Air Conditioner or Heat Pump Diagnostic Procedures List.

For each component in an A/C or heat pump system that might have failed, this article lists articles useful to diagnose and repair the problem.

Major Areas to Investigate for Problems With Air Conditioning Systems & Equipment

For air conditioner or heat pump troubleshooting that can be done by many building owners or occupants,

Start Here: DIAGNOSTIC GUIDE A/C or HEAT PUMP - quick easysteps to diagnose an air conditioner or heat pump.

Or for more detailed troubleshooting and procedures used by trained HVAC repair technicians, can see our list of detailed diagnosis and repair procedures given below.

A/C Heat Pump Repair Topic Areas

• AIR DELIVERY: SUPPLY & RETURN DUCTS
• AIR FILTERS
• AIR HANDLER / BLOWER UNIT
• COMPRESSOR / CONDENSER UNIT
• CONDENSATE HANDLING
• CONTROLS & SWITCHES
• COOLING / EVAPORATOR COIL
• ELECTRICAL PROBLEMS
• HEAT PUMPS & BACKUP HEAT
• OPERATING CONDITIONS / PROBLEMS
• PARTS - AIR CONDITIONING or HEAT PUMP - what are the parts of an air conditioner or heat pump, and what do the do?
• RATED COOLING CAPACITY
• REFRIGERANT CHARGE

Air Delivery: Supply & Return Ducts

The air conditioning system operates, both indoor blower and outdoor compressor are running, but the does not produce cool air, or not enough cool air: see

• AIR FLOW TOO WEAK
  
• COOL AIR SUPPLY IMPROVEMENT

The air conditioning system operates but there is not enough cool air flow at the registers: see these diagnostic guides:

• DUCT SYSTEM DEFECTS - home - problems with the air duct system, air filters, supply registers, return air registers
• ASBESTOS HVAC DUCTS how to recognize asbestos materials on heating and cooling air ducts and equipment
• BALANCING AIR DUCT FLOW, how to balance the air flow in a heating or cooling system
• DUCT ROUTING & SUPPORT errors in duct routing and support can cause poor air flow or air quality and mold problems
• FIBERGLASS HVAC DUCTS fiberglass contamination of indoor air, fiberglass as a possible mold source
• GOODMAN GRAY FLEXDUCT deterioration of flex duct, cause, cure
• LEAKY DUCT CONNECTIONS, how to diagnose and correct leaky ducts
• OWENS CORNING FLEXDUCT deterioration of flex duct, cause, cure
• REGISTERS & DUCT LOCATIONS, improper placement can reduce air conditioning capacity or may be unsafe
• RETURN AIR, INCREASE, poor air flow cause and cure, how to increase return air and thus cooling air flow
• RETURN AIR REGISTERS & DUCTS , how to inspect for defects
• SUPPLY DUCTS & REGISTERS, how to inspect for defects
• UNDERSIZED RETURN DUCTS, how to recognize under-sized air conditioning ducts or inadequate return air
• TRANSITE PIPE AIR DUCT ASBESTOS RISKS as a possible source of asbestos, blocked ducts, reduced air flow, mold
• UNSAFE DUCT OPENINGS as a possible source of air quality issues or carbon monoxide poisoning hazards
• VIBRATION DAMPENERS and asbestos hazards at the air conditioner air handling unit or blower
• WET CORRODED DUCT WORK, water and rust as indicators of duct conditions, possible mold or other pathogenic health hazards
• ZONE DAMPER CONTROLS - discusses manual and automatic air duct zone controls

Air Filters

• Dirty air conditioning filters, - a source of increased operating costs. Very dirty filters can eventually block the fan itself, leading to more costly repairs.
  
  The filters should be changed monthly when the system is in use. Failure to properly filter dust from the return air supply can load the fan or evaporator coil with dust and prevent proper system operation. Installing a filter is normally a minor expense.
  
  Dirty, blocked evaporator coils can result in improper system operation, very low output temperatures, low air flow (compensated sometimes by increased fan speed), and sometimes frost on the coil and failure of system components.
  
  See AIR FILTERS for HVAC SYSTEMS,
  and
  
  also AIR HANDLER / BLOWER UNITS.
  
