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AC Won't Turn On | The Complete Guide to Air Conditioning + How to Make Sure it Works Efficiently

AC won't turn on

It happens to everyone; the AC won't turn on, or the air conditioning system stops working on the hottest summer day. Fortunately, there are things you can do to prevent that from happening. Nonetheless, if you are reading this guide, it probably happened to you.

If your HVAC system turns on but doesn't reach its set temperature. Check out this guide.

This guide covers possible reasons why your air conditioning system isn't turning on or isn’t working properly and includes tips on how to fix and prevent these problems in the future. We also discuss signs to look out for to help catch and avoid future air conditioning failures.


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Signs of a forthcoming air conditioning failure

Table of contents for this section:

AC Safety switch (float) keeps filling with water

If the AC safety switch (float) keeps filling with water, it is a sign that the system isn’t draining properly. Air conditioning systems generate condensation as they operate. Sometimes this condensation happens in the wrong places or does not drain properly. Both can cause damage to the HVAC system or water damage in the home.

Too much condensation or condensation in the wrong place can indicate refrigerant, drainage, ducting problems, or restricted indoor or outdoor coils. It can also be because of dirty filters. Any signs of excess water can also indicate that the indoor coil is freezing.

If you recognize any signs of water or your AC won’t turn on or stops working, we recommend consulting with an HVAC professional to diagnose the issue properly.

The air conditioner makes a loud buzzing noise when starting

If your air conditioner won’t turn on or the AC unit makes a loud buzzing noise when starting, it is a sign of a faulty AC capacitor. In other cases, it can be a sign the AC unit is working hard, which causes strain on the motors. The strain on the motors will cause them to make noise and fail sooner. The two biggest motors of the HVAC system are the indoor blower motor which moves the air throughout the system, and the compressor motor, which compresses the refrigerant in the refrigeration process.

When the outside ac unit makes a loud noise when starting, there is a good chance the compressor's capacitor is failing, and if the indoor AC unit is making a loud buzzing noise when starting, there is a good chance the blower wheel motor capacitor is failing.

Although capacitors fail over time, other underlying issues can cause capacitor or motor failures. Dirty air filters, ducting problems, refrigerant problems, and indoor and outdoor coil restrictions can all cause strain on the HVAC system motors causing them to fail sooner.

Air from the HVAC grille (vent) exits at a high velocity and is noisy

Some HVAC systems have high-velocity HVAC ducting and grilles by design. However, in most residential and commercial applications, air flowing at a high velocity indicates high static pressure. Often times this also causes the system to be noisy. Both noise and high velocity can be irritating and cause discomfort.

High static pressure results from too many air restrictions within the air conditioning system and can be caused by ducting problems, improper system sizing, a restricted indoor coil, dirty filters, or too many closed grilles (vents).

AC is blowing warm air

If the AC in the home is blowing warm air, it can indicate that the refrigeration cycle isn’t happening properly and can be a reason why your AC won’t turn on. It is important to understand the underlying to why an AC is blowing hot air.

The AC turns on and off frequently

An AC system that turns on and off frequently is prone to fail sooner. The energy and strain on the motors are greater when the AC cycles on and off every few minutes rather than when it cycles correctly. There are several reasons why an air conditioner keeps kicking on and off.

When the compressor turns on and off frequently, it can indicate a few common AC problems such as drainage, thermostat, or refrigerant problems, an improperly sized system, indoor or outdoor coil restrictions, or a restricted air filter.

Each system is unique, and the nature of each problem can cause the air conditioner to keep kicking on and off in a unique way. For example, in some cases, the AC system will cycle on and off every 10 minutes, and in other cases, it can start and stop after a few seconds. You can read more about the various reasons in the links above.

The AC won’t turn off and runs continuously

Some AC systems can run continuously. For Example, a high-efficiency variable speed system is designed to run all the time with the ability to adjust its cooling capacity. This in-depth guide lets you learn more about the types of HVAC systems. However, in most cases, that is not the case, and if the AC won’t turn off, it is a sign that something is wrong with the AC system.

Standard AC systems that run continuously can have other underlying problems that cause the thermostat not to reach its set temperature. If that is the case, it can mean the HVAC system is improperly sized, the thermostat is faulty, the indoor or outdoor coils have restrictions, or there are ducting or refrigerant problems.

In some cases, just the AC blower fan won’t turn off, which can be a sign of any of the above issues. In other cases, when it is 100 degrees outside, and the system runs continuously, it can indicate an undersized HVAC system. As a rule of thumb, the higher the cooling capacity, the faster the AC system can cool a space. Although it is important to keep in mind that a system that is oversized is also not ideal because it will cycle frequently. You can learn more about this and HVAC system design here.

The thermostat keeps turning off and on or isn’t working right

A faulty thermostat can also be a reason the AC won’t turn on or isn’t working properly. The thermostat may be turning off and on, not turning off at the set temperature, keeps turning off, the display may not be working, and the screen goes blank, or it is not getting power at all.

There are many reasons that may cause any of the above symptoms in a thermostat. In many cases, it is a simple fix, such as a disconnected wire or replacing the thermostat. And in other cases, it can indicate problems with the drainage of the AC system.

Deep dive into air conditioning problems

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Dirty or restricted air filters and air restrictions

Dirty filters block the airflow within the HVAC system and throughout the home, causing air restrictions within the air conditioning system. Restricted airflow can cause many problems with the air conditioning system due to high static pressure.

An air restriction caused by a dirty filter can cause an indoor coil to freeze. The indoor air conditioning coil freezes because the air conditioning system needs airflow to work correctly. You can learn more about how an air conditioning system works, but in short, the refrigerant temperature in the coil gets below freezing temperatures. When the humidity in the air makes contact with the coils, it condensates and freezes. A dirty air filter restricts the warm ambient airflow, which helps prevent the coil from freezing.

If the coil is restricted completely, this can prohibit any airflow through the coil. This prevents the liquid refrigerant from “evaporating” in the evaporator coil and, as a result, can cause liquid refrigerant to make its way back to the compressor and damage it.

In addition to freezing the indoor AC coil, a dirty filter can cause the blower wheel fan motor to die. The restriction in the air filter results in more strain on the motor as it forces air through the HVAC system.

Replacing an air filter is easy and should be done as part of your air conditioning maintenance. You can connect with an HVAC pro or do it yourself. You may wonder why your air filters get dirty quickly and how often you should replace them. As a rule of thumb, 1-inch filters should be replaced every 1-3 months, and 5-inch filters should be replaced about every 6-12 months. However, this greatly depends on how often the HVAC system works and the home conditions.

Frozen coils on AC (Refrigerant problems)

It is not uncommon that an AC unit will freeze in the summer. In fact, the summer is when the AC unit is on and will usually freeze. If the type of system you have is a heat pump, it is also possible for the system to freeze in the winter. What causes an AC to freeze is typically a refrigerant or airflow problem. There are two common refrigerant problems, too much refrigerant or too little refrigerant. Both are not good and can cause your air conditioning system not to work correctly.

When an air conditioning system is too low on refrigerant, it can cause the AC coils to freeze or not cool the home at all. Frozen AC coils can also be referred to as a frozen AC lines. When the refrigerant pressure is low, it is also a sign that there may be a refrigerant leak in the system. When an air conditioning system has too much refrigerant, it can cause the system not to cool properly and even cause a refrigerant leak because of high pressures.

You can learn more about how the refrigeration cycle in an air conditioning system works, but in short, refrigeration is about heat transfer and pressure changes. Think of an aerosol spray that gets cold as you use it. A fast pressure drop happens, and the substance (refrigerant) inside gets cold. The same concept applies to refrigeration in a closed system. If the pressure is high, the refrigerant won't be as cold, and it can cause a leak because the high pressure exceeds the system design. If the pressure is too low, it can cause the system to freeze because of the low temperatures and moisture build-up on the coils. If there is a significant leak, the refrigerant will escape entirely, and the system won't provide cooling at all.

Frozen AC coils

Always contact an HVAC pro in the event of refrigerant problems. Refrigeration is a science; many factors, such as airflow and outdoor temperatures, contribute to proper refrigeration within an HVAC system. And because refrigerants contribute to global warming and ozone depletion, they must be handled by an EPA-licensed professional. Click here to learn more about frozen AC coils.

There is no power to the thermostat

The thermostat is what we see and think about when it comes to HVAC. It is what tells the system to go on or off. Fortunately, nowadays, thermostats don't have many moving parts. However, there are some rare cases where an HVAC thermostat has no power or doesn’t work. For example, it may not have power, or it may be flashing or blinking when the heat or cooling mode is on.

A thermostat may have faulty temperature sensors or incorrect wiring, or the thermostat may have gone out due to electrical shorting, surges, or other internal issues. And other times, the thermostat works; however, some of the settings may be confusing, which can lead one to believe that something is wrong with the thermostat.

If you suspect the thermostat isn’t working or has power, ensure that it is wired securely, or try resetting it. Just be careful because the electrical lines are live 24 volts. If everything seems wired correctly, it could be a drainage problem causing the HVAC system to disconnect power to the system via the “safe-t-switch.” If that is the case, you should connect with an HVAC pro to diagnose the source of the problem.

Restricted or frozen indoor AC coils

The AC’s indoor (evaporator) coil can get restricted from dirt, dust, and debris, microbial growth, or the evaporator coil can freeze up. A restricted or frozen AC coil is not good for your HVAC system or indoor air quality.

When the indoor AC coil is restricted, it limits air flowing through it. This airflow restriction raises the static pressure and negatively affects the system's performance. It can also cause the air conditioning system to not work by damaging the blower wheel or compressor motor.

You can learn more about the negative effects of high static pressure on an HVAC system, but to give you an idea of the concept, imagine blowing into a straw with no restriction, now imagine blowing into the straw again; however, this time close of the end of it a bit. The second time your lungs will work much harder to force the air out of the straw. The same concept applies to the air conditioning system. That means when the static pressure is higher, the blower wheel fan motor needs to work harder, increasing strain and energy consumption. The airflow restriction can damage the compressor because it will not allow the refrigeration process to happen correctly. You can learn more about how the refrigeration cycle works within an air conditioning system. But in short, the warm ambient air flowing through the indoor (evaporator) coil evaporates the liquid refrigerant to a gas. An airflow restriction can prohibit the refrigerant from entirely evaporating into a gas. And because liquid cannot compress, any liquid refrigerant that makes its way to the compressor will damage it.

Microbial growth on an indoor coil will not only cause restriction in the system but will also taint indoor air quality. Although there is an air restriction, some air can still flow around the coils and into the home. That air can pick up microbial organisms and negatively impact indoor air quality.

Most newer air conditioning systems have safety switches to prevent severe damage to the air conditioning system due to restricted coils. However, when the indoor coil gets restricted, it is a sign of other underlining issues such as restrictive filtration, expansion valve problems, ducting problems, or refrigerant issues.

If your AC evaporator coil is freezing up or you suspect you have a restricted AC coil, an HVAC pro can diagnose and fix the problem.

Restricted outdoor coils

Restriction can occur in the outdoor (condenser) coil from dirt, dust, debris, or obstructions around the system. When the outdoor coil is restricted, it prohibits the refrigeration process from working correctly and can negatively affect the system's performance.

You can learn more about the refrigeration cycle within an air conditioning system. But in short, the outdoor (condenser) coil condenses the refrigerant from a gas state to a liquid form. It does so with the help of coils and a fan.

After the refrigerant exits the compressor into the outdoor (condenser) coil, it is a high-temperature gas. The ambient air from outside cools and condenses the gas refrigerant into a liquid refrigerant as it flows through the coil.

If the outdoor coil is dirty or restricted, the outdoor air won't be able to flow freely through the coils, which prevents the refrigerant from being able to condensate properly in the refrigeration cycle. This can prevent the refrigerant from having a good pressure and temperature drop when it enters the evaporator coil. Resulting in warmer temperatures blowing from the AC. Additionally, the high pressures will cause the HVAC system to consume more energy. The fan motor will need to work harder to pull air through the coils, and the compressor motor will work harder because the refrigerant pressure will be higher, eventually damaging the compressor motor.

Condenser coils are located outdoors and can easily get dirty. Consider cleaning your outdoor coil or cleaning it professionally to ensure your air conditioning system operates correctly.

Showing symptoms of a bad expansion valve

The expansion valve is a critical part of the refrigeration cycle. Below we talk about the types of expansion valves and the symptoms of a bad expansion valve. There are three common types of expansion valves; fixed, thermal, and electronic.

In the refrigeration cycle, the expansion valve acts as a metering device that reduces refrigerant pressure as it enters the evaporator coil. The reduction in pressure brings the refrigerant to a near-freezing temperature in the evaporator coil (think of the aerosol spray mentioned in the refrigerant problems section).

The refrigeration cycle won't happen correctly with a faulty expansion valve. It can either let in too much refrigerant or not enough. A few symptoms to look for when the expansion valve lets too much refrigerant into the evaporator coil is that the air coming out of the AC is warmer or not as cold as it should be. This can eventually cause damage to the compressor. A few symptoms that the expansion valve is not letting enough refrigerant in is that the evaporator coil can be colder than it should be, causing it to freeze.

An expansion valve could be faulty or show signs that it has gone bad if it was installed incorrectly or the system is charged with too much refrigerant, or in some cases, it may come defective from the factory.

Faulty motors

A typical air conditioning system has three motors, the blower wheel fan motor, the compressor motor, and the outdoor fan motor. If these motors fail, the air conditioning system won't work correctly.

The compressor motor is used to compress the refrigerant and is a vital part of the refrigeration process. Without a compressor motor, there will be no cold air from the system.

The blower wheel fan motor is the motor that forces the air through the system. If the blower wheel fan motor does not work, air will not flow through the system.

The outdoor fan motor helps the refrigeration process. The HVAC system may still work if the outdoor fan motor is not working. However, the refrigeration process won't work effectively, which can cause the air flowing through the system to feel warm, can damage the compressor and will cause the system to consume more energy.

Most HVAC system motors operate with the help of a capacitor. It is not uncommon that the motors are working fine; however, the capacitors have gone bad. A faulty AC capacitor should be ruled out before diagnosing a faulty AC motor.

Often a faulty motor is caused by other underlying issues, such as refrigerant or ducting problems, a restricted indoor or outdoor coil, or even a dirty air filter.

Signs and symptoms of a bad AC capacitor

An air conditioning system has an electrical component called a capacitor. A capacitor gives the motors within the air conditioning system an extra boost when powering on. Below are some bad AC capacitor symptoms to look for.

A capacitor works by storing energy, similar to a battery. However, it can release that energy in a burst rather than over time, like a battery. A faulty capacitor will prevent any motor that relies on it from working correctly.

Capacitors can go bad over time due to heat. However, other underlying issues, such as restricted outdoor (condenser) coils or refrigerant problems, can strain the motors causing the capacitor to go bad sooner.

We go in-depth about AC capacitors in this guide. However, one of the telltale signs of a bad capacitor is a loud humming noise from the system while on or during start-up or if the system appears to be on, but no cold air is coming from the vents.

Connect with an HVAC professional if you think you have a faulty air conditioner capacitor. Air conditioning capacitors require expertise. Every capacitor has a microfarad rating that must match the HVAC system's requirements. Plus, you should determine the underlying issue as to why the capacitor failed in the first place.

**Air conditioning capacitors store HIGH VOLTAGE and should always be handled by a professional.

Electronic problems

Every air conditioning system works with electronics. Some types of air conditioning systems have more complex electronics, and some have fewer.

The electronics connect and allow communication between three main components of the air conditioning system; the outdoor unit (condenser), the air handling unit (air handler or furnace), and the thermostat.

An electronic problem can be bad wiring, control board failures, faulty transformers, faulty AC capacitors, or a faulty motor. If the problem is with the field wiring, an HVAC pro can most likely fix the problem on the spot. However, other times, wiring built into a component of the air conditioning with a wiring harness is faulty. In that case, the HVAC professional must order wiring that is specific to the HVAC system.

In the event of a control board problem, it can take a few weeks to fix. AC systems can have a few different control boards specific to the type of system, which needs to be diagnosed and ordered by an HVAC pro.

A transformer is an electronic component that ensures parts of the air conditioning system receive the correct voltage. Typically in air conditioning systems, transformers convert 120 volts to 24 volts. A faulty transformer must be diagnosed and replaced by an HVAC pro and is usually done on the same day.

Connect with an HVAC pro if you suspect you have an electrical problem with your air conditioning system. Consider completing yearly HVAC maintenance on your system to help prevent electrical problems when you need your AC the most.

**Air conditioning systems use high voltage. Always connect with an HVAC pro for electrical problems**

Drainage problems

AC water drainage lines (P-Trap)
Air conditioning water drainage lines (P-Trap)

As an air conditioning system works, it creates condensation. This moisture build-up is normal. The problem is when the moisture doesn't drain properly.

Water that gets built up should make its way to the drain line and out of the home. The drain can often get clogged by dust, dirt, debris, or even insects or spiders that lay eggs or create nests.

When that happens, water gets backed up and can eventually flood, causing the air conditioning system to leak visually. When a "Safe-T" switch is in place, a clogged AC drain line will trigger the switch, which depending on how it's wired, will shut off power to the AC system and thermostat to prevent further flooding and damage.

