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Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28 - User Manual

Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28
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Manufacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obligations.
PC 111 Catalog No. 534-80211 Printed in U.S.A. Form 48PG-10SI Pg 1 9-05 Replaces: New
Book 1 4
Tab 1 a 6 a
Installation, Start-Up and Service Instructions
CONTENTS
Page
SAFETY CONSIDERATIONS ..........................1,2
INSTALLATION ......................................2-17
Step 1 Provide Unit Support .........................2
ROOF CURB
ALTERNATE UNIT SUPPORT
SLAB MOUNT
Step 2 Remove Shipping Rails .......................2
Step 3 Rig and Place Unit ............................2
POSITIONING
ROOF MOUNT
INSTALLATION ONTO CURB
Step 4 Field Fabricate Ductwork .....................9
Step 5 Make Unit Duct Connections..................9
VERTICAL CONFIGURATION
HORIZONTAL APPLICATIONS
Step 6 Install Flue Hood and Inlet Hood ............ 10
Step 7 Trap Condensate Drain ..................... 10
Step 8 Install Gas Piping ........................... 10
Step 9 Make Electrical Connections.................11
FIELD POWER SUPPLY
FIELD CONTROL WIRING
Step 10 Install Outdoor-Air Hood................... 12
MANUAL DAMPER ASSEMBLY
Step 11 Position Optional Power
Exhaust or Barometric Relief Damper Hood ......... 16
Step 12 Non-Fused Disconnect .................... 16
Step 13 Install All Accessories ..................... 16
PRE-START-UP ....................................... 18
START-UP...........................................18-45
Unit Preparation ...................................... 18
Compressor Mounting................................ 18
Refrigerant Service Ports ............................. 18
Crankcase Heater(s) .................................. 18
Compressor Rotation................................. 18
Internal Wiring........................................ 18
Subcooler Heat Exchanger (SHX)..................... 18
Evaporator Fan ....................................... 18
Condenser Fans and Motors.......................... 19
Return-Air Filters ..................................... 19
Outdoor-Air Inlet Screens............................. 19
Gas Heat.............................................. 19
Optional EconoMi$er IV............................... 39
Operating Sequence .................................. 44
SERVICE............................................46-57
Cleaning .............................................. 46
Lubrication ........................................... 47
Manual Outdoor Air Damper .......................... 48
Economizer Adjustment .............................. 48
Evaporator Fan Service and Replacement ............ 48
Evaporator Fan Performance Adjustment ............. 48
Evaporator Fan Belt Tension Adjustment ............. 48
Condenser Fan Adjustment........................... 49
Verify Sensor Performance ........................... 49
Economizer Operation During Power Failure.......... 49
Evacuation ........................................... 49
Refrigerant Charge ................................... 50
Gas Valve Adjustment ................................ 50
High Altitude.......................................... 51
Main Burners ......................................... 51
Filter Drier ............................................ 52
Protective Devices.................................... 52
Relief Devices ........................................ 52
Page
Control Circuit, 24-V .................................. 52
Replacement Parts.................................... 52
Diagnostic LEDs ...................................... 52
TROUBLESHOOTING .............................. 57-61
Unit Troubleshooting.................................. 57
EconoMi$er IV Troubleshooting....................... 60
Phase Loss Protection ................................ 61
UNIT START-UP CHECKLIST ........................CL-1
SAFETY CONSIDERATIONS
Installation and servicing of air-conditioning equipment can
be hazardous due to system pressure and electrical compo-
nents. Only trained and qualified service personnel should
install, repair, or service air-conditioning equipment.
Untrained personnel can perform the basic maintenance
functions of cleaning coils and filters and replacing filters. All
other operations should be performed by trained service per-
sonnel. When working on air-conditioning equipment, observe
precautions in the literature, tags and labels attached to the unit,
and other safety precautions that may apply.
Follow all safety codes. Wear safety glasses and work
gloves. Use quenching cloth for unbrazing operations. Have
fire extinguishers available for all brazing operations.
Before performing service or maintenance operations on
unit, turn off main power switch to unit. Electrical shock
could cause personal injury.
Puron (R-410A) refrigerant systems operate at higher pres-
sures than standard R-22 systems. Do not use R-22 service
equipment or components on Puron refrigerant equipment.
If service equipment is not rated for Puron refrigerant,
equipment damage or personal injury may result.
1. Improper installation, adjustment, alteration, service,
or maintenance can cause property damage, personal
injury, or loss of life. Refer to the Users Information
Manual provided with this unit for more details.
2. Do not store or use gasoline or other flammable
vapors and liquids in the vicinity of this or any other
appliance.
What to do if you smell gas:
1. DO NOT try to light any appliance.
2. DO NOT touch any electrical switch, or use any
phone in your building.
3. IMMEDIATELY call your gas supplier from a neigh-
bors phone. Follow the gas suppliers instructions.
4. If you cannot reach your gas supplier, call the fire
department.
48PG20-28
Single Package Rooftop Units
Electric Cooling/Gas Heating with
Electromechanical Controls and PURON® (R-410A) Refrigerant

