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Air Conditioner or Heat Pump Diagnosis:
This article is the start of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains
how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperaturer, or an air conditioner that won't start.
First try our air conditioner checklist just below - try these steps before calling your HVAC service company.
If not enough cool air is provided by your air conditioner, or if the air temperature is not cool enough, or if you just can't get your A/C unit running, this article helps diagnose and correct the problem with step by step things to check and links to more detailed explanation when you need it.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
What to Check First if the Air Conditioner Output is Inadequate or A/C Won't Run
These Simple Checks May Enable Inexpensive Repair of Lost Cooling Output from an Air Conditioner
If you don't see information you want, ask us for it using the comments box On this page.
If your air conditioner or heat pump is a split system or mini split system design (using a wall-mounted cooling or heating unit) our troubleshooting diagnostics are at SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS.
Is there no cool air at all coming out of the supply registers?
Or is there air blowing out of the supply registers but it's not cool enough?
Here we explain how to diagnose loss of air conditioning cool air flow or cool air temperature.
Before ordering an expensive air conditioner service call to restore lost cooling capacity, here are a few simple steps
to perform. Some of these can be done by any homeowner, others may require a bit more expertise.
Below we provide first a very basic Air Conditioning or Heat Pump Diagnostic Checklist, followed by more detailed descriptions of common problem cooling system topics.
Following the checklist we provide more detailed descriptions, photos, and links to in-depth air conditioning inspection, diagnosis, and repair or improvement advice.
Where to start: check that the air conditioning equipment is turned on, the thermostat is calling for cooling, and that the blower unit or air handler is actually blowing air through the ductwork. Details of how to proceed in figuring out what's wrong with your air conditioner begin just below.
Air Conditioning Checklist for Lost Cooling: What to Do First in Air Conditioner / Heat Pump Tests
If the air conditioning system is not running at all:
Check the Room Thermostat Temperature Setting: Set the thermostat to at least 5 degrees below room temperature. Our elderly mom has no patience with switches and controls. She regularly calls her air conditioning service company with a service request, sometimes late at night, because she has simply failed to set the temperature on the thermostat lower than the room temperature. Don't drive your A/C like our mother.
Check that the Room Thermostat is set to "Cool"not "Off" or "Heat". If the thermostat is not set to "cool" it is simply turning off your A/C.
Check that electricity is on for the equipment. Check all switches and controls, including service switches, including outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel.
For example, condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner. There are several switches and controls, both manual and automatic that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch, an electric motor overload or overheat switches, and a condensate tray spillage detector switch.
See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system. A bad or failed starter capacitor could also be leaving your system shut down, failing to start a blower, fan, or compressor motor. Some hard-to-find switches could be keeping your air conditioner from starting, such as
If the air conditioning system is still not working, or if the A/C system is running but cool air is not coming out of the supply registers you probably need to call a service technician, but below are some things to check further yourself.
If At Least Some of the Air Conditioner or Heat Pump Equipment is Running but Cool Air is Not Being Delivered -
5 Quick Diagnostic Air Conditioning / Heat Pump System Checks to Get the Air Conditioner Running Again
The basic air conditioning diagnostic checks just below are followed by more detailed investigation of why the air conditioner is not working. But unless you already have a good idea of what's not working, start with these five steps.
If the air conditioning system is still not working, or if the A/C system is running but cool air is not coming out of the supply registers you probably need to call a service technician, but here are some things to check further yourself.
Just after these checks we discuss what to do if the air conditioner is running but air flow is too weak - A/C FLOW TOO WEAK
or what to do if the air conditioner or heat pump runs but in cooling mode the air supplied is too warm - LOST COOLING CAPACITY
Step 1: Check the Outside Air Conditioning /Heat Pump Compressor-Condenser unit
Step 2: Check the Air Conditioning /Heat Pump Indoor Air Handler Unit
Is the indoor blower unit running? If not be sure that the electrical power switch at your furnace or air conditioner air-handler is in the "on" position.
Typically the air handler or blower unit is indoors inside the basement, crawl area, or attic.
Sometimes there is more than one switch, such as one right at the unit and another at the entry to the room where the equipment is located, or even upstairs on a higher floor if your air handler is in a basement or crawl space.
Make sure that the blower compartment cover or door is properly closed or an interlock switch may be keeping the system "off".
