Why is My Air Conditioner Blowing Hot Air?

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If your indoor air conditioning (AC) is spewing hot air, the most probable cause is that the refrigerant has leaked out. 

However, there are a number of other possible reasons that may explain why your air conditioner not blowing cold air but running. See our comprehensive list of common issues below, in order of most likely causes to least likely, to identify what your particular AC unit may be experiencing, and whether or not the issue has a DIY solution. 

AC Unit Components and How They Work

This gets a little technical, but some of our readers enjoy learning the physics and process of how their system works. A brief understanding of the process might clarify why a refrigerant leak or other issue will affect cooling – and how it can be resolved.

Throughout this article, we refer to different AC unit components. For your understanding, here is a brief description of each component’s function:

  • Fan: Aka the blower fan, it pulls in warm air from a room and pushes out cooled/cold air
  • Evaporator (coil): Evaporates refrigerant liquid to absorb heat out of the air
  • Compressor: In the outdoor unit of your AC system, it increases the pressure of refrigerant gas, raising its temperature, so heat can be “squeezed” out of it
  • Condenser (coils): Also outside, it releases heat outside and reconverts refrigerant into a liquid
  • Expansion valve: Restricts refrigerant liquid flow and drops refrigerant pressure 

An AC unit contains liquid refrigerant which absorbs the hot air in the home with the help of fans. The refrigerant liquid passes through the evaporator coil, which turns it into a low-pressure gas. That gas is then compressed, so that its temperature increases. It is sent to the condenser coils where the heat is released outside and the refrigerant is turned back into a liquid. 

The liquid passes through the expansion valve, dropping to a temperature lower than the room, and it proceeds through the evaporator coils a second time to become cool air that is ejected into the room. This process repeats while the AC unit is on.

This helpful video explains the process just described. 

With the basics covered, why is your AC blowing hot air?

1. Most Likely: Refrigerant Leak

There is a fixed amount of refrigerant, aka Freon, in your unit, so if any of it is missing, a professional will know it immediately by testing refrigerant pressure with a meter.

The hot air you are feeling in your home is likely the result of the refrigerant leaking out due to one of the causes listed below. Remember, it is the refrigerant that carries heat out of your home. The less refrigerant pumping through the AC system, the less heat it can remove. If all the refrigerant leaks out, your AC will blow warm air. 

There is no one cause for a refrigerant leak, as it could be the result of several mishaps. Here are some potential causes your unit may be experiencing: 

  • Your AC unit was poorly assembled by the manufacturer. This is less common among main AC brands, but, of course, no company has a perfect track record.
  • A connection between refrigerant lines and coils may have come loose due to your AC unit’s age and how often you use it. These connections sometimes corrode too. This is especially common in coastal areas with salt spray and where heavy pollution results in “acid rain.”
  • Wear-and-tear on the external AC unit, which can be the consequence of turning your internal AC on high immediately after being off for the winter. This is why a central air conditioner often “dies” immediately or shortly after the cooling season begins.

How to fix the problem: Unfortunately, a refrigerant is required to purchase refrigerant and charge an air conditioning system with it. Refrigerant can be dangerous in the hands of a non-professional. Contact your HVAC (heating, ventilation, and air condition) technician for assistance. If you don’t have an AC company you prefer, use our Free Local Quotes service for no-cost, no-obligation estimates from licensed, pre-screened AC contractors in your area.

How to prevent the problem in the future: Have a HVAC technician regularly check your unit. We suggest scheduling six-month or annual checkups. Waiting longer than a year can risk AC unit issues.

2. Thermostat on Heat Mode 

tthermostat on heat mode

The reason for your AC unit’s malfunction may be that it is not malfunctioning at all. A simple and common mistake among homeowners is to forget that their thermostats were set on “Heat” in the winter, needing only to be changed to “Cool” for the AC. D’oh! 

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 happen all around. Practical precautions can help, such as writing yourself a note at the start of the winter to switch the thermostat settings in the spring or summer. 

3. Dirty Air Filter

dirty air filter

Air filters are a vital component to the AC unit. If your unit’s filter is dirty, such as with dander, dust, or debris, the pollutants will greatly reduce the air flowing through the AC unit.

Less airflow means less heat passing over the indoor coil and being removed. This results in warm air flowing out of your ducts and grates. 