Table of Contents

Questions and Answers

  • M
    Matthew MahoneyAug 13, 2025
    Why does my Carrier Heating System compressor operate continuously?
    • C
      Carlos VasquezAug 13, 2025
      If the compressor operates continuously, first check and replace the dirty air filter. If the unit is undersized for the load, decrease the load or increase the unit size. You may need to reset the thermostat if it is set too low. Also, there may be a low refrigerant charge, so locate the leak, repair it, and recharge. If there is air in the system, recover the refrigerant, evacuate the system, and recharge it. Finally, the condenser coil might be dirty or restricted, so clean the coil or remove the restriction.
  • K
    Kyle CrawfordAug 16, 2025
    What to do if the compressor and condenser fan of my Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28 will not start?
    • B
      Bradley JamesAug 16, 2025
      If the compressor and condenser fan of your Carrier Heating System won't start, it could be due to several reasons. First, check for a power failure and call the power company if needed. Replace any blown fuses or reset tripped circuit breakers. Defective components like the thermostat, contactor, transformer, or control relay should be replaced. Ensure sufficient line voltage and correct any incorrect or faulty wiring by checking the wiring diagram. Also, make sure the thermostat setting isn't too high; lower it below room temperature.
  • M
    Manuel Page DVMAug 20, 2025
    How to troubleshoot low suction pressure in my Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28?
    • B
      brandi81Aug 20, 2025
      If the suction pressure of your Carrier Heating System is too low, start by replacing the dirty air filter. Check for refrigerant leaks, repair them, and recharge the system if the refrigerant charge is low. Remove any restrictions in the metering device or low side. Inspect the TXV, ensuring the bulb is tightly mounted to the suction line, and replace the TXV if faulty. Increase evaporator airflow, checking and replacing the filter if necessary. Reset the thermostat if the temperature is too low in the conditioned area. Finally, replace any restricted field-installed filter driers.
  • C
    clarkdonaldAug 25, 2025
    What to do if my Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28 Heating System compressor cycles too often?
    • M
      Micheal LamAug 25, 2025
      If your Carrier Heating System's compressor cycles more than it should, it might be due to a refrigerant overcharge or undercharge, in which case you should recover the refrigerant, evacuate the system, and recharge to the nameplate value. The compressor itself could be defective and need replacement, after determining the cause. Insufficient line voltage or a blocked condenser could also be the issue; determine the cause and correct it. Other potential causes include a defective overload (determine cause and replace), a defective thermostat (replace it), a faulty condenser-fan motor (replace it), or a restriction in the refrigerant system (locate and remove the restriction).
  • L
    Lauren BarryAug 28, 2025
    What to do if Carrier Heating System compressor won't start but condenser fan runs?
    • J
      Jon DelgadoAug 28, 2025
      If the compressor won't start but the condenser fan runs in your Carrier Heating System, check for faulty wiring or loose connections in the compressor circuit and repair or replace as needed. The compressor motor might be burned out, seized, or have an internal overload open, requiring you to determine the cause and replace the compressor. A defective overload could also be the problem, so determine the cause and replace it. If the compressor is locked out, determine the cause for the safety trip and reset the lockout. Finally, check if one leg of the 3-phase power is dead, and replace the fuse or reset the circuit breaker, determining the cause.
  • T
    Teresa ObrienAug 31, 2025
    Why is the suction pressure excessive in my Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28?
    • Z
      zhallAug 31, 2025
      To address excessive suction pressure in your Carrier Heating System, first check for and eliminate any sources of high heat load. Inspect the TXV, ensuring the bulb is tightly mounted to the suction line, and replace the TXV if faulty. If the refrigerant is overcharged, recover the excess refrigerant.
  • S
    samantha85Sep 4, 2025
    What causes excessive head pressure in a Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28 Heating System?
    • M
      Mandy MedinaSep 4, 2025
      If your Carrier Heating System has excessive head pressure, start by replacing the dirty air filter. Clean the dirty condenser coil. If the refrigerant is overcharged, recover the excess refrigerant. Inspect the TXV, ensuring the bulb is tightly mounted to the suction line, and replace the TXV if it's stuck open or closed. If there is air in the system, recover the refrigerant, evacuate the system, and recharge. Finally, determine and correct any restrictions to condenser air or air short-cycling.
  • M
    mia57Sep 8, 2025
    What causes low head pressure in Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28?
    • E
      Eric RamirezSep 8, 2025
      If the head pressure in your Carrier Heating System is too low, check for refrigerant leaks, repair them, and recharge the system. Also, inspect for and remove any restrictions in the liquid tube.