  See OPERATING TEMPERATURES.
• AIR FILTERS for HVAC SYSTEMS - air filters are critical not only for building health and comfort but to keep an adequate air volume moving through the air conditioning or heat pump system
• AIR FILTER EFFICIENCY - what determines how effective an air filter will be
• GASKETED AIR FILTERS, leaks, debris in the return air plenum,
• HOW AIR FILTERS WORK
• MERV, HEPA Definitions
• MERV Air Filter Ratings
• WASHABLE AIR FILTERS
• AIR FILTER LEAKS or MISSING COMPLETELY - what happens if we simply omit the air filter - some costly problems develop
• AIR FILTER LOCATION - where should air filters be located
• AIR FILTERS, OPTIMUM INDOOR - what is the absolute best, most effective way to clean indoor air
• AIRBORNE PARTICLE SIZE DEFINITIONS & TYPES
• AIRBORNE PARTICLE SIZE TABLE
• DIRTY AIR FILTER PROBLEMS - what happens when the air filter gets dirty - how often should filters be changed?
• FIBERGLASS & AIR FILTERS - how these filters work, fiber shedding, fiber sizes
• SOURCES FOR AIR FILTERS - where to obtain air filters of various sorts, how effective are other air cleaners such as free-standing air purifier units? Are some of them dangerous? Maybe. Are claims exaggerated? Usually.
• Legionella BACTERIA & HVAC EQUIPMENT how to remove or prevent bacterial growth in air conditioning systems, cleaning condensate trays, cooling towers, use of chlorine, bromine, disinfectants in air conditioners

Air Handler Unit

Inside cooling or evaporator coil defects

that can be seen by eye: dirty coil, blocked coil, frost on the coil, improperly sized evaporator coil, improper evaporator or cooling coil placement in the system.

Air flow across the air conditioning evaporator coil:

if airflow is weak for any reason (dirty coil, duct system defects, blower fan defects, dirty blower squirrel cage fan), the air conditioning system will not operate properly.

Some experts write that there should be between 350 and 400 cubic feet of air per minute (CFM) moving across the evaporator (cooling) coil for each ton of air conditioner capacity.

One ton = 12,000 BTUH so if your AC unit is a 24,000 BTUH unit it is a "two ton" unit and needs to see 700 to 800 CFM of air across the evaporator coil.

Some home inspectors and air conditioning service technicians carry a small airflow meter that can actually measure this number with fair accuracy.

The same tool is nice for comparing air flow and balancing air flow at various building supply ducts and registers.

See COOLING COIL or EVAPORATOR COIL
and

also DIRTY COOLING COIL / EVAPORATOR COIL - how dirt and debris appear, what to do about a dirty evaporator coil.

Evaporator coil cleaning

often requires cutting refrigerant lines, removal of the coil and other components for cleaning, and reinstallation, pulling a vacuum on the refrigerant lines, and recharge with refrigerant. Such service and repair may involve significant expense, although there are some "in place" cleaning methods using foams and sprays that are a simpler procedure.

• AIR HANDLER / BLOWER UNITS which describes problems with the air handler, air filters, and the cooling coil itself
• ADDING A/C: RETROFIT SIZING when adding air conditioning to an existing warm air furnace heater
• AIR HANDLER IS DIRTY
• AIR HANDLER LEAKS, RODENTS, PATHOGENS
• AIR HANDLER MOLD CONTAMINATION
• BLOWER FAN AUTO ON CONTROLS - for air conditioning or heating blower fan units
• BLOWER FAN OPERATION & TESTING - problems with the blower fan on furnaces, heat pumps, and air conditioners
• BLOWER FAN WON'T START - trouble with the blower fan in the indoor air handler unit
• BLOWER FAN WON'T STOP - LIMIT SWITCH - sometimes the indoor blower fan never stops - due to a bad fan limit control
• BLOWER FAN WON'T STOP - THERMOSTAT SWITCH - other reasons the blower fan may keep running
• BLOWER LEAKS, RUST & MOLD - damage that can prevent the blower from working or may be blowing contaminants into the occupied rooms
• DIRTY HVAC COIL CLEANING PROCEDURES,
• FAN, AIR HANDLER BLOWER UNIT - diagnose and fix problems with the indoor furnace or air conditioner blower unit fan and fan motor
• FROST BUILD-UP on AIR CONDITIONER COILS
  

Compressor / Condenser Unit

• COMPRESSOR / CONDENSER REPAIR - home - problems with air conditioner compressor/condenser units - us this link to find our full list of HVAC condenser/compressor diagnostic & repair articles.
• BURNED-OUT COMPRESSOR - how we diagnose a compressor motor that is burned out
• COMPRESSOR MOTOR REPLACEMENT - steps in installing a new air conditioner compressor
• COMPRESSOR PRESSURE READINGS - what are the usual system pressures in an air conditioner or heat pump?
• CONDENSING COIL REPAIR REPLACE
• COMPRESSOR NOISES - diagnosing and curing a noisy air conditioner compressor unit
• FAN, COMPRESSOR / CONDENSER UNIT - diagnose and fix problems with the outdoor compressor/condenser fan and fan motor
• HARD STARTING - how to fix an air conditioner compressor motor that won't start
• INSTALLATION ERRORS
• TIGHT or SEIZED AC COMPRESSORS - how to diagnose a compressor that needs replacement

