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• Air conditioning compressor hard-starting
• Air conditioner compressor defect diagnosis

This chapter of "How to Inspect the Central Air Conditioning or Cooling System" discusses the diagnosis and repair of hard-starting air conditioning compressors. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

HARD STARTING - How to Diagnose & Repair Air Conditioner Compressor Hard Starting

• Stuttering or on-off at startup: A "hard starting" compressor may stutter or begin to cycle-on then stop, then restart. If a compressor is frequently tripping the circuit breaker (or blowing the fuse) which protects its circuit, compressor and wiring diagnosis and repair are needed.
• Low line voltage: Abnormally low line voltage may also be causing a compressor to "hard start". Air conditioner supply voltage is typically checked using a VOM (voltage ohms meter) right at the air conditioner service disconnect box near the compressor/condenser unit. Voltage should be within 10% of the required number (on the compressor/condenser data tag). A service technician may also check line voltage again at the compressor terminals when the compressor has reached normal operating state to be sure that there is no voltage drop.
• Starting an air conditioner against compressor head-pressure: When an air conditioning compressor has been running long enough to reach its normal operating condition, it has pumped refrigerant to a high pressure condition in the compressor head. When an A/C system compressor is running and is unexpectedly shut off, perhaps by a human testing a thermostat or switch, if the system is immediately turned back on, some compressors, particularly older window and wall units, may be unable to re-start against this high head pressure. Simply waiting a few minutes for pressures to equalized may be all that's needed. So if the compressor is not starting in this condition we simply turn it off and wait. If this problem is happening often, a service technician may install one of several versions of "hard start kit".
• Do not just install a larger fuse or circuit breaker as doing so risks a fire or equipment burn up. Do not bypass fuses such as by installing copper tubing in place of fuses as some A/C service people recommend. This is a crazy electrical hazard risking fire, shock, and equipment burn-up. If new circuit breakers are to be installed to replace an older fuse-protection on an air conditioning circuit, the electrician should remove any obsolete, unused electrical equipment or devices to avoid future confusion or possible shock hazards.
• A "hard start kit" is basically a capacitor which gives an extra electrical "jolt" to the compressor motor to get it moving. A starter relay may also need to be installed or replaced. Some air conditioning compressor brands do not usually need a hard start kit. GE, Trane, and Manurope are examples that do not usually take this kit. See   A/C CAPACITORS for photos, wiring diagram, and installation instructions for air conditioner compressor, fan, blower, refrigerator motor, freezer motor, or other electrical motor starting booster capacitors.
• Tight or Seized Air Conditioner Motors can be hard to start and may make a "humming" noise while trying to start-up and the compressor motor may fail to start at all, eventually tripping the circuit breaker or blowing the circuit fuse. It might be possible to get a slow or hard-starting tight or even seized AC compressor motor going again - see TIGHT or SEIZED AC COMPRESSORS for more details on how this is done. But in any case such a compressor is probably near the end of its life.
• End of air conditioning compressor life may be near: A compressor which has difficulty starting might be fixed by installing a "hard start" kit, but depending on the reason for hard starting it's possible that the entire compressor will have to be replaced soon. On a more optimistic note, some hard start kit manufacturers assert that installing a "hard start kit" on a compressor will extend its life. "By assisting the compressor in starting at up to ten times faster than normal, Kickstart(R) hard start devices significantly reduce the amount of damaging heat that is generated in the motor windings with each and every start. Over time, this reduced stress on insulation, wiring, and other critical components of the compressor has the effect of increasing its reliability and extending its useful life." -- www.kickstartoem.com

AIR CONDITIONING SYSTEMS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNIT
CLEANING & Legionella BACTERIA
COMPRESSOR CONDENSER
  BURNED-OUT COMPRESSOR
  CAPACITORS for HARD STARTING MOTORS
  CONDENSING COIL FINS, SUPPORT PADS
  HARD STARTING COMPRESSOR MOTORS
  INSTALLATION ERRORS, COMPRESSORS
  MOTOR OVERLOAD RESET SWITCH
  NOISY COMPRESSOR UNITS
  PRESSURE READINGS, COMPRESSOR
  REPLACING A COMPRESSOR
  TIGHT or SEIZED AC COMPRESSORS
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
  FROM MODEL #
  FROM EQUIPMENT RLA #
  COOLING RULES OF THUMB
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
FAN AUTO ON Thermostat Switch
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
  What to check first
  Compressor failure diagnosis
  Duct & Air Handler diagnosis
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
  Air Conditioning System Temperatures
  Instruments Used to Measure A/C Temperatures
  Procedures for Making Temperature Measurements
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS
Air Conditioning "How To" Books

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AIR CONDITIONING SYSTEMS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNIT
CLEANING & Legionella BACTERIA
COMPRESSOR CONDENSER
  BURNED-OUT COMPRESSOR
  CAPACITORS for HARD STARTING MOTORS
  CONDENSING COIL FINS, SUPPORT PADS
  HARD STARTING COMPRESSOR MOTORS
  INSTALLATION ERRORS, COMPRESSORS
  MOTOR OVERLOAD RESET SWITCH
  NOISY COMPRESSOR UNITS
  PRESSURE READINGS, COMPRESSOR
  REPLACING A COMPRESSOR
  TIGHT or SEIZED AC COMPRESSORS
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
  FROM MODEL #
  FROM EQUIPMENT RLA #
  COOLING RULES OF THUMB
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
FAN AUTO ON Thermostat Switch
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
  What to check first
  Compressor failure diagnosis
  Duct & Air Handler diagnosis
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
  Air Conditioning System Temperatures
  Instruments Used to Measure A/C Temperatures
  Procedures for Making Temperature Measurements
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS
Air Conditioning "How To" Books

Technical Reviewers

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).

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