How to Diagnose & Repair Loss of Air Conditioner / Heat Pump Cooling Capacity or an Air Conditioner that is Not Working InspectAPedia® -
How to diagnose & repair loss of air conditioning cooling capacity or has stopped working - Air conditioner repair checklist: before calling an air conditioning service technician check these items
How to diagnose & repair loss of heating or cooling capacity from an A/C or heat pump system - weak air flow or air flow not at the right temperature
Air conditioner repair checklist: before calling an air conditioning service technician check these items
Air conditioner compressor defect diagnosis, How to diagnose and fix an air conditioning system that is not working
Causes & cures of air conditioner or heat pump compressor hard starting or short cycling on and off
Questions & Answers about air conditioner system diagnosis & repair
This article discusses
how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity or an air conditioner that won't start. These articles also diagnose lost heating capacity if your HVAC system uses a heat pump. If you don't see information you want, ask us for it using the comments box on this page.
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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 provide links to other air conditioning system diagnostic articles too.
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
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. 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.
First try our air conditioner checklist just below - try these steps before calling your HVAC service company.
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.
Watch out:
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 a Float Switch on Condensate Tray or a blower MOTOR OVERLOAD RESET SWITCH
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 too warm.
Step 1: Check the Outside Air Conditioning /Heat Pump Compressor-Condenser unit
Check the outside compressor condensing coil for damage or blockage.
A blocked condensing coil can cause the compressor to overheat, stop running, or become noisy and work poorly. A/C Compressor Problems
If you cannot find your air conditioner controls and switches see A/C - HEAT PUMP CONTROLS & SWITCHES - list of controls and switches found on air conditioners and heat pumps
Also check the compressor/condenser outdoor fan and fan motor
At BURNED-OUT COMPRESSOR we explain that if the condensing unit fan is not working the compressor itself may shut down or even be ruined by overpressure or over temperature. Also see NOISES, COMPRESSOR CONDENSER where some noise problems are traced to the cooling fan .
Step 2: Check the Air Conditioning /Heat Pump Indoor Air Handler
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: 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
Step 4: 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.
At 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.
Step 5: 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.
How to Diagnose Air Conditioner Output Cool Air Flow Too Weak, Too Cold, or Too Warm
Air Conditioner Blower fan unit not moving enough air: too little air coming out of your air conditioning ducts?
Check the condition of the blower unit: if it's dirty the blower may be spinning but not moving much air. See DIRTY A/C BLOWERS for details.
Dirty filters or iced coils or crimped or disconnected air ducts can also cause loss of cool air or too little cool air coming out of supply registers.
Air flow that is too slow for any reason (such as a dirty filter or dirty blower fan assembly blades) can cause first, air temperatures that are abnormally low coming out of the air conditioner, and eventually a reduction in air flow as coil ices over. see A/C Cooling Coil Icing.
Air flow that is too fast for any reason (improper fan motor, speed, fan belt or pulley size, duct design, duct registers removed, etc) can produce air that is not cool enough and can prevent proper air dehumidification (oversized A/C system). A/C Not Dehumidifying.
Check and Fix Existing Air Conditioning Problems Before Trying to Improve its Cooling Capacity
Watch out: It could be a mistake to spend on any costly air conditioning improvement if the original system actually already had the needed capacity, but simply needed maintenance (like replacing a filter - A/C Filter Problems) or repair (like repairing a leaky duct - A/C Air Duct Problems).
FIRST: review all of the air conditioning performance diagnosis suggestions here at -
LOST COOLING CAPACITY to be sure there is not something that needs to be repaired or corrected with the system. For example we have seen cool air output significantly improved by finding and fixing a disconnected cool air duct that no one had noticed in a hard-to-access crawl area.
Certainly some central air conditioning systems, especially retrofit designs, have less than optimum duct design, such as inadequate central return air registers, room doors that are so tight that it is difficult to blow cool air into the room when the door is closed, inadequate supply ducts or less than optimum supply register placement such as only in floors. But redesigning and installing for an optimum air conditioning duct system can be costly, so it might be worth taking some other steps.
SECOND: make sure that other energy-savings steps have been taken in the building, such as proper insulation, sealing un-wanted drafts, etc. See ENERGY SAVINGS in buildings - those suggestions apply to both heating and cooling costs.
If your HVAC service technician is sure that the present cooling system is working as intended, then if you are not considering supply or return air duct improvements, or a replacement air conditioner system of higher capacity, you might consider a stronger blower fan (discuss what you could do with your HVAC tech), or booster fans individual registers.
Watch out - just boosting air conditioner air flow by installing a higher capacity blower fan is not that simple - if the blower moves air through the duct system faster than the design speed for the system ,the cooling coil may be insufficient in capacity, the air will not be dehumidified (and the building will be less comfortable - A/C Not Dehumidifying), or you might get annoying duct or equipment noises. Discuss these possibilities with your HVAC service company.
Air Conditioner Filter Clogging Problems Cause Weak Cool Air Flow
Clogged Air Conditioner filters can lead to lost cooling capacity first, because the clogged filter reduces the air flow
through the system, meaning that you'll feel less air flow at the supply registers than was previously present.
Air Conditioner Compressor Problems - noisy or hard starting air conditioner compressors
Compressor problems - Air Conditioner Compressor: problems such as an aging air conditioner compressor motor that is at or near the end of its life may be unable to
properly compress the returning refrigerant gas to a sufficiently high pressure. A service technician will need to evaluate and test the system
and if needed, replace the compressor.
Since this is a costly repair, be sure to ask why the compressor failed and to correct
any underlying cause (such as low voltage). Variations in line voltage can lead to improper compressor operation and loss of cooling output.
If you have a hard-starting air conditioner compressor that "hums" but doesn't start, it may be possible to get more life out of the compressor motor with a simple starting capacitor (rather than a costly whole new compressor motor.) See and HARD STARTING COMPRESSOR MOTORS and CAPACITORS for HARD STARTING MOTORS.
We first mentioned at NOISES, COMPRESSOR CONDENSER that we had a field report from a reader who explained that a noisy outdoor compressor unit was, according to his HVAC service technician, traced to a blocked, clogged outdoor condensing coil. We speculate that perhaps the compressor was running hot and that correcting air flow across the condensing coil corrected that condition.
COMPRESSOR/CONDENSER COOLING - Can Cooling Down the Air Conditioner Compressor Motor to Keep the A/C Working?
Several readers have described a step of cooling down an overheated air conditioning or heat pump compressor motor to get it running again producing cool air indoors. One reader used a garden hose to spray the outdoor unit (after all it's intended to tolerate being rained-on). Another used a watering can. A third, buckets of water.
All three reported that this step got their cooling system cooling again when before either the compressor had stopped running entirely or it was running but the system was not producing cool air. But for several reasons this is not an effective durable repair:
The "repair" achieved by pouring water over a hot compressor is not lasting and if you wet electrical components it could cause damage or be dangerous
It's hot standing outside dumping water on your A/C unit and you're not supposed to have to do that
The loss of cooling means there is some problem to be found and fixed
While we wait for more comments from air conditioning service technicians and experts (CONTACT us), here are some interpretations of what may be going on:
The A/C compressor motor may be at or near end of life, running hot. An air conditioner motor that is overheating may turn itself off on thermal reset. In this case when the motor cools down it may automatically reset itself and start up again - for a while. Sometimes you can get a compressor motor starting again on its own by adding or replacing a starting capacitor - see CAPACITORS for HARD STARTING MOTORS. Also see MOTOR OVERLOAD RESET SWITCH. Also see HARD STARTING COMPRESSOR MOTORS.
The A/C compressor motor may be overheating due to its location: for example in hot sun. Most equipment will tolerate this condition but an older unit or very hot sunny conditions may lead to thermal overheating and an overload circuit trip-off.
The A/C system compressor/condenser fan may not be running or may be damaged, failing to adequately cool the refrigerant gas in the condensing coils. Watch the fan to see that it is running while the compressor motor is on, and perhaps even before and after the compressor motor starts or stops. [But not indefinitely.]
The A/C condenser unit coils may be blocked, bent, dirty, damaged. It may be possible to clean the coils of leaves, dirt, grass kicked up by nearby mowing, etc. and thus improve the condenser operation and start making cool air again. COMPRESSOR & CONDENSING COIL
The air conditioning or heat pump system may be very low on refrigerant. You'll need a service technician to check and repair this condition. REFRIGERANT LEAK DETECTION
Air Conditioner Cooling Coil Icing Problems & Coil Leaks
Cooling Coil Ice-Up on the air conditioner air handler or evaporator coil: A second result of the reduced air flow due to a dirty air conditioning filter can cause
the evaporator coil (the cooling coil) in the
air handler to become blocked by ice, stopping or significantly reducing cool air output from the system.
A visual inspection
of the cooling coil in the air handler can quickly show whether or not it's ice-covered. If the coil is iced-up and blocked,
turn off the cooling system entirely until the ice has all evaporated and cleared. Be sure that your condensate drain is
not blocked and that the water from melting ice will be properly disposed-of.
If you replace all dirty filters on the
system and remove ice from an iced coil and the coil ices up again when the system is turned back on, you may have the next
condition in this list.
Cooling Coil (Evaporator Coil) Leaks, Condensing Coil Leaks: if your A/C system used to work fine but now is blowing only warm air, it is possible that you have lost refrigerant in the system. A refrigerant leak can occur anywhere in the refrigerant piping system, in the cooling coil, or in the condensing coil.
You'll need an HVAC technician to find and repair the leak problem. If the leak is in a refrigerant line it can usually be soldered and sealed and the system re-charged. If the leak is in a cooling coil or condensing coil, the coil probably needs to be replaced. We explain this topic in more detail below at Air Conditioner Refrigerant Problems - Refrigerant Leaks, Low Refrigerant, Wrong Temperatures.
Air Conditioning System Dehumidification Problem Diagnosis & Cures
Details about inadequate dehumidification by central air conditioning and heat pump systems are found at DEHUMIDIFICATION PROBLEMS. Excerpts are below.
Air Conditioning Dehumidification Problems: air conditioning system cools but does not dehumidify the room. The most common cause of inadequate dehumidification by an air conditioning system is the installation of a cooling unit which has too much capacity, or is "over-sized" for the space it is being used to cool. What happens is simple:
If an air conditioning compressor unit is oversized (too many BTUH of cooling capacity) what happens is it cools the room so quickly that the system does not move enough total volume of air across the cooling coil to remove much moisture before the room temperature has dropped to the A/C cut-off point.
In other words, an air conditioner needs to run longer, and move more total volume of air through itself to drop room humidity than it does to just cool the air.
"Bigger" cooling capacity or higher BTU capacity for an air conditioning system is not necessarily better, and it can actually prevent the air conditioner from adequately dehumidifying indoor air.
Also see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs and see RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.
Air Conditioning Air Duct Problems and Cool Air Flow Defects - Poor Air Flow
Duct problems: Damaged Air Conditioning Ducts such as ducts which have been improperly routed and are crimped, crushed, or have
excessive bends can reduce cool air flow in an otherwise properly functioning system.
One client had us drive a considerable distance to repair her apartment's central air conditioning system after having had several unsuccessful service calls.
Apparently
no one had managed to get into a rather tight and hard-to-enter attic crawl space where the cooling ducts were routed.
We found that the main cooling duct had become disconnected.
The attic was nice and cool but no cool air was being blown
into the living area. Check the condition of the duct system for blocked ducts, loose leaky connections, closed dampers,
crimps and bends, before calling your service technician.
See DUCT SYSTEM DEFECTS for details of various A/C supply and return air duct and register problems and how to detect and correct them.
Air Conditioner Won't Start - electrical problems
As we stated earlier, check that electricity is on for the equipment, everywhere. 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.
Electrical problems: air conditioning system won't start: it may sound silly, but is the air conditioner turned on?
Has the cooling thermostat been set to "cool" and the temperature set below ambient room temperature?
Do both the outdoor compressor/condenser unit and the indoor blower fan/evaporator coil unit have electrical power?
Has the air conditioning electrical wiring been physically damaged or cut? Photo courtesy of Tim Hemm.
Are the power switches on at these units, are the fuses good, are the circuit breakers in the "on" position, and is the thermostat set correctly? See A/C - HEAT PUMP CONTROLS & SWITCHES and THERMOSTATS for some diagnostic tips.
Air Conditioner Won't Start or Stays Off Due to Condensate Pan Switch
Condensate pan switch lockout: an attic or other air conditioner air handler condensate drip tray or drip pan located under an air handler is installed to catch air handler condensate leaks if the normal condensate drain system fails. Some condensate pans have their own separate overflow drain (a proper installation) or share their drain with the normal condensate drain (an improper installation).
But on some air conditioning air handlers the installer may provide a condensate overflow pan switch rather than a separate pan drain line. In this installation the switch is designed to turn off the air conditioning system if it finds condensate water in the overflow pan. The idea is to shut down the air conditioner before there is a more costly leak into the building insulation or ceiling.
Condensate leaks into the condensate pan can shut down the air conditioner.
In tracking down an air conditioner condensate leak, I found that the the condensate pump drain line, a small-diameter PVC pipe, was clogged with water mold.That was why my air conditioner wasn't kicking on. It would of ended up costing at least a hundred dollars just for a service tech to fix something as simple as that. I had to take the pump apart and clean it. I took off the PVC drain line coming out of inside air conditioner and blow and clean it all out too. - Jacob Behrends, FL
Refrigerant problems: Improper air conditioner refrigerant charge - too little, too much, too leaky: an air conditioner system which has lost some (but not most) of its refrigerant
will sometimes run too cold at the evaporator coil, leading to coil icing and loss of cool air delivery in the home.
If the filters are
clean and the coil ices-up this condition may be present. A service technician will need to evaluate and test the system and if needed,
adjust the refrigerant charge.
Low refrigerant level in the air conditioning system: Watch out for refrigerant leaks that lead to a repeat of this problem. An air conditioner or refrigerator is a sealed system that should not normally "use up" refrigerant. If the cooling system is low on refrigerant because it has a leak, it is much smarter to find and fix the leak than to simply keep adding refrigerant. If you keep adding refrigerant to a cooling system you're leaking possible contaminants into the environment as well as wasting money.
Abnormally low output air temperature: A refrigerant leak in an air conditioning system may show up first as abnormally low system output air temperature, followed by rising air temperatures, followed by just plain old warm air coming out of the system, as the amount of refrigerant that has been lost increases.
Loss of most refrigerant from an air conditioning system means that the cooling coil will not get cool at all.
Abnormally high output air temperature: A service technician will need to evaluate and test the system and if needed,
find and fix the refrigerant leak, evacuate the system, and install the proper refrigerant charge. See A/C REFRIGERANT LEAK DETECTION for diagnostic details.
Don't keep adding refrigerant. Refrigerant leaks should be found and repaired. It may be easier to keep adding refrigerant, and sometimes a refrigerant leak can be hard to find, but a proper repair is to find an fix the refrigerant leak, not just to keep adding refrigerant. See REFRIGERANT LEAK DETECTION. See A/C REFRIGERANT LEAK DETECTION for details.
Improper refrigerant charge - too much can also lead to improper air conditioning system operation and in
some cases can damage the compressor (called liquid-slugging the air conditioner compressor). A service technician will need to evaluate and test the system and if needed,
adjust the refrigerant charge. In this case the cooling coil is probably not going to ice-over, it just won't get cool.
Air Conditioner Won't Start - Blower Fan Will Not Turn on or Will Not Turn Off
Thermostat problems: Air Conditioner Thermostats:
Air conditioner won't turn on, or fan won't turn on or won't turn off.
See THERMOSTATS for a discussion of how air conditioner thermostats work and how the air conditioner blower fan controls work.
Air Conditioner Air Handler Blower motor Won't Start?
See MOTOR OVERLOAD RESET SWITCH just to be sure that the air handler blower fan motor has not shut off on thermal overload. And if your blower fan is driven by a fan belt and an electric motor, of course check to see that the drive belt is in place and un-damaged. If the blower fan belt is broken the electric motor will run just fine (you may be able to hear it) but the blower fan assembly itself won't be turning.
Air Conditioner Compressor Diagnosis: How to Diagnose & Repair an Air Conditioner Compressor Which has Lost Cooling Capacity
A compressor which appears to have lost cooling capacity can be diagnosed by a service technician who can connect the appropriate test
gauges to the system. Lower than normal discharge pressure and higher than normal suction vacuum will indicate this problem.
But before assuming that something is wrong with the air conditioning compressor, some basic investigation is in order.
Unless there is an obvious indication of a compressor problem (noise, hard starting, compressor
won't start), the service technician, to be thorough and economical, will inspect the system in an order, checking the easy and least-costly
problems first, such as presence of electrical power, proper setting and operation of system controls, condition of filters, condition of duct work, operation
of blower fans, before moving on to check the compressor itself by looking at the air conditioner operating temperatures, pressures, and
current draw in Amps.
Cooling Capacity of the Air Conditioning Equipment
The cooling capacity of an air conditioning equipment refers to the ability of the compressor/condenser (usually outside) and the air handler/evaporator (usually inside)
to deliver cool air to the occupied space.
Briefly,
the compressor/condenser draws refrigerant gas from the building air handler, compresses it
and cools it back to a liquid refrigerant, and the air handler/evaporator coil permits liquid refrigerant
to evaporate inside a cooling coil, across which the fan blows building air to cool and dehumidify it.
The particular
combination of this equipment has a cooling capacity, usually rated in BTUh or thousands of BTU's of cooling capacity per hour,
documented on equipment data tags discussed at RATED COOLING CAPACITY.
Also see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs and see RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.
Air Conditioner Long "on" cycle and Insufficient Cooling - Loose or Worn Compressors
A longer than normal "on" cycle combined with little or no cool air conditioner output could be due to an inefficient
air conditioning compressor or one which has lost its ability to "compress" the refrigerant due to internal wear.
This condition can be diagnosed by an air conditioning service technician who will install air conditioning
manifold gauges onto the system to check the compressor suction vacuum and discharge or "high side" output
pressure. If gauge ports are not installed on the air conditioner compressor unit the technician cannot
make this test without cutting the refrigerant lines to install gauge ports (adding to the cost of this diagnosis).
Low air conditioner motor amperage draw
Low amperage draw: unlike a high-amp current draw which indicates that the compressor is danged internally in a way that its
piston(s) is(are) tight in the cylinder, a low-amp current draw, if below normal, may confirm internal wear on
the compressor parts, and would support the diagnosis that the compressor is worn and inefficient. Where there are no gauge
ports to actually measure compressor low side and high side vacuum and pressure, this simple electrical test is a useful
first step.
If you have the opposite concern, that is the A/C compressor is turning on and off too frequently, see SHORT CYCLING AC COMPRESSOR.
Abnormal air conditioner compressor pressure readings
Refrigerant line pressure readings which are abnormal (probably too low) on the high pressure side (compressor output) or
on the low pressure side (compressor input or suction line) can indicate a problem with the compressor's ability to
develop normal operating pressure ranges and thus will affect the cooling capacity of the air conditioning system.
For more details on air conditioner refrigerant line pressures and how they are examined see COMPRESSOR PRESSURE READINGS
Details about air conditioning compressor functions, how air conditioner compressors work, what goes wrong with air conditioner compressors, and how to measure and diagnose air conditioner compressors are provided at COMPRESSOR CONDENSER.
Cooling Capacity of the Air Conditioning Duct System
Even if a very high BTUH capacity cooling system is installed, if the duct system is defective the ability of the system
to deliver cool air to the occupied space can be severely or even totally lost.
Duct System Efficiency (in percent) describes
the percent of cooled air produced by the A/C equipment which is actually delivered to the occupied space. This number is less than
100% because of air flow restrictions and losses in the duct work.
Duct Delivery Effectiveness is the percent of cooling
capacity which is delivered through the registers into the occupied space. Registers themselves restrict air flow.
See DUCT SYSTEM & DUCT DEFECTS - see links at left.
Cooling Capacity of the Whole Air Conditioning System
So the ability of the entire A/C system to cool a building or rooms in it requires that both the cooling equipment and the
duct system be in proper working order.
Details about duct system and air handler diagnosis, including duct defects, air filter defects, and air handler problems are provided at AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself.
How to diagnose and fix an air conditioning system that is not working
If your air conditioning system won't work, follow our diagnostic guides
At LOST COOLING CAPACITY, our focus is on the case in which the air conditioning system seems to be "running" but not enough cool air, or no cool air at all is being delivered to the occupied space. Sketch from Carson Dunlop.
At OPERATING DEFECTS we take you through the major air conditioning problem symptoms and how to get the air conditioning system working again.
At A/C - HEAT PUMP CONTROLS & SWITCHES we explain the many electrical switches and controls that control an air conditioner or heat pump system. You'll need to check these if your air conditioner won't start.
See our complete list of air conditioning system diagnostic and repair guide articles just below.
Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply,
or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or
air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.
If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.
Five causes of a home air conditioner compressor short cycling on and off too rapidly - Short Cycling Air Conditioner Diagnosis & Repair
Joe said: short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!
Matthew said: short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old
DanJoeFriedman (mod) said to Joe and Matthew (see questions below):
Loss of air conditioner refrigerant - a refrigerant leak in the system. You may be able to repair the short cycling problem temporarily by recharging the air conditioning system but the proper repair is to find and fix the leak. Otherwise you will have to keep repeating the costly service call to just add refrigerant. See REFRIGERANT LEAK DETECTION.
A/C Coil Icing - the evaporator coil (cooling coil) is iced over (such as due to improper refrigerant charge or dirt or a reduced air flow due to a dirty filter) - take a look in the air handler to see if the coil is blocked by ice or dirt.See FROST BUILD-UP on AIR CONDITIONER COILS
Oversized air conditioner - if the short cycling has always been a problem since the day the system was installed, there is a good chance that the unit is too big (too many BTUS) for the space being cooled. A more subtle version of this same problem is that you've done something like closing doors or adding a partition that had the effect of reducing the size of the space being cooled. Sometimes we can mitigate this problem by running the blower fan at a lower speed or by opening interior doors to increase the size of the space being cooled or even by moving the thermostat. See AIR CONDITIONER BTU CHART and OPERATING TEMPERATURES
A/C control problem - it's less likely, but a damaged control board or switch could also be causing rapid equipment on-[off cycling.
Compressor damage or compressor start-troubles: - I've seen these other causes of air conditioner short cycling: if someone manually turns the air conditioner thermostat up and down or on and off too frequently, a hard-starting compressor may find that it has been shut down with high internal head pressure inside the compressor. Normally that head pressure bleeds off over time, making it easier for the compressor to re-start (against low head pressure) the next time it turns on. See HARD STARTING COMPRESSOR MOTORS
But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.
We have also see or a damaged compressor internal refrigerant valve causing high head pressures;
In sum, you need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.
If your air conditioner or heat pump has the opposite problem, staying on too long, see LONG-ON CYCLING AC COMPRESSOR.
My Air Conditioner won't turn off - what to do?
Carla said: My outside air unit will not turn off on its own. I have to manually turn the breaker off to turn it off. Does anyone know why or how I can fix it?
DanJoeFriedman (mod) said:
Carla if your A/C won't turn off it could be that the thermostat is set to a temperature that the system cannot reach - due to lost cooling or due to a setting below the capability of the system. If your A/C won't turn off even if you set the thermostat to a temperature that is above the current room temp, then the thermostat or an A/C control board or switch is bad and needs replacement - in that case you need a service call from a trained HVAC Technician. See OPERATING TEMPERATURES and also CONTROLS & SWITCHES, A/C - HEAT PUMP
Why is my A/C not producing enough cool air?
Amanda said: A/C not producing enough cool air - I had my air conditioning system gassed up last week - $210. ! The unit is not producing enough cold air. The unit is set on 72 and does great at nights but during the day where the temp. outside is reaching mid 90"s it's getting up to 80 in the house. Is my duct work screwed up? How do you repair trailer metal ducting?
DanJoeFriedman (mod) said:
Amanda: if your system is not cooling there could be any of a number of problems - see the article above as a place to start. If your basic complaint is that the A/C temperature at the supply registers is cool enough but the volume of air flow is too weak, we'd start by:
Install a new clean air filter
Check that the air supply registers are open
Check that ductwork has not become disconnected or crimped or crushed
See DUCT SYSTEM & DUCT DEFECTS where we list quite a few steps in diagnosing poor air flow out of an HVAC duct system
Weak air conditioner air flow - is there a relationship between refrigerant charge level and weak cool air flow rate?
Les said: Weak air conditioner air flow: Our A/C was serviced two months ago and the repairman said it had a leak. $400 later it was recharged with coolant and now the ac is doing the same thing. Very little pressure coming out of vents and no cold air coming out. Does anyone know what I can do for the weekend? It is stifling!!!! See A/C Air Duct Problems
DanJoeFriedman (mod) said:
Les: A leak that was fixed by a re-charge is not as good a repair as a leak that was fixed by finding and fixing the leak - you'll just have to keep adding refrigerant.
But weak air flow out of the vents would not be due to a refrigerant leak; more likely a clogged filter or crushed or disconnected ductwork, or a blower fan problem.
Warm A/C suction line (should be cold) and zone control dampers that stopped working
JMONTE said: warm A/C suction line question: After my condenser is turned on for about 4 minutes the suction line starts to get warm to the touch. can you tell me what the problem may be
DanJoeFriedman (mod) said:
JMONTE:
If the HVAC suction line gets warm, you may be out of refrigerant, or the system may be running in heating mode if it's a heat pump. See OPERATING TEMPERATURES.
Becky, If a motorized HVAC zone control damper is not opening or closing, most likely the motor has failed, or the thermostat that operates that zone control is off or set in an incorrect position. See ZONE DAMPER CONTROLS
Becky said: motorized air conditioning zone dampers not working
My house has "zone" control with dampeners to close off the upstairs over the garage room unless that thermostat is on - the room is not cooling. I have located the damper under the house. What are some causes for the damper not opening and how to repair them? condensation, motor to damper? silicon glued properly?
Can Clogged Air Filters Affect the Room Thermostat?
(May 13, 2011) Jim said: If air conditioner filters are clogged will it cause the thermostat to shut off?
Jim: clogged A/C filters won't cause a room thermostat to shut off. The thermostat responds to room temperature. However clogged A/C filters that reduce air flow, cause coil frosting, or otherwise reduce or stop the flow of cool air into the room where the thermostat is located would mean that the thermostat would remain "un-satisfied" and should mean that the thermostat says "on" - continuing to call for cooling. See AIR FILTERS for HVAC SYSTEMS
Could Removing the Air Filters Cause the Electric Motor in the Air Handler to Draw Higher Amps?
Question: got one for you. i put an ammeter on my air handler and it read 8.25 amps, I removed the filters and it went up to 9.75 lmao at the situation the amperage should have gone down. what gives here
Reply: causes of variations in electric motor efficiency and current draw measured in amps
Lost: this amps variation is beyond my expertise but here are two interesting explanations of amps or current variations on an electric motor that I found when researching the question:
1. Voltage variations and current draw at electric motors: If your supply voltage is varying from your power company that can show up as higher amps draw on the motor (though it's a suspicious coincidence to see it exactly when you removed the filters and supposedly reduced the load on the motor). Quoting from motorsanddrives [dot] com: "The effect of low voltage on electric motors is pretty widely known and ... The amount of power the motor draws is roughly related to the voltage times current (amps). Thus, when voltage gets low, the current must get higher to provide the same ... To summarize the situation, low voltage can cause high currents"
2. Load variations and electric motor efficiency: A second possible source of seeing higher amps or current draw on your blower motor when you pulled out the air filters and thus reduced the load on the blower motor might be illuminated by this U.S. DOE pamphlet "Determining Electric Motor Load and Efficiency" - Quoting: "Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is
usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of
5 to 10 hp; peak efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below
about 50% load."
Questions & Answers regarding this article
Questions & Answers about air conditioner system diagnosis & repair
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Timothy Hemm, Yucala, CA, contributed photographs of electrical wiring and equipment installed in California buildings. Mr. Hemm can be contacted at TimHemm@yahoo.com
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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
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"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, Toronto, have provided us with (and we recommend)
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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