Condensate Handling

• CONDENSATE HANDLING, HVAC - home - cooling air produces condensate that must be disposed-of properly
• CONDENSATE DRAINS, proper routing and potential health concerns
• CONDENSATE LEAKS
• CONDENSATE PUMPS, where and how to use them
• DRIP TRAY DEFECTS can cause leaks into a building interior and may be a mold source

Controls & Switches

• CONTROLS & SWITCHES on A/C or HEAT PUMP - air conditioner controls and switches - begin here if your A/C won't start.
  
  Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week.
  
  Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control and some refrigeration class instructions opine that 80% of problems on these systems are electrical in nature.
  
  So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
• SAFETY CONTROLS what safety controls and switches are required on air conditioners?

Cooling / Evaporator Coil

• COOLING COIL or EVAPORATOR COIL - problems at the evaporator coil can cause total loss of cooling capacity (or heating with a heat pump)
  • DAMAGED COOLING COIL
  • DIRTY COOLING COIL / EVAPORATOR COIL - how dirt and debris appear, what to do about a dirty evaporator coil
  • DIRTY HVAC COIL CLEANING PROCEDURES - a review of methods for cleaning a dirty A/C coil
  • FROST BUILD-UP on AIR CONDITIONER COILS - the cause and cure of ice on the A/C cooling coil
• EVAPORATIVE COOLING SYSTEMS - how can a "swamp cooler" or evaporative cooling system best deliver cool air in a building?

Electrical Problems

• A/C CAPACITORS - using starter capacitors on an air conditioner compressor or other electric motors
• AIR CONDITIONER WON'T START - is it turned on? What else keeps the system from starting?
• The air conditioner fuse keeps blowing or the circuit breaker trips: could mean a compressor which needs repair or replacement, or it could mean a problem with the unit's electrical wiring, such as use of aluminum wiring combined with bad electrical connections.
  
  Have your service technician check the condition of the electrical circuit first (that's cheaper than a new air conditioning compressor) and then check the condition of the compressor.
  
  See COMPRESSOR CONDENSER for air conditioner compressor diagnosis and repair;
  
  and if your air conditioning system electrical wiring is aluminum, review the aluminum wiring hazards
  
  at ALUMINUM WIRING HAZARDS & REPAIRS since a serious fire hazard could be present.
• The air conditioner operates but our electrical bills are too high. See
  
  DIRTY HVAC COIL CLEANING PROCEDURES, SEER RATINGS and
  
  DUCT SYSTEM DEFECTS
• ELECTRIC MOTOR DIAGNOSTIC GUIDE - diagnostic procedures for electric motors used on oil burners, air handler blower assemblies and fans, air conditioners, heat pumps, well pumps, etc.

Heat Pumps & Backup Heat

• BACKUP HEAT for HEAT PUMPS - types of backup heat, diagnosing back up heat pump heat not working properly, types of backup heat; problems with backup heat; begin here if your heat pump is not providing enough heat or if your air conditioning system provides heat when it should be providing cooling
• HEAT PUMPS - home - includes heat pump terms, definitions & terminology, diagnosis & repair articles specifically for heat pumps
• HEAT PUMPS, GROUNDWATER - what are the COP's and EER's of groundwater based heat pump systems?
• HEAT PUMP STRATEGY - INDOORS? - can we locate a heat pump or water heating heat pump indoors to scavenge heat?
• HEAT PUMP THERMOSTAT, OUTDOOR - failures in an outdoor component can cause odd heat pump operation or noises
• REVERSING VALVE on HEAT PUMPS - when this control fails the system may work only in heating mode, or only in cooling mode, but not both

Operating Conditions, Noised & Defects

• AIR CONDITIONING SYSTEM ODORS - diagnose smells from ductwork or from the air handler or at the compressor unit
• AIR CONDITIONER TEMPERATURES - what are the normal input and output temperatures and air temperatures & where are the measured?
• LOST COOLING CAPACITY - what to do when not enough cool air comes out of the system - use this link to see our key diagnostic articles if your A/C unit is not cooling or the air flow is too weak.
• OPERATING DEFECTS - major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volumes
• The air conditioning system will not operate at all: power may be off, controls may be improperly set, or the system may be inoperative. (Confirm that the system has electrical power and that all of its control switches and thermostat are set to on and cooling positions.
  
