standard for the industry in exibility, reliability and performance. This UL certied component is
engineered to optimize Heating, Cooling and Dehumidication operation while communicating valuable
information to the end user. Special features include 2-Stage operation with varying speed control to
optimize latent and sensible capacity, built in remote communication (MODBUS) for monitoring and/
or control, optimized head pressure control and the ability to function autonomously without the need
for an off the shelf thermostat. The Marvair PCB comes equipped with LEDs to monitor lockouts for
independent circuits, which drastically reduces troubleshooting time and system downtime. Lockout
contacts are also provided along with the alarms being transferred via MODBUS.
This chapter provides the necessary information for installing and operating the Marvair PCB.
The diagram below identies the inputs, outputs and connections for the Marvair PCB.
1 2 3 4 5
6
7
8
9101115
16
17
18
121314

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Item Description
1Outdoor Motor Control Signal Output
2Indoor Motor Control Signal Output
3
P 1/2- Pressure for Circuit 1 and Circuit 2 (Respectively)
FS1 – Freeze Stat for Circuit 1
FS2 – Indoor Temperature Input
4Modbus Communication
3 Wire [A, B, COM] and Parallel RJ-11 Port
5
6
Fan Cycle Control Input (only Applicable for EC Outdoor Motors)
Hum – Humidity Control Input (Connect to R Node)
Y 1/2 - Cooling Request for Stage 1 and Stage 2 Cooling Operation
7Modbus Network Address. Set All 4 to OFF for Local Control
8Potentiometers for Indoor and Outdoor Speed
9Energize or De-Energize Test Mode
10 Set Speed of Indoor Motor for Y1 and Y2 Operation
11 Set Baud Rate. 19.2k Between Top and Mid. 9.6k Between Mid and Bottom
12 Set Indoor Motor Control Signal Type
13 Set Outdoor Motor Control Signal Type
14
Thermostat Inputs:
W2 – Heat Request
G – Indoor Fan Request
ON – Connect Respective Terminal to R Node.
Off – Open Circuit
15
Pressure Switch Inputs for Respective Circuit
HP – High Pressure Switch
LP – Low Pressure Switch
Switch to Be Closed for Cooling Operation. Switch to Be Connected to “R” Node
16 24 VAC Power Input to PCB.
17
Digital Outputs (24 VAC): The PCB Makes and Breaks R.
RH – Reheat
EH – Electric Heat
CC 1/2– Compressor (Respectively)
18 Alarm Contacts
2.2 Installation and Replacement
The PCB is factory installed. To install a replacement PCB, use the six mounting holes along with the
appropriate screw size to rmly secure the board to the control box. After this is achieved, follow the
wiring diagram and pin conguration for the respective system for appropriate operation. Ensure that
the terminals used do not make any unwanted electrical connection (via strands etc.) with any other
terminals. Please allow a 1” creepage distance between the board and all other adjacent electrical
components.
2.3 Operation
LED Status Indicators
Color Type Status Description
Green Power Constant On 24 VAC power has been applied
Red
Status 1
and
Status 2
Constant On Normal Operation
1 Blink High pressure switch has opened twice
2 Blinks Low pressure switch has opened twice
3 Blinks Freeze stat (optional) - Indoor coil temperature is below 35°F (1°C)
Continuous Flash of Both LEDs Insufcient voltage to the board. Less than 20 Volts

17 Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Power
The Marvair PCB requires 24 VAC to operate. When the board is sufciently powered, the “PWR”
status light on the PCB illuminates “Green.” If there is insufcient power to the board, the “STATUS
1” and “STATUS 2” ashes continuously. Insufcient power to the board will result in no outputs being
energized.
Setting the Speed for the Y1 and Y2 Operation for Indoor Motor
Put Bridge Jumper between the “Y1 pin and center pin” shown in the gure below. Use the potentiometer
marked “IBM SPEED” to set the required speed for rst stage cooling (Y1 request). Note that the type
of control signal required by the motor must be set and the appropriate signal terminations must be used.
This jumper will be factory installed. In replacing the PCB, verify the necessary signal and congure
the board accordingly. Only 2 of the 3 pins should be used for the necessary congurations. DO NOT
CONNECT ALL 3 PINS.
4
1 2 3
1Set for 0-10V Signal.
Jumper To Remain On The Appropriate Terminal For Correct Operation
2Set For PWM Signal.
Jumper To Remain On The Appropriate Terminal For Correct Operation
3Bridge Jumper Position for Setting Y1 Speed
4IBM SPEED Potentiometer
Figure 3a. Setting the Speed for the Y1 and Y2 Operation for Indoor Motor
To set second stage cooling (Y2 request) speed, put Bridge Jumper on the “Y2 pin and center pin”
shown in the gure below. Use the potentiometer marked “IBM SPEED” to set the required speed.
2
1
1Bridge Jumper Position For Setting Y2 Speed
2IBM SPEED Potentiometer
Figure 3b. Second Stage Cooling Speed

