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Background and Identification
Air conditioning (also called “AC” or “A/C”) is the process of controlling the temperature and humidity of air. You might not have known, but modern air conditioning was actually invented by Willis Carrier in 1902 to control the humidity of the air in a printing factory. The cooling effect of the machine also happened to increase worker productivity in the hot summer, so demand for this new invention quickly increased. Over time, air conditioning became more common in houses and cars to improve the comfort of the people inside. Before Carrier’s invention, ice was often harvested and stored during cold months to cool the air in the summer.
The modern electric air conditioner utilizes a special chemical called a refrigerant to move heat from inside a space to the outside. Early refrigerants, called “chlorofluorocarbons” or “CFCs” did their job well but caused significant environmental damage. CFCs released into the atmosphere severely damaged the ozone layer and ended up creating large holes in the layer near the north and south poles. Since the ozone layer—a region in the Earth’s atmosphere filled with the chemical “ozone”—shields the planet from most of the sun’s damaging UV radiation, scientists predicted an increase in cancer in humans and skin damage to other animals like whales due to the ozone hole. After this effect was discovered in the 1970s and 1980s, CFCs were banned and replaced with more environmentally-friendly refrigerants. This steady shift towards even better chemicals continues today and the ozone layer is predicted to recover before the year 2100.
Not all air conditioning systems use refrigerants, but this page will only cover repair information for traditional systems. Other systems work differently and require some different repair techniques. The most widespread type of air conditioning without refrigerant is called an “evaporative cooler” or, more commonly, a “swamp cooler.” You can learn more about swamp coolers on this page from How Stuff Works.
For AC repair, you may need to find the brand and model of your air conditioner. You can usually find this information on the condenser unit located outside your building. It is shaped like a metal cube and has vents on the sides. Find a sticker on the unit that has the brand’s logo and displays the (sometimes lengthy) model number. You can use this information to find some help online for your specific device. If your air conditioner is smaller (e.g. it sits in your window or has a tube connecting it to a window) the sticker will likely be on the outside of this all-in-one unit.
I find that I can troubleshoot and repair something more effectively if I (somewhat) know how it works. If you want to learn a bit about how your AC system works, see the section below titled “How AC Works.” There’s also some information there on the different types of AC systems in use and some basic terminology if you end up needing to call an AC repair person. For car AC specifically, see the bottom of the page.
The thermostat batteries are dead
The simplest fix for AC issues is to replace a dead battery in your thermostat. Of course, this troubleshooting step only applies for people who have a thermostat, and even then not all thermostats require batteries. Older thermostats (like this or this) without a little screen on the front don’t need batteries; new thermostats that let you program AC and heat for a specific time of day often require batteries. You can take your thermostat off the wall by pulling it straight out away from the wall (if this doesn’t work, push it up and then out). The thermostat is mounted on a plastic plate on the wall, so it’s made to be easily removed. If it requires batteries, they will be easy to find on the back side of the unit. Thermostats usually require AA, AAA, or CR2032 coin-cell batteries that you can find at a local store.
The air returns are blocked
If you have ducts in your house/office, you’ve probably noticed that wherever the ducts poke out of the wall/ceiling/floor they are covered by a grate. Most of the grates have a little lever on the side that lets you close them off, but at least one grate cannot be closed. These grates that are always open are called “returns.” The AC unit needs a steady supply of air to work properly, and it sucks this air in through the returns. Thus, if you block the returns you will starve the AC of air and the evaporator will eventually get so cold it starts to freeze. If you’re having issues with the system, make sure the return(s) in your building aren’t blocked by boxes or furniture, and give them a good clean with a vacuum to remove any built up dust and animal fur.
The air filter is dirty
Most AC systems have a filter somewhere that removes dust from the air so it doesn’t clog up the fans and evaporator fins. You should replace/clean this filter about once a month to once every two months depending on how dirty your space is. If you look at the filter and it’s absolutely coated with dirt, replace/wash it right away.
Disposable filters (mostly found in split systems) are usually made from some kind of fabric in an accordion-like pattern held together at the edges by cardboard. If you remove one of these filters, replace it with a new one from the store. Washable filters (more often found in mini-split systems, portable units, or window units) have a border made from a tougher material like plastic or metal. You can simply clean these filters with a vacuum or spray them with a hose (let it dry out after) and then put them back in place. You can see the sections below for examples of how to find the filter on different types of AC units.
Replace the filter in a split system
Here’s a video overview of the filter-changing process.