Learn more about preventing and unclogging an AC drain line. Or connect with an HVAC pro to get it done for you.

Air conditioner safety switches

Many newer air conditioning systems have safety switches installed to prevent damage to the HVAC system's components. Additionally, many systems have a "Safe-T" switch that is field installed to avoid water damage in the home in the event of a clogged drain line. In many cases, the AC safety switch keeps filling with water which can indicate a clogged drain line.

Each manufacturer designs the safety switches differently, but the purpose is to prevent damage to the HVAC system. The various HVAC safety switches typically monitor the refrigerant's pressure, temperature, or both to avoid liquid refrigerants entering and damaging the compressor.

You should connect with an HVAC pro if your air conditioning system isn't working because of a safety switch. The HVAC safety switches will trigger because of refrigerant problems, expansion valve problems, drainage problems, ducting problems, or a restricted indoor or outdoor coil.

Ducting problems

It may be hard to believe, but ducting is the root cause of many problems in most HVAC systems. It is one of the few things that isn't mechanical but causes most air conditioning problems. That is because the ducting is unique for each home and is usually designed by the HVAC professional installing the system.

Every air conditioning system has specifications that determine how much airflow is required for the system to work properly and how much restriction or static pressure the system can handle to operate correctly.

If the ducting is undersized, it can cause airflow restriction problems and high static pressure. Which can cause the system to freeze, damage motors, cause refrigeration cycle issues, and causes the system to run less efficiently.

If the ducting is oversized, it may feel like little to no air is coming out of the supply registers (vents), and air won't flow around the home correctly.

In addition to improper duct sizing, ducts can get disconnected or damaged in the attic or within the walls. This can cause leaks in either the supply (conditioned air) or the system's return (unconditioned air) side. If the leak is on the supply side, then conditioned air will flow into the walls or attic. And if the leak is on the return side, then the air will not circulate properly throughout the home and will suck in filthy air from the attic or other unconditioned spaces.

A static pressure test and visual inspection need to be done to determine if there are ducting issues. A static pressure test involves drilling a few holes and inserting pressure probes to measure the pressure along different points in the system. You can connect with an HVAC pro for a static pressure test or learn more about Air Design if you are looking to redo the ductwork of your air conditioning system.

Oversized or Undersized AC System

Every air conditioning system needs to be sized to match the heat load requirement for the home. The heat load is determined by the cubic area of the space and other factors, such as the number of windows and insulation. Below we will explain what happens if the AC unit is over or undersized if it is better to oversize or undersize the AC system, and how to help an undersized unit.

When the system is undersized, it will run continuously, trying to cool the home to the set temperature on the thermostat. And when the system is oversized, then it will reach the set temperature too soon, causing the system to cycle on and off frequently. Additionally, oversized systems also fail to remove humidity effectively because they shut off too often. All in all, both are not ideal.

When in doubt, it is better to oversize a system. That is because the system needs to be able to cool the home adequately.

If you have an undersized system, you can help the system by reducing the heat load. This can be done by improving insulation, upgrading windows and doors, and reducing outdoor air leakage around the home.

In the case of an oversized system, it is possible to add a ventilation system, which brings in unconditioned air from the outdoors. This will help improve indoor air quality and increase the system's heat load, allowing the HVAC system to cycle correctly.

If you are considering replacing your HVAC system, check out the Air Guide. We provide in-depth insights about air conditioning systems and the top things to consider when buying a new HVAC system.


There are endless reasons why an air conditioning system won't work, and it can be frustrating when it happens to you.

In this guide, we covered the many reasons the AC system won’t work properly or turn on. We went over the signs to look out for to prevent a forthcoming air conditioning failure, such as signs of water, a noisy system, draft winds, warm air flow, frequent system cycling, and thermostat issues. Then, we dove deep into the most common causes to air conditioning system failures.

Now, you should feel confident in knowing what signs to look for in a faulty air conditioning system and be knowledgeable when connecting with an HVAC pro if you need one.

To prevent your AC from not turning on or causing issues in the future. And to ensure your air conditioning system works efficiently, it is best to maintain your AC system regularly.

In the meantime, you can contact us or comment below if you have any questions.

What air conditioning problem brought you to this article?

  • Signs of water

  • The system is noisy

  • The air is coming out too strong or too soft

  • The air coming out is warm

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