Trying to pull air through a dirty decreases your unit’s efficiency, slows or stops the cooling process, and, if the filter is not cleaned or replaced, will shorten your AC and blower fan lifespan. 

How to fix the problem: Replace or clean the air filter. Most are disposable. A few can be cleaned with mild detergent and water. If yours is disposable, and you don’t have a new filter handy, use a nylon brush to brush off a layer of dust and debris. Do it outside, because it’s a messy job. Then get a replacement filter the next chance you get.

In either case, there is no need to phone a professional. 

How to prevent the problem in the future: Check on the filter from time to time. A regular checkup will help you remember to clean the filter when it starts getting dirty, and it will help you judge when it is necessary to replace the filter.

4. Dirty Evaporator Coils 

dirty evaporator coil

The hot air may be the result of either or both coils being filthy. If the coils are not cleaned, it inevitably leads to heat absorption and cooling complications, higher temperatures surrounding the coils and the possibility of the coils freezing.   

How to fix the problem: You can call a HVAC professional to clean the coils if you prefer, but it would save you money making this a DIY project. 

Here is a basic list on how to clean the coils yourself:

  • Turn the AC off on your thermostat.
  • Find the access panel on your internal AC unit and remove the screws/fasteners to take the panel off.
  • Blow compressed air across the coils to remove the larger debris and loose dirt. Make sure to blow the air in the opposite direction of the system’s normal airflow. Pro Tip: If using high-pressure compressed air, direct the air through the fins or at a 90-degree angle to prevent damage.
  • To clean the coil, you can either use a brush or a commercial cleaner.

Brush method: Use a soft-bristled brush to avoid damaging the fins and coils. Apply the brush to the coils and fins to scrub away the hardened dirt. 

Product method: Follow the instructions and cautions written on the bottle. Spray the cleaner on the coils as directed and allow the cleaner to foam until the cleaner and the dirt drain away. Reapply the cleaner as much as the instructions permit or until the coils are spotless. 

  • For a visual step-by-step for cleaning an indoor coil, see this video.

Cleaning the outdoor coil is a similar process. First, remove the fasteners holding on the cabinet, and lift the cabinet off the unit. You can then use compressed air or a hose to remove the dirt and debris. Using a soft brush and cleaner will improve the cleaning.

How to prevent the problem in the future: Regularly clean or change the air filter. If the air filter gets dirt buildup, the evaporator coil will eventually get dirty. 

The outdoor coil, the condensing coil, should be hosed off periodically. For example, if cutting the grass throws lawn clippings onto the condensing unit, hose them off before they can clog the coil.  If you’re comfortable with coil cleaning, there is no need to phone a professional.

It is imperative to clean the evaporator coil at least once a year.

5. Frozen Evaporator Coils

frozen evaporator coil

A dirty AC filter can also cause the evaporator coil to freeze. This happens because there is not enough airflow passing the coils to evaporate the condensation that the coils produce during the cooling process.

Another factor is how cold the evaporator coil becomes. With proper airflow, warm air flowing over it will keep it above freezing. Not enough airflow means the coil will literally get freezing cold and the condensation on it will freeze. 

When the coil is frozen, it can’t absorb heat out of the air, and the AC blowing warm air is the result.

How to fix the problem: Turn the AC off for about an hour or two so that the evaporator coils can thaw out, and clean or replace the filter. You can also try turning your thermostat to “Fan” mode. This will move warm air over the coil without the AC function on. The warm air will speed up the thawing process. 

If the filter is cleaned, the coils should not freeze again. However, if the coils do end up freezing a second time, it may be due to an internal problem with your AC unit. In this case, contact a HVAC technician for further inspection and assistance.  

How to prevent the problem in the future: Regularly clean your AC filter and indoor coil, and keep your thermostat on auto around 70 to 72 degrees Fahrenheit during the first few weeks you use the AC in the warmer seasons. 

6. Wiring Issues

There may be frayed, cut, or damaged wires among your internal or external AC unit. If this is the case, the AC system will perform poorly and might break down.

Your unit’s malfunction may be the result of a wire being frayed from overheating; however, this is a rare occurrence. Usually, house pets chew on the internal unit wiring if it is exposed, or an outside animal, such as a mouse, chews on the external unit’s wiring. 

How to fix the problem: If the coating around the wires is missing, you may be able to salvage the wires by wrapping electrical tape around the unprotected areas. This should be a short-term fix. It will be in your best interest to contact a professional to replace them. 