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Output Termination for Indoor Motor Control Signal
Note: Follow data (wiring and signal control signal type) of the appropriate motor to setup the PCB
1 2
1
3 4
1
1Indoor Motor PWM Signal Output. Reference Wire Connected to “GND” and
the Signal Connected to “PWM”
2
3Indoor Motor 0-10V Signal Output. Reference Wire Connected to “GND” and
the Signal Connected to “0-10V”
4
Figure 4. Output Termination for Indoor Motor Control Signal
Setting the Speed for Outdoor Motor
The Outdoor Motor runs at constant speed dictated by the potentiometer. See gure below.
3
12
1Set for 0-10V Signal. Jumper To Remain On The Appropriate Terminal For Correct Operation
2Set For PWM Signal. Jumper To Remain On The Appropriate Terminal For Correct Operation
3Outdoor Motor Speed Potentiometer
Figure 5. Outdoor Motor Speed Setting
Output Termination for Outdoor Motor Control Signal
Note: Follow data (wiring and signal control signal type) of the appropriate motor to setup the PCB.
1 2
1
3
1 2
4
1
Note that there are redundant (2 of each) output signal terminations for each motor.
1Outdoor Motor PWM Signal Output. One Wire Connected to “GND” and the
Other Connected to “PWM”
2
3Outdoor Motor 0-10V Signal Output. One Wire Connected to “GND” and the
Other Connected to “0-10V”
4
Figure 6. Output Termination for Outdoor Motor Control Signal

19 Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Communication
The Marvair PCB comes equipped with MODBUS communication standard. There are 2 adjacent
MODBUS communication ports connected in parallel. That is, both ports transmit the same information.
The difference between the ports is the physical connection. One port is for RS-485 (3 wire shielded
cable is recommended) and the other port is RJ-11. The board allows you to set the Baud Rate at 9600
bits per second or 19200 bits per second. The board also allows 15 different MODBUS addresses based
on the position of the “NET ADDRESS” DIP switches. The Net Addresses are written in Binary (see
table below) with switch 4 being the least signicant bit. THE NUMBERS ON THE DIPSWITCHES
DO NOT CORRELATE TO MODBUS ADDRESSES. The gure below shows the positions of the
various components necessary to set up communication on the on the PCB.
1 2
3
4 5
Note: No Parity; Serial Data Bit = 8; Stop Bit = 1
1RJ-11 Termination
2RS-485 Termination
3MODBUS Network Address DIP switches. See Addressing Table
4Baud Rate - 19200 BPS: Between Upper and Middle Pin
Baud Rate - 9600 BPS: Between Lower and Middle Pin
5
Figure 7. Communications SetUp

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Marvair MGA3090-3120 I&O Manual 03/2021 Rev.6
Address DIP Switch 1 DIP Switch 2 DIP Switch 3 DIP Switch 4
0 [Local Control] OFF OFF OFF OFF
1OFF OFF OFF ON
2OFF OFF ON OFF
3OFF OFF ON ON
4OFF ON OFF OFF
5OFF ON OFF ON
6OFF ON ON OFF
7OFF ON ON ON
8ON OFF OFF OFF
9ON OFF OFF ON
10 ON OFF ON OFF
11 ON OFF ON ON
12 ON ON OFF OFF
13 ON ON OFF ON
14 ON ON ON OFF
15 ON ON ON ON
Table 9. MODBUS Network Address DIP Swith Positions
2.4 Sequence of Operation
IMPORTANT
All equipment should go through the recommended commissioning/start up sequence
to ensure safety and system reliability. This document is only valid if the system is
used as intended.
This section denes the manner and method of control of the HVAC system. It will cover the
following operations and protections.
1.0 Blower Operation
2.0 Cooling Operation
2.1 Mechanical Cooling
2.1.1 Partial Capacity - 50-60%
2.1.2 Partial Capacity - 50%
2.1.3 Full Capacity
3.0 Heating
3.1 Electric Heat
4.0 Dehumidication
4.1 Independent Reheat Output
5.0 Refrigeration Protection
5.1 High Pressure Lockout
5.2 Low Pressure Lockout
5.3 Low Voltage
5.4 Anti-short Cycle
6.0 Additional Features
6.1 MODBUS communication
6.2 Modulating Head Pressure Control
6.3 Freeze Stat Operation
6.4 Onboard Thermostat