With a split system that supplies cool air to the entire building through ducts, the filter is usually located by the air handler (a.k.a. the metal box where all the ducts in the building converge). Here’s a picture from Wikipedia of a fairly standard setup. The filter will be located in a duct just to the side of the air handler or even in the bottom or top of the air handler itself. Sometimes the slot that holds the filter is covered by a metal strip that you can temporarily remove. If you can’t find the filter here, it might also be found behind the grate on one of the big air intakes in your building. (See above for how to identify which grates count as air intakes). If you have multiple air intake returns, replace the filter on all of them.
Finally, make sure you turn off the AC unit before replacing the filter and turn it back on afterwards. It’s bad for the unit to run without a filter in place. You can turn off power to the unit by flipping the switch on the side of the air handler or setting your thermostat to “off” instead of heat or cool.
Replace the filter in smaller systems
In other AC systems (e.g. a mini-split, window unit, or portable unit) the filter is always part of the unit itself. First, turn the unit off by unplugging it or via the remote. Open a hatch on the front or side of the unit and remove the mesh filter. You can wash the filter with water or clean it with a vacuum. Let it dry and then put it back in place. Here a few demonstration videos for each different type of AC unit:
The AC is leaking water (drain line issue)
An important part of any AC system is the evaporator, a series of metal fins that work to suck up heat from the air in your building. The evaporator fins can only do this because they are fairly cold. A side effect of this situation is that water will turn to liquid on the evaporator fins and drip down into your AC unit. If the water doesn’t have somewhere to drain out, it will pool up and start to leak out. If you notice that your AC unit is leaking water, these simple fixes can save you from having to call an AC repair service.
For small window units, the evaporator sits in the part of the unit on the outside of the building. The water is supposed to drain from a hole in the bottom of the unit. If you can, look at the bottom of the unit and try to clean the drain hole if it is clogged (note: not all units have a drain hole). A window AC unit should be tilted slightly backwards so that water inside runs outside instead of dripping into your room. If your unit is tilted forward, try reinstalling it at a better angle. Some people online say that you can drill a new hole in the unit to drain water, but I don’t recommend this practice because the unit usually needs at least some water to run at an optimal level.
For portable units, some models need to be manually drained. The drain has a temporary plug and is often located at the back of the unit near the bottom. You can lift the unit onto a chair or table and then remove the plug to drain water into a bowl, or take the unit outside and remove the plug (see this video from LG). If your room has a drain on the floor, you can buy some flexible tube and use it to move the water from the AC unit to the drain (as shown in another LG video).
For a split system or mini-split, the extra water goes out a hose or PVC pipe. On a mini-split system, this hose comes out of the bottom or back of the unit attached to your wall. For a split system, the hose is attached to a duct near your air handler/furnace (in simple terms, it’s near the area where all the ducts converge). As shown in this YouTube video, you can pull the hose off the fitting it is attached to and blow through it to clear a clog. If this doesn’t solve the issue, try taking the fitting itself off the drain hole (this probably requires a wrench) and clean the inside it with a brush or towel.
PSA: How to turn off power to the AC
For the rest of the troubleshooting steps below, you should turn off the power to your AC unit before working on it. The high voltage used for AC equipment is dangerous, but you will be completely fine if you follow some sensible precautions. For small AC systems like a window unit or portable unit, you can remove power to the device by simply unplugging it from the socket on the wall.
If you have a split or mini-split system, a big part of the system is the condenser unit (a cube-shaped metal box with vents on the sides) which sits outside. The condenser unit gets power from your building and has a power disconnect box nearby which you can easily flip open. To remove power for the condenser, either flip the switches inside the disconnect box or pull out on the handle inside to remove the metal contacts and disrupt the circuit (as shown in this video). Most of these service boxes are easy to open, but sometimes they are specifically made to be “tamper-proof.” As a backup, I would also recommend turning off power for the AC circuit at your breaker box as described in this article.
If you have a split system and you’re working inside at the air handler/furnace, you can turn off power to this portion of the system by flipping the light switch that’s usually installed on the side of the unit. Of course, it’s never a bad idea to also turn off power at the break box as well.
The condenser unit is dirty
If you have a condenser unit outside (the metal cube that gets loud in the summer), make sure there are no leaves piled up next to the sides. If the area around the unit is not clear, the condenser fan inside can’t pull in air and the unit will overheat.
Every once in a while, you can also use a hose to clean the fins in the condenser and remove the dirt that builds up over time. The fins sit just behind the outer vents, so you can spray water through the vents and hit the fins behind. Please don’t spray the unit with pressurized water! In fact, don’t use anything stronger than the jet of water you can make by holding your thumb on the end of your hose. Try to spray the water in a straight up and down motion. The fins inside run vertically, so moving the water in the same direction keeps them from getting bent.