How to prevent the problem in the future: For the internal unit, it would be wise to cover the wiring with split plastic tubing like this designed for the purpose. Until you get the tubing, elevate exposed wires from a pet’s reach. 

For the external unit, building a solid fence around it will likely deter animals from going near or tampering with the wiring. 

 7. Breaker Switch 

The internal and external AC units have their own circuit breakers that trip (that is, shut off) if there is a disconnect in the electric current, which is known as a short circuit. The breakers shut off as a safety precaution since a short circuit could otherwise cause a fire. 

If the breaker trips for the external AC unit, your internal unit will still power on, but it will not be able to cool the hot air it sucks up. 

How to fix the problem: Go to your fuse box and switch the breaker “on.” If you do not know which breaker to look for, it is usually the only one in the group switched off. 

If the breaker does not stay in the “on” position, or if the power for the AC unit does not return once the breaker is switched, then your unit is likely damaged or experiencing an electrical issue. To resolve this, contact a HVAC technician or an electrician. 

How to prevent the problem in the future: AC units require a lot of energy and can easily trip breakers. To prevent this, contact a HVAC technician or an electrician to see if your breaker can handle your AC system’s current draw. 

8. Least Likely: Damaged Compressor

The compressor is at the heart of the AC process; if it fails to work, then the entire process collapses. The compressor’s breakdown is typically the consequence of another underlying issue. For this reason, be prepared to find other damaged areas in your AC unit. 

Unfortunately, a number of things can cause the compressor to malfunction: 

  • Dirty condenser coils. This is when dirt builds up on the coils, preventing them from transfering the heat to the outside. The system is forced to constantly run in order to cool down, which is taxing on the compressor and causes it to fail. 
  • Blocked refrigerant lines. If the refrigerant lines are blocked, the increased pressure and temperature will damage the compressor. 
  • Refrigerant leaking. If this substance leaks, there will be less of it flowing through the lines. This means the compressor will have to work harder, which will eventually break it down. 
  • Electrical problems. If your AC unit experiences electrical issues, it can result in acid buildup, damaging the compressor. Unfortunately, you may find other areas of your AC unit being damaged from the buildup.  
  • An inadequate amount of oil lubricant. The AC unit will not work adequately if it is not lubricated enough. Most bearings used today are sealed, so cannot be lubricated. If your AC is really old, this might be an issue. 
  • Anomalies, such as manufacturing or installation mistakes. If the manufacturer assembled your AC unit poorly, some of the mistakes can cause overt and unnecessary stress on the compressor. For instance, if your refrigerant lines are too large or small for your system, then your compressor will have a shorter lifespan. Also, quality installation is vital to a system’s longevity. Get estimates from several air conditioner installers, and choose one with a record of quality installation. Our Free Local Quotes is a quick, convenient way to start the process. 

How to fix the problem: Sadly, the compressor cannot be fixed and requires an expert eye to determine what is breaking it down. In other words, there is no way a DIY project can be made out of this. 

If you suspect the compressor is the reason your AC unit is spewing hot air, contact an HVAC professional to diagnose the problem. No matter what the underlying cause is, however, expect one of the following outcomes to happen:

  • You must replace the compressor. 
  • You must replace the entire outside condensing unit.
  • You must replace the condensing unit and the evaporator coil. 
  • You must replace the entire HVAC system. 

How to prevent the problem in the future: You can rest assured, as the reason this falls last on our list is because it does not often occur. This said, we do advise starting the following procedures to prolong the compressor’s lifespan:

  • Clean the air filters so that no pollutants affect your HVAC system or hinder the airflow.
  • Clean the system’s air ducts so that there is no buildup.
  • Schedule a HVAC system maintenance checkup. 

HVAC Checkups 

As mentioned throughout this article, the most surefire way to avoid most of these problems is by arranging inspections with your HVAC technician. We suggest waiting no longer than a year for each inspection, as you may otherwise risk severe problems with your AC system, which will cost you more than having regular inspections.

You might also void your warranty! That’s something most homeowners don’t know about warranties. Most companies require regular maintenance to keep the warranty intact

HVAC technicians will give you an understanding of your AC unit’s condition and lifespan by testing the unit’s moving parts and checking on the refrigerant, while also locating and repairing any damages that may exist. 

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