Step 3: Weak air flow: Check the Air Conditioning /Heat Pump Air Filter(s)
If your air conditioning equipment is running but little or no cool air is coming out of the supply registers, check that your air filter(s) have not become blocked with dirt and debris.
Usually the air filter is right at the air handler or blower unit, or your air filter may be installed behind a grille covering a central warm-air return that sends air back to the air handler.
See A/C FILTER PROBLEMS for details as dirty air filters are a common cause of weak air flow and inadequate cooling.
If the air conditioner or heat pump is running but air flow is still too weak you may want to see our list of causes and cures for weak air flow at A/C FLOW TOO WEAK
Step 4: Weak air flow: Next Check the Air Conditioning /Heat Pump system duct work & air supply registers
In ceilings, walls, or floors, where cool air is supposed to be delivered to various rooms in the building, be sure that the register is in the "open" position (you will be able to see through it into the ductwork) and be sure you haven't blocked the supply registers with a carpet or furniture.
Look for a duct damper or register that has been closed; look for a flex-duct section that has become disconnected, bent, or crimped or squashed. Remember that a duct may have become disconnected in an attic or crawl space.
Look for an air filter that has come loose and blown into the ductwork, clogging it.
In our photo at above/left you can see a ceiling air supply register that has leak stains around its opening - further investigation for a condensate leak or roof leak is needed.
Also see RETURN AIR, HVAC - inadequate return air will reduce the supply of cool air in cooling mode or warm air in heating mode.
Step 5: No air flow at all: Check the Air Conditioning /Heat Pump indoor air handler blower fan assembly
If the indoor air handler blower fan itself won't start, see BLOWER FAN OPERATION & TESTING. Check for lost power, a fan motor that wont' start or has tripped a motor reset button or needs a start / run capacitor.
Check for a blower fan that is loose on the motor drive shaft or for a broken or lost blower fan drive belt on pulley-driven blower systems.
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
The air conditioning system operates but does not produce cool air, or not enough cool air. see LOST COOLING CAPACITY
The air conditioning system operates but there is not enough cool air flow at the registers. see DUCT SYSTEM DEFECTS.
The compressor was short-cycling, that is, turning itself on and off rapidly, perhaps every few seconds or minutes rather than producing a normal on-cycle of 10 minutes or longer. 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.
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. see AIR HANDLER / BLOWER UNITS.
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.
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. See DIRTY COIL CLEANING PROCEDURES,
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.
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.
OTHER AIR CLEANERS - how effective are other air cleaners such as free-standing air purifier units? Are some of them dangerous? Maybe. Are claims exaggerated? Usually.
CLEANING & Legionella BACTERIA how to remove or prevent bacterial growth in air conditioning systems, cleaning condensate trays, cooling towers, use of chlorine, bromine, disinfectants in air conditioners
Questions & answers or comments about air conditioner system diagnosis & repair
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
ACHR News Magazine,
2401 W. Big Beaver Rd., Suite 700
Troy, MI 48084, Tel: 1-800-837-8337, Email: firstname.lastname@example.org, Website: http://www.achrnews.com, Mike Murphy, Publisher,
Kyle Gargaro, Editor-in-Chief
email@example.com. This HVACR news magazine offers excellent heating, air condtioning and refrigeration systems education and troubleshooting help.
Timothy Hemm, Yucala, CA, contributed photographs of electrical wiring and equipment installed in California buildings. Mr. Hemm can be contacted at TimHemm@yahoo.com
A/C - HEAT PUMP CONTROLS & SWITCHES: 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. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
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 volume
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
Thanks to reader and research scientist Cyril Roberts, Barbados, for technical discussion and investigation of air conditioning system dehumidification problems (April 2009).
Thanks to readers Beth & Dennis for asking about how to improve an inadequate air conditioning system supplying cool air through crawl space ducts and floor registers. (May 2010).
Thanks to reader William Smith for discussing cooling coil leaks and lost cooling capacity diagnosis - June 2010
Thanks to reader Jacob Behrends, FL for discussing how a clogged condensate drain line can overflow condensate into a condensate pan that in turn may contain a safety switch that shuts down the whole air conditioning system. August 2010.
Determining Electric Motor Load and Efficiency, U.S. Department of Energy, web search 08/01/2011, original source: http://www.p2pays.org/ref/40/39569.pdf [copy on file at InspectAPedia.com]
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. 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 & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones