Why is my A/C Blowing Hot Air and How to Fix It (A/C Troubleshooting Guide)

A mid-summer tragedy, so often told;

As if it's not already frustratingly hot enough for your liking today, it seems almost impossible to keep the inside of your home cool. You check the thermostat for confirmation; "Is it hot, or is it just me?" You ask yourself only to find out the temperature is, in fact, soaring well above the preferred setting.

At this point, you run to one of the AC registers -- Is it even blowing cold air right now? After placing your hand over it, you find out to your amazement, it's blowing warm summer air.

You may not know where to turn at this moment. What exactly is going on? Why is my central air conditioner blowing hot air? Is this something you can fix yourself, or do you need to call a professional?

Chill out! We're here to help. We have outlined everything you need to know in this article!

So, without further ado, let's dive into the Ultimate AC Troubleshooting Guide for 2021!

Understanding the Types of AC Split Systems

Let's tackle the basics first for a second. HVAC is the system utilized by homeowners across the world to keep one's home at a comfortable temperature all year long. There are primarily two different types of systems, each of which possesses its own set of advantages.

Split Systems

Central A/C Split System

The split system model has the condenser and compressor in an outdoor cabinet, made of metal usually. Simultaneously, the system will have an indoor cabinet that houses the evaporator coil on top of the furnace/air handler. The air handler then blows air over the evaporator to push air through the ducts throughout the home.

Ductless Mini-Split A/C Split System

A ductless mini-split system will also have a condensing unit on the outside of the home, and will have one or many indoor units that house the evaporator coil and blower motor. The indoor units will be mounted close to the ceiling and do not require ducts. These units are typically used when running ductwork through the house is impossible or too expensive to do.

Common Causes for an AC Blowing Warm Air

There root cause of unwanted, warm air blowing from your registers can be one of many. We will discuss each culprit and how to troubleshoot moving forward! Let's do this!

A Dirty Condenser Coil

Your air conditioning system as a whole has two major types of coils (as well as other components): the evaporator coil and condenser coil. An evaporator or "indoor" coil is, of course by the name, located indoors above one's furnace/air handler. Symmetrically speaking, the condenser coil is your unit's counterpart to the evaporator coil.

Likewise, a condenser coil repels all outdoor heat that's blown over the coil. It's located outside, on the condensing unit and like its sister coil, both are commonly constructed out of copper, paired with light aluminum fins. It is oftentimes referred to as "the warm coil."

Although both types of similar coils can accumulate dust and dirt over time; the condenser coil is located outdoors, its contamination risk is increased due to grass clippings, leaves, pollen, and other outside debris.

What Happens When a Coil Gets Dirty?

When a coil gets dirty, it prevents heat transfer, which is critical to an efficient cooling system. This will in return overtax the compressor, shortening its lifespan, and increasing its overall working temperature. The coils need to stay free of all debris to allow the appropriate flow of air throughout the HVAC system.

What Are Signs and Effects of Filthy Coil?

The first symptom you'll notice affecting your AC's health is a significant reduction cooling system's efficiency. This can cause your air conditioner to take longer to cool the home, to feel as if it's not cooling at all, and may even make your home feel more humid.

When your AC is severely working harder and it can't keep up with the moisture in the air, you may even start to feel HOT air blowing in your rooms. If that happens, it's best to turn it off and call a professional for help. Alternatively, there are steps you can take to keep your condenser coil clean and in healthy shape.

How Can I Avoid Condenser Coil Disaster?

Easily enough, take a garden hose to your compressor once a month to rinse all the accumulated dust and debris. You can also thoroughly clean them once a year with a non-acid, non-alkaline-based cleaner. Make those coils shine!

A Broken Condenser Fan Blade

As simple and cheap as the fan blades in your condenser are, they surprisingly create a popular problem amongst AC units. The smallest bit of damage or imbalance to the blades can create some serious cooling issues, yet they are fairly simple to replace or repair if done promptly. Just don't run an AC with bad blades, this can cause wear on the motor and create more difficult and vital problems moving forward.

Fix it quick. It's not worth waiting for more damage and having to pay for more costly parts and repairs.

Sudden Signs of a Fan Blade Gone Bad

This is actually a rather easy problem to diagnose. When fan blades in the condenser start to develop issues, they make a ton of noise! Bent blades scraping against the metal of the machine can cause a distinct scraping sound to come from the condenser.

Fan blades will also crack over time, especially with the wear and tear of being unbalanced or bent. This type of damage is easy to spot out; through the top of the fan cover on the condenser cracks, bends, and loud and major issues will be very apparent.

What Are My Next Steps? Can I Fix the Fan Blade?

If just one blade is dragging or bent, one may assume that you can just bend it back for a quick fix. This is not the case. If one blade is bad, the entire unit will need to be replaced since the blades are connected.

This is a process that involves great precision and accuracy when installing and replacing blades on an AC unit. The consideration of revolutions-per-minute and horsepower for blade's master motor, sizing specifications for the condenser and its blades within the unit, knowing the manufacturer's intended direction for blade rotation; it's usually not a DIY project and should be left to a professional, to be honest. Once a technician can decipher what parts are needed and how to install them, with respect to the unit's make and model, the blades can be replaced carefully followed by a performance test.

If your blades aren't really damaged or bent in any way, yet out of alignment a bit, a technician may be able to balance the old blades without needing to replace them. This can also involve cleaning the blades of debris just like the coils mentioned above; the blades can also accumulate a coating of dust and other debris from outdoor contaminants.

A Failing Compressor

One of the most vital parts of your air conditioning unit is the compressor, also known as "The Heart of the System." Without the compressor, the unit would not be able to function at all, which is why it's crucial to keep it maintained in tip-top shape.

What exactly causes an AC compressor to fail?

Unfortunately, there's not just one solitary cause for an AC compressor to fail. AC experts have found there to be multiple sources to the same problem. Don't sweat it; we have them outlined here!

Here are the five most probable causes of compressor failure:

  • Inadequate Refrigerant Charge- If your AC unit begins to leak refrigerant or lacks the correct amount of fluid, your compressor will have to work harder to pump refrigerate throughout your whole system. This will make it very difficult for your Ac to cool the whole space and will eventually cause it to break down.
  • Dirty Coils- As mentioned above, as grime and dust cakes onto your condenser coil, your until will have a difficult expelling enough heat from the system. This will cause a compressor to inevitably overheat and fail.
  • Contaminants- Pollen, leaves, gravel, moisture, sand, and even bird droppings can cause damage to one's compressor. Pay attention to where your unit is located and take the necessary steps to keep it clean and safe.
  • Inadequate Oil Lubricant- If there aren't enough lubricants (oil) in your AC system, just like an automobile engine, it will cause major cooling issues and eventually the machine will break down. With that in mind, it may be wise to call a professional to check fluid levels at the beginning of your warmer seasons.
  • Clogged Suction Lines- Any blocked or kinked suction lines will certainly wreak havoc on your HVAC system. If the problem isn't fixed immediately, the unit will not cool effectively and can cause major damage to more serious parts of the machine.

How Can I Prevent Compressor Failure?

Some AC systems have a special feature known as a high-pressure switch. This is used to prevent the compressor from overheating and really comes in handy during the summer months. Alternatively, if you don't have this feature on your AC unit, regularly cleaning/replacing the air filters to the system will surely aid the battle against dust and air contaminants.

Bad AC Contactor

The contactor is a great part to become familiar with for one clear reason. It's an AC component that needs to be replaced periodically and is a manageable fix.

But What Is an AC Contactor?

Simply put, the contactor is what controls the flow of electricity into the AC unit. When you turn your air conditioner off, the contactor blocks power. On the other hand and on the same token, switching your AC on will cause the contactor to push power into the unit.

This works in conjunction with your thermostat. When your home is cool enough and reached the user-determined temperature, the contactor will close the flow of power turning the AC unit off.

Faulty AC Contactor Signs

Contactors are replaced every now and then because as their efficiency diminishes, the whole unit is exposed to overheating. As an overwhelming result, this will cause your AC to not kick on at all with no moment's notice. Let's go over the symptoms to pay attention to when it comes to an AC contactor:

  • The condenser never kicks off
  • The air conditioner hums and never turns on
  • The air conditioner makes a weird chattering noise

How Do I Test an AC Contactor?

First things first, you must shut off the power to the AC unit at the thermostat and breaker box. Failure to do this can lead to the risk of electrocution. Once the power is completely shut off, you can venture over to your condenser unit.

If you look along the side of the unit; whichever side has the wires connected, that's the AC's control panel. Use a screwdriver to remove the side cover. You will then see a collection of colored wires connected to a flat black rectangle that's positioned vertically; congrats, the black box you are seeing is the contactor.

Unscrew the contactor with the wires still attached. Once you have the contactor lose and ready to remove, you can proceed to remove the wires as imagined. You can test if the contactor is working properly with a multimeter to decide if you need to buy a replacement or not.

Your multimeter should read between 5 and 20 when connecting to the low voltage terminals on the contactor. If you end up getting a lower number than that, or no reading at all, the contactor is non-functioning and needs to be replaced.

After all is said and done with, you can rewire the contactor to the machine, reinstall the condenser's side paneling, and kick the power on to the AC unit. See? That wasn't too difficult of an AC repair, now was it?

Bad AC Capacitor

When speaking of electro-mechanical machines, capacitors are an essential component; and this is no different for your air conditioning system.

The simplest way to explain what it does? Capacitors are strictly used to "store" energy in an electrostatic field. When attached to a motor, a capacitor will stabilize the voltage and provide a mandatory jolt of power to start the motor initially. In an AC unit, capacitors are attached to three major motors: the compressor motor, the outdoor fan motor, and the blower motor.

The blower motor will have a run capacitor inside the furnace/air handler. The outside condenser will sometimes have up to 3 capacitors, but most commonly will only have 1; a run capacitor that drives the compressor and the condenser fan motor. Sometimes these will be 2 separate capacitors, usually due to field modifications. Sometimes, in order to eliminate the stress of startup, you will see a start capacitor; Usually it is the black plastic capacitor.

Malfunctioning capacitors are one of the main dilemmas an AC owner will experience in their unit's life span and is the most common call for help at the beginning of summer. Thankfully, if not ignored and caught at the right time, capacitors can be easily swapped causing no harm to the rest of the AC.

A Refrigerant Leak

Surely, the word refrigerant isn't completely alien to you in correlation to air conditioning systems. It is a vital part of keeping your home cooler during the roasting months of the year. But what exactly is it?

It Is Not a Fuel

Let's clear up a common misconception people have about refrigerant first; It is not a type of fuel. Refrigerant isn't like gasoline, where it "uses up" the fluid to combust and run. The source of energy for an AC unit is solely electricity, of course.

Your AC should circulate the same amount of refrigerant from the time it was first installed, until its last day of replacement unless there is a leak that needs to be addressed. The primary role of a refrigerant in an air conditioner is to provide heat exchange. It shifts between a gas and liquid state, never dissipating (keeping the same "charge") absorbing the heat from inside and then releasing it outdoors.

Why Is Losing Refrigerant Dangerous?

As mentioned, air conditioning units have a set charge of refrigerant. Losing even a fraction of the charge can jeopardize the performance of the AC unit and ultimately cause the system to lose its cooling capacity. But that is the least of your problems; refrigerant matters can get much more challenging.

Critical damage to your evaporator coil and compressor is certain when you are running an AC with the improper amount of refrigerant within. With the compressor, being such a vital organ to the AC body; it's often more cost-effective to replace the entire air conditioner instead.

Introducing: The Evaporator Coil

When the average person thinks about the core parts of an air conditioning system, usually they imagine the large box-unit outside, vents, and a thermostat. Yet, one of the most important components in the whole system is the evaporator coil.

The evaporator coil can be found inside the internal unit or near the air handler, where the blower fan is usually located. It is likely made of aluminum, copper, or sometimes steel because these metals make excellent heat conductors. With this part being located deep within the internal unit, it is recommended to contact a professional for skilled labor and guaranteed safety.

A Dirty Evaporator Coil

If you haven't been able to tell already based on this article; Dirt is the arch-enemy of all air conditioners . All parts that get tarnished from debris, dust, and dirt, suffer consequences that can affect the whole AC system. Your evaporator coil shares the same weakness alike.

Considering the fact that evaporator coils absorb the heat in the whole home as air is drawn through the return ducts, this sets the stage for a series of AC disasters waiting to happen when the signs are left unnoticed.

Let's go over the common symptoms of a dirty evaporator coil now, so you don't have to worry.

Inadequate Cooling

Have you, by chance, noticed a different temperature of the air being pushed through the registers than what you are used to? With debris and gunk built up on the coil, the AC will often still switch on but not do a very exceptional job.

The AC Is Constantly Running

One would think that a thermostat will intelligently turn the air on and off, in comparison to the temperature inside the home, as if the AC is meant to run in cycles. This is true, but not the case with a dirty evaporator coil. The AC system will just run and run until completely wearing itself out.

An Iced Over Evaporator Coil

A frozen coil is one of the worst things that can happen to your air conditioning system. This happens as a result of leaving a dirty coil untouched for too long.

Since dirt restricts the amount of heat that can be absorbed, condensation begins to form on the coil. This unwanted water will then turn to ice causing the air conditioner to malfunction and conclusively break down.

There are plenty of causes for a frozen coil and we have outlined them so you can stay ahead of the troubleshooting curve:

  • Low refrigerant charge
  • A dirt blower wheel
  • poorly functioning blower motor
  • lack of airflow (from putting furniture in front of vents or closing registers in unused rooms
  • Forgetting to change dirty filters
  • Using wrong sized filters
  • Installing thicker/beefier filters that cause more restriction (due to covid-19, used for stronger filtration)

Check Your Blower Motor Safely

It is recommended that you check the capacitor on the blower motor and make sure that the blower wheel spins smoothly and freely. Please make sure you power off the AC system before examining! Due to the dangers and aptitude needed, it is sensible to hire a professional to repair a blower motor and its capacitor.

How to Clean Your AC Coils

If you decide you want to tackle the project yourself, the first step in cleaning your AC coils is unmistakably obtaining access to them. Again, before doing anything, make sure you turn the air conditioner off at the thermostat and at the breaker box. Then, you can remove the screws or fasteners and loosen the side panel, making sure to place the panel and screws aside where they won't get lost.

Next, use one of the following methods to clean your AC evaporator coils:

1. Using compressed air

If there isn’t a huge accumulation of dirt on your coils, it can usually be removed with the use of compressed air to blow the dirt off the evaporator coil. You’ll want to direct the compressed air in the opposing direction of the normal airflow over the coil. Also, make sure that you use a constant airflow across the coil, wear eye protection, and use a shop vacuum to tidy up the dirt and debris as it becomes dislodged.

2. Using a brush

This can be a useful technique for removing small amounts of dirt from the coils, also providing you more control over the pressure and areas that are being cleaned. With this method you’ll use the brush directly on the coils to sweep the dirt away, scrubbing if necessary for the more troublesome dirty spots. You’ll want to use a soft nylon brush, avoiding hard bristles or wire brushes as they can cause damage to the fins.

3. Using commercial cleaners

You’ll have choices when it comes to the selection of cleaners available for cleaning your evaporator coils. After selecting your favored brand, follow the directions that come with the cleaner. Let the cleaner sit and foam until both the foam and debris drain away. Reapply as needed until the coils are free of nasty buildup.

4. Using mild detergents and water

If you would rather not use a commercial cleaner, a mild detergent, and water work just as adequately sometimes when cleaning coils. Mix a simple detergent and warm water in a spray bottle. Spray the solution onto the evaporator coils and give it a few minutes to soak in. Wipe away any loosened debris with a soft brush or cloth.

5. Heavy-duty cleaning

If your evaporator coils are heavily soiled, you may need to use heavy-duty cleaning chemicals and tools like a steam cleaner or pressure washer. It may also indicate you need to take apart more of your AC unit than just a regular cleaning, such as the extraction of the coil, cutting of the refrigerant lines, and then reassembly afterward.


A thermostatic expansion valve (TXV) is a more complicated part to understand, so brace yourself for this one. To begin, the TVX's purpose is to regulate the rate at which refrigerant flows into your evaporator coil.

There are three different types of charge that a TXV can hold:

  • Maximum Operating Pressure Charge (MOP)
  • Universal Charge
  • MOP charge with ballast (the standard for Danfoss expansion valves with MOP)

MOP Charge

An expansion valve with a MOP charge is normally used on factory-made AC units where section pressure limitation on starting is mandatory, such as in the transport sector and in air conditioning systems.

Expansion valves with MOP have a very diminutive charge in the bulb. This means the element or valve must be located in a warmed area than the bulb. Otherwise, the charge will migrate from the bulb to the element and prevent the expansion valve from functioning.

Universal Charge

Expansion valves with universal charge are alternatively used in most refrigeration systems where there are no pressure limitations required. The bulb can be located warmer than the element or even at a higher evaporating pressure.

MOP Charge With Ballast

Expansion valves equipped with MOP ballast charges are used in refrigeration systems with what's called "high dynamic" evaporators. Plate heat exchangers with high heat transfer and air conditioning systems are prime examples of machines that use this type of valve.

Thermostat Failure and Troubleshooting

A programmable thermostat has become the most common for controlling the home temperature amongst modern homeowners. It is simple to make some costly mistakes when it comes to using a programmable thermostat and it shouldn't be ruled out when troubleshooting AC system problems.

Setting the Wrong Temperature

Truly, setting the correct temperature on one's programmable thermostat is the solution to feeling comfortable at home. Nevertheless, nowadays a lot of people tend to overheat or cool their homes. This leads to wasted energy.

While comfort cannot be defined completely, the official cold weather plan for the US suggests living room temperatures of 70°F, whereas bedrooms and all other occupied rooms should be at 64°F. Rooms should never exceed 75°F.

The Thermostat Is Set to Heat Mode

The reason for your AC unit’s failure may be that it is not actually malfunctioning at all. A common mistake among homeowners is to overlook that their thermostats were set on “Heat” in the winter, needing only to be changed to “Cool” for the air conditioner to kick on and cool the home.

How to fix the problem: All you need to do is change the thermostat setting.

How to prevent the problem in the future : Life gets busy and distractions occur all around. Practical forethoughts can help, such as writing yourself a note as a reminder to change the setting in the warmer months.

Cranking up/down the Thermostat to Heat up/Cool down a Room Quickly

We've all been there. You come home to a freezing house, maybe after being soaked in a rainstorm or after shoveling snow in Mid-February. It appears completely reasonable to turn up the thermostat. On the contrary to popular belief, that won’t help you feel warm and comfortable any quicker.

A thermostat has no authority over how swiftly your house cools down or heats up. All it does is establish the final temperature at your comfort level, like some type of temperature limiter. It allows the AC or heat to be fully on until the decided temperature is reached, at which point the thermostat will turn the HVAC system off.

Allowing the AC to Run Constantly

For improved energy efficiency, the most beneficial thing to do is increase your temperature when you’re not at home. The amount of cool air your home loses is connected to the contrast in temperature between inside your house and outside, so the hotter it is outside, the more cool air you lose.