Summary

SAFETY CONSIDERATIONS

INSTALLATION

Covers unit installation steps and procedures.

START-UP AND SERVICE PROCEDURES

Covers preparation, operation, and maintenance.

TROUBLESHOOTING GUIDE

Common problems and their solutions.

INSTALLATION STEPS

ELECTRICAL CONNECTIONS

Make Electrical Connections

Connecting power and control wiring.

PRE-START-UP CHECKS

Compressor and Fan Checks

Verifying rotation, wiring, and refrigerant ports.

START-UP PROCEDURES

UNIT CHECKS AND GAS HEAT

Gas Heat Operation Verification

Procedures for checking gas heat system.

ECONOMISER IV SYSTEM OVERVIEW

EconoMi$er IV Components and Sensors

Location and description of EconoMi$er IV parts.

Controller Wiring and Modes

Setup and operation of the EconoMi$er IV controller.

ECONOMISER IV CONTROL MODES

DAMPER AND VENTILATION SETTINGS

Demand Control Ventilation (DCV)

Using DCV for ventilation control.

DEHUMIDIFICATION AND OPERATING SEQUENCES

Unit Operating Sequences

How the unit functions in cooling and heating modes.

ECONOMISER IV OPERATION AND SUBCOOLER

Cooling and Heating with EconoMi$er IV

Operation modes with EconoMi$er IV.

SERVICE AND COIL CLEANING

General Cleaning Guidelines

Basic cleaning procedures and recommendations.

Coil Cleaning Procedures

Detailed steps for cleaning coils using specific methods.

UNIT COMPONENT MAINTENANCE

CONDENSER FAN AND REFRIGERANT SYSTEM

Refrigerant System Evacuation

Procedures for evacuating the refrigerant system.

REFRIGERANT CHARGING PROCEDURES

PROTECTIVE DEVICES AND SYSTEM PROTECTION

Compressor & Fan Motor Protection

Overload, overtemperature, and lockout protection.

UNIT TROUBLESHOOTING

Cooling Service Analysis

Common cooling issues and solutions.

Gas Heating Service Analysis

Common heating issues and solutions.

ECONOMISER IV TROUBLESHOOTING

ECONOMISER IV FUNCTIONAL CHECKS

ELECTRICAL SYSTEM PROTECTION

UNIT START-UP CHECKLIST

Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28 Specifications

General IconGeneral
Model Number48PG20-28
Cooling Capacity20-28 tons
Heating TypeGas/Electric
Compressor TypeScroll
RefrigerantR-410A
Voltage208/230/460/575V
DimensionsVaries with model. Consult factory.
WeightVaries with model

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