  see CONTROLS & SWITCHES
  
  If the indoor air handler blower unit does not start
  
  see: AIR CONDITIONER WON'T START
  
  If the outdoor compressor condenser does not start,
  
  see: COMPRESSOR / CONDENSER DIAGNOSTICS
  
  see COMPRESSOR CONDENSER
• The compressor was noisy, during startup, indicating an operating problem.
  
  see COMPRESSOR NOISES.
• The compressor squealed at startup, indicating that service may be needed promptly to prevent possibly costly damage to the system. This repair/maintenance item should not be deferred.
  
  see COMPRESSOR NOISES.
• DEHUMIDIFICATION PROBLEMS - air conditioner cools but does not dehumidify - indoor humidity remains too high
• SHORT CYCLING AC COMPRESSOR - what causes the air conditioner compressor to keep turning on and off too quickly?
• OPERATING TEMPERATURES - what are the normal operating temperatures at the air handler or compressor/condenser unit?
  

Refrigant Gas Charge, Leaks, Repairs

• REFRIGERANTS in AIR CONDITIONERS & HEAT PUMPS: lists historic and currently in-use air conditioning refrigerants and gives their characteristics
  • REFRIGERANT LEAK DETECTION - how to find where refrigerant gas leaks are happening
  • REFRIGERANT LEAK REPAIR - how to fix air conditioning or heat pump refrigerant gas leaks

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-07-25 by Brandon - AC Compressor won't run

I have a outside heat pump unit and the heat works , but when I turn on A/C the compressor doesn’t come on. The plug to compressor was unplugged so I plugged it in and unit sounded like it shorted out. Any thoughts

On 2020-07-25 by (mod)

That sounds to me as if a relay or control board has shorted out or worse, a short circuit occurred in the compressor motor itself.

On 2019-10-03 by Anonymous - musty odor from the air conditioner

A musty smell when unit comes on

On 2019-10-03 by (mod)

Anon: check for mold and water in the air handler supply and return plenums and in the ductwork.

See details at HVAC SYSTEM ODORS 

On 2019-07-28 by Craig

15 y/o heat pump condensing unit fan spins at slow speed then shuts off and starts again in ac mode. Blower in attic pushs warm vs cold air.

On 2019-07-28 - by (mod) -

Could be a bad fan motor or motor capacitor or control board.

Take a look at CONDENSER FAN WON'T START

Keep me posted, Craig.

On 2019-07-19 by Rick

My control board has power going to transformer but no power out. Is it a bad transformer?

On 2019-07-19 - by (mod) -

It certainly could be a bad Transformer, or something as simple as a loose or broken connector. I would check all the connectors in and out before replacing the parts.

...

Continue reading at COMPRESSOR / CONDENSER DIAGNOSTICS, or see AIR HANDLER / BLOWER UNIT DIAGNOSTICS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• AIR CONDITIONING & HEAT PUMP SYSTEMS - home
• AIR CONDITIONER / HEAT PUMP OPERATION - how does an air conditioner or heat pump actually work?
• COMPRESSOR / CONDENSER REPAIR - home
• COMPRESSOR / CONDENSER DIAGNOSTICS
• CONTROLS & SWITCHES on A/C or HEAT PUMP - what and where are all of the controls?
• DIAGNOSTIC GUIDE A/C or HEAT PUMP - first steps to diagnose an air conditioner or heat pump
• DIAGNOSTIC DETAILS A/C or HEAT PUMP - detailed list of diagnostic procedures - you are on this page
  • AIR CONDITIONER WON'T START
  • BURNED-OUT COMPRESSOR
  • CAPACITORS for HARD STARTING MOTORS
  • CONDENSATE PAN OVERFLOW SWITCH
  • CONDENSER FAN WON'T START
  • ELECTRIC MOTOR DIAGNOSTIC GUIDE
  • FURNACE FAN WONT START
• DUCT SYSTEM DESIGN SIZE & DEFECTS - home
• MANUALS & PARTS GUIDES - HVAC - home
• SEER RATINGS & OTHER DEFINITIONS - for air conditioners and heat pumps
• SPLIT SYSTEM AC / HEAT PUMP REPAIRS

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DIAGNOSTIC DETAILS A/C or HEAT PUMP at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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Citations & References

In addition to any citations in the article above, a full list is available on request.

• Timothy Hemm, Yucala, CA, contributed photographs of electrical wiring and equipment installed in California buildings. Mr. Hemm can be contacted at TimHemm@yahoo.com
• In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
  
  CONTINUE READING or RECOMMENDED ARTICLES.

• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
  
  Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The HOME REFERENCE BOOK - the Encyclopedia of Homes and to use illustrations from The ILLUSTRATED HOME .
  
  Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.

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