Also, before you spray the condenser with water (see the PSA above). You can see this whole cleaning process in a YouTube video called “15 Air Conditioning Maintenance Tips.” It’s great!
The refrigerant line insulation is missing
In a split system or mini-split system the condenser unit is connected to your building with two refrigerant pipes; one pipe sends warm liquid into the building and the other pipe carries cool gas back outside. You can distinguish these two pipes because the gas return pipe (a.k.a. the “suction line”) is much thicker than the other pipe. The suction line needs foam insulation around it to keep the gas inside from turning to a liquid before it reaches the compressor. This foam, however, can break down over time (due to sun, rain, animals, etc.) and should be replaced if it becomes thin or starts breaking off.
You can 100% add new insulation yourself. Simply measure the diameter of the suction line (the diameter of the pipe itself not including the insulation) and look for “suction line insulation” in the same size at a store or online. It looks like a pool noodle with a cut down one side. When you get back, turn off the power to the condenser unit as described in the PSA above. Rip off the old, damaged insulation, test-fit the new insulation around the suction line, and cut it to the length of the pipe. The cut in the insulation has a seal on each side which you peel off to expose glue. Pinch each side of the cut together to complete the insulation seal. The whole process is shown well in this DIY video on YouTube.
A fuse is blown
If your AC doesn’t start up, you might want to check if a fuse is blown somewhere. A fuse is a small electrical part placed into a circuit for safety reasons. Under normal circumstances, the fuse allows power to flow through it, but if too much current passes by, the fuse “blows” and stops letting power through. You may sometimes need to replace a blown fuse to restore power flow in your AC.
Fuses come in a few different styles. You can see on this UPS Battery page that fuses most often look like a long cylinder (sometimes with a glass center) or a flat rectangle with two metal pieces poking out. Here are some common locations of fuses:
- Split system: The electrical panel outside by the condenser unit sometimes has fuses inside as shown in this demonstration video. Some panels don’t have fuses and only have a breaker switch. With a split system, you might also have a fuse on the circuit board of your air handler. The air handler is a big box inside that all your air ducts go into. This YouTube video shows the location of this fuse and what to do if the fuse keeps blowing after you replace it.
- Mini-split: Check the fuse by the condenser as you would for a regular split system. The evaporator inside may also have a fuse on its main circuit board as shown in this video.
- Window unit and portable unit: If these do have a fuse, it will likely be found on the main circuit board. Some units do not have a fuse.
Extra tip: if you look at a circuit board in any unit and it has black scorch marks, you may have found the issue. For some models, you can order just a replacement circuit board online. Take a picture of the existing circuit board to document where each wire is connected to the board. Remove the wires and transfer them to the same place on the new board.
The breaker is tripped
As you know, the AC requires power to operate. If there is an electrical issue, the switch in your breaker box which supplies power to the AC will trip and stop supplying power. A tripped switch is easy to spot because it will not be in either the on or off positions. You can see in this HVAC School article that a switch that is on/off will be pressed to one side while a tripped switch will be stuck in the center.
Look for a short circuit
If no part of your AC is working because of a tripped switch, you can try turning the power back on. If the switch keeps tripping, however, you should investigate the issue; don’t just keep turning it back on. The first suspect to look for are wires where the insulation is missing. If this uninsulated wire comes in contact with another piece of metal it can create a short circuit and trip the breaker.
For small AC systems like a portable unit or window unit, look for damage to the power cord. If you can open it up and find the electronics, look for any wires that appear damaged. Make sure to unplug the machine before inspecting the electronics! If you have an outdoor condenser unit, remove power to the device (see the PSA above) and unscrew the side panel to find the wires inside. Again, look for damaged wires that could cause a short circuit. You can fix one of these wires by cutting it on either side of the damaged portion. Use wire strippers to remove half an inch ( a few cm) on the insulation on both sides of the cut wire and twist the two back together with a wire nut. You can see this general procedure in this Sears PartsDirect video. Of course, (see the PSA above) before you cut the wire.
Look for another issue
The breaker switch can also trip if the AC is pulling too much power. This happens when some part of the system is working too hard. For example, if the filter is very dirty, the evaporator fan will have to work very hard to push the same amount of air and will use extra energy. Some of the issues below can lead to a tripped breaker.
The condenser fan isn’t starting
If you have a condenser unit outside, it has a fan in it behind a wire grille. The purpose of the fan is to blow air across the fins in the condenser, allowing the refrigerant in the unit to dump out all the heat it absorbed while inside the building. If the condenser fan isn’t running, the condenser will start to overheat and the AC system will not produce cool air inside.
Thus, if you run your AC and it doesn’t seem to cool down your building at all, go outside and check the condenser fan. On the first time you start up the system after not using it for a while, the fan will occasionally not start spinning. As a first test, try poking a stick or screwdriver through the grille on the condenser unit and give the fan a shove. If it starts up and keeps spinning, that’s a problem solved! If, however, it doesn’t spin when you shove it or it needs you to jumpstart it this way multiple times in a row, you probably have a bigger issue. Read on…
Replace a bad capacitor
The most common cause for the fan not starting is a bad capacitor. You see, once the fan is started, it can run indefinitely, but it needs a little kick of energy to get through the first rotation. The capacitor is like a small temporary battery that sits inside the condenser unit and provides this required surge of power to the motor.
This video on YouTube (I highly recommend watching it) shows an excellent procedure to replace the capacitor in a condenser unit and get the fan running again. In summary, the capacitor is located with the rest of the wiring behind a panel on the side/top of the unit. (see the PSA above) and then remove this panel. You can buy a new capacitor at a store or by looking online, but make sure you find a replacement with the rating in farads. The rating of the capacitor will be written somewhere on the side with the units of uF or MFD (for microfarads). The capacitor also has a max voltage rating (written on the side with the units V), and the new capacitor should have an equal or higher rating. As an example, let’s say the existing capacitor reads “40 uf + 6%” and “50/60 Hz 370 V.” I would look for the information in bold and buy a new capacitor with a rating of exactly 40 microfarads and at least 370 volts.
Some capacitors in a condenser unit have two leads (the parts that stick out where you can attach wires), while others have three. A capacitor with three leads is actually two capacitors in a single case; one of the capacitors is for the condenser fan and the other is for starting the compressor. If you are replacing a three-lead capacitor, it will be labeled with the rating of each capacitor inside (in microfarads), so buy a new one that matches the rating for both.
Before you remove the existing capacitor, find a screwdriver with a rubber or plastic handle (to insulate your hand from the electricity) and use the metal part to connect the leads temporarily (do one pair at a time for a three-lead variety). This procedure discharges any remaining energy in the capacitor so you don’t get shocked. Next, for a two-lead capacitor, remove the wires from the existing capacitor and place them on the leads of the new one. The orientation doesn’t matter. For a three-lead capacitor, each lead will be labelled as “comm” for the ground, “herm” for the compressor (because the compressor is hermetically sealed), and “fan” for the fan, obviously. Move the wires to the same lead on the new capacitor one at a time (if you take them all off at once, you’ll get them mixed up).
Replace the fan motor
If replacing the capacitor doesn’t fix the fan issue, you will need to replace the fan motor. This video on the subject shows how you can confirm that the fan motor is dead using a multimeter. You can see this excellent DIY video for instructions on how to replace the motor, but it mostly involves unscrewing the fan grille, removing the wires and pulling them out with the motor (take a picture of where each wire went before you pull them off), and moving the fan blades to the new motor. To pull the fan blades off the old motor, you have to remove a screw on the side of the motor shaft and pull the blades up. Try using some oil to act as lubricant if the blades won’t come off the shaft easily. Also, (as described just above) before you touch any wires in the condenser).
The new motor needs to exactly match the old one in horsepower (that’s watts for people using the imperial system) as well as the rated RPM (rotations per minute). Additionally, when everything is put back together, make sure that the fan is sucking air up. Airflow for the condenser needs to come in the sides and out the top near the fan. Lots of replacement motors come with small connectors on the side that you can use to reverse the direction of the fan to achieve the correct airflow. See this video for more information on wiring the new motor and this other video for a tip on how to secure those reversing connectors with a PVC pipe so they don’t get chopped off by the fan blades.
The blower fan isn’t working
Much like the condenser fan described above, the blower fan can sometimes die or require a capacitor replacement. Whereas the condenser fan moves air across the fins of the condenser, the blower fan moves air across the fins of the evaporator. If either fan dies or won’t start, your AC will not cool the air effectively.
In a split system, the blower fan is located in the air handler or furnace. This machine looks like a big box where all the ducts in your building converge. This video from Word of Advice shows the full process of installing a new fan and changing the capacitor. Try replacing the capacitor first and move on to the more drastic measures afterwards. Remember to (see the PSA above) and (described in the condenser fan section above) before working on the fan. The same considerations from the condenser section apply when buying replacement parts; match the new capacitor and/or motor ratings exactly to their old counterparts.
How AC Works
The air conditioning system in your house or car works on the same basic principles as a fridge. Whereas a fridge moves heat from the space where the food is stored to the exterior air, an AC unit moves heat from the space you want to cool down into the air outside where the extra heat doesn’t bother anybody. The central chemical that allows us to move the heat around is called the “refrigerant.” Refrigerants are specially made to have the exact properties required to make this heating/cooling cycle work.
To explain further, I need to clarify one simple rule of physics:
- When a gas cools into a liquid it releases energy into the surrounding space. In the reverse case, liquid turning into a gas absorbs energy from the space around it.
The four components
There are four key components in an AC system: the compressor, condenser, expansion valve and evaporator.
At the beginning of the cycle, the refrigerant is in the gaseous state. It moves through a machine called a “compressor” which spits it back out at a very high pressure. Since the temperature of a gas increases when it is under high pressure, the refrigerant gets really hot. Hotter, in fact, than the air outside even on a warm day.
The refrigerant travels along a pipe to a part of the system called the “condenser” (this part sits outside your house or outside the cabin of your car). The condenser is basically a big set of metal fins connected to the outside of the refrigerant tube. The very hot refrigerant dumps a bunch of its heat into these fins, where it gets blown away into the outside air. To help make this transfer faster, the condenser usually has a fan nearby which blows more air across the fins, supplying new air every second for the refrigerant to dump its heat into. In the condenser, the refrigerant cools down enough for some of it to turn into a liquid. As I mentioned above, the gas turning into a liquid releases yet more heat from the refrigerant into the outside air.
The refrigerant now moves from the condenser to an “expansion valve” where its pressure drops significantly. With the decrease in pressure, the refrigerant cools down more and becomes colder than the surrounding air.
Finally, the cold refrigerant goes into the “evaporator” (this part is inside the space with you). The evaporator is made from another set of fins with another fan blowing air across them. In the reverse of what happened before, the refrigerant sucks heat out of the air, making the air much colder. With all the heat it pulls in, the refrigerant turns back into a gas and uses even more heat from the air to do so. After moving through the evaporator, the refrigerant goes back to the compressor to start the cycle over again.
In summary, the refrigerant is pressurized and heated up by the compressor. In this hot state, it goes to the condenser where it dumps the extra heat out into the air and turns into a liquid. It gets even colder and less pressurized when it goes through the expansion valve and is then able to suck heat from your house/car when it goes through the evaporator.
You can also find excellent explanations of this refrigeration cycle in the following YouTube videos:
- How a Home AC Works For Dummies (basic explanation)
- Personal “air conditioners” aren’t what they seem (more advanced explanation; skip to 4:31)
Types of AC units
All conventional AC systems use these four components, but the location of each depends on the type of system that you own. Knowing which type of unit you have makes repairs easier. In all the scenarios, keep in mind that the evaporator sucks heat from the air which the condenser then dumps outside.
The most common setup in houses is called a “split-system.” In this configuration, the compressor and condenser sit outside in a cube-shaped metal box with vents on the side (here’s a picture on Wikipedia). This whole box is called the “condenser unit” because it contains the condenser. You know you have this type of system if you walk around the outside of your building and you find one of these units. It will be connected to the building by two pipes which carry the refrigerant from this condenser unit to the evaporator inside.
Usually the evaporator sits indoors in a metal duct. If you follow the two refrigerant pipes from outside, you will find that they lead into the duct and are connected inside to the evaporator. Since the evaporator in this duct gets fairly cold, water in the air will actually turn to liquid on the fins and run down into a “drain pan” sitting inside the duct. The outside of the duct usually has another hole with a tube coming out so that this extra water can drain out.
The fan that pushes air past the fins of the evaporator is located nearby, usually in a metal box called the “air handler.” If you open up the air handler, you can identify the evaporator fan because it is almost always shaped like a big hamster wheel. If you also have a gas furnace for heating your building in the winter, the furnace serves double duty and replaces the air handler. In that case, the blower fan is located somewhere inside the furnace and moves air for both the heating and cooling system depending on the season. You can see a great breakdown and explanation of all these pieces in this YouTube video.
Mini-split system (a.k.a. ductless)
The mini-split system is almost the same as the split system except that it doesn’t use any ducts. This setup uses the same condenser unit outside, but the refrigerant pipes from it connect to a