Your Thermostat May Not Be Level

While most modern thermostats use some sort of an electrical temperature sensor, an older thermostat uses mechanical switches, typically a bimetallic strip with a mercury switch. With a standard mercury switch, when the temperature in the room changes, the coil expands or contracts, shifting a glass bulb with mercury inside. Depending on where the mercury is inside the tube, it will either close or open the circuit by attaching the positive and negative leads at the end of the tube.

For this to work as planned, the thermostat is required to be level. So break out your level and check how close it really is. If the thermostat is circular, you may need to remove the cover and look for leveling lines to ensure it's actually level. With a newer thermostat, being level is mainly for aesthetic purposes and won't affect the thermostat's performance.

A Defective Temperature Sensor

Another explanation for a thermostat to be reading the incorrect temperature in your home is a broken temperature sensor. This one can be difficult to troubleshoot and you may have to eliminate other possible problems before getting a better idea of what the issue is.

You can also always try recalibrating the thermostat. This is done differently depending on what type of thermostat you have, such as mercury or magnetic switch.

Some thermostats with mechanical switches have a calibration screw that you must turn until the contacts of the bimetallic strip close. Wait for a second and turn the screw until the contacts are opened.

How Do I Properly Use My Thermostat?

There are different types of thermostats and it is important to know how to use each one to cool your house efficiently. We will go over the popular types of thermostats below; let's dive in!

Manual Thermostats

Manual thermostats offer mere functionality and ease of use. There is no need to program a manual thermostat. Instead, you can control your AC system with a flick of a switch or twist of a dial for immediate cooling.

Programmable Thermostats

For an enhanced version of functioning, use a programmable thermostat that allows you to set personalized heating schedules specifically suited for your needs. So, if you need your cooling system to come on at a certain time every morning or evening, or at different times on a weekday or weekend, you can program the thermostat as easily as you would set your watch.

Smart Thermostats

Smart thermostats, like other smart devices, allow you to remotely manage your home’s temperature via mobile or internet-connected devices. This unique ability makes handling more convenient, offering greater control of heating and cooling, and thus, energy savings.

Uncommon Causes That Shouldn't Be Overlooked

Among many other unique issues you can come across when dealing with your home air conditioning not working, these electrical issues can cause some problems when troubleshooting AC issues:

A Circuit Breaker Tripped

Circuit breakers are intended to shut off when they’re receiving an electricity overload protecting your home from a fire. The reason breakers trip could be due to using too many appliances at once, or maybe an appliance is too old and uses too much electricity. Regardless of the reason, if the breaker that grants power to your AC unit trips, your air conditioner won’t turn on.

Check to see if you need to reset your breaker.

There’s May Be Wiring Issue in Your Home

Overall, how many problems do you have with electric appliances in your home? Do your lights flicker on and off frequently? Do appliances make a humming noise when you first plug them in? If you often encounter any of these, you’ll want an electrician to fix the underlying cause.

And if it has impacted the unit and your central AC is not working, you’ll need an HVAC professional to run a diagnostic test.

You Could Have a Blown Fuse

To find out whether your fuses are operating correctly, you’ll need a multimeter to do a continuity test. Test both fuses. If the multimeter beeps, your fuses are running properly. If one of them doesn’t beep, you can purchase a replacement at your closest hardware store at a decent price.

Why Is My Central Air Conditioner Blowing Hot Air? Now You Know!

Well, there you have it! We have discussed the most common issues to troubleshoot your malfunctioning air conditioning system.
After asking yourself " Why is my central air conditioner blowing hot air" you now have the answer and a good idea of what you need to do next.

Now you know how to diagnose AC issues like a pro! When doing AC repairs, browse our site for the parts you need to get the job done!

About the Author

James Clark

James Clark is the HVAC Controls Specialist and Ecommerce Manager for Technical Hot & Cold. He has been with the company since 2014 and enjoys helping homeowners save money by providing help articles that walk them through various DIY HVAC repairs. In his spare time, he's playing music with his children and spending too much time working on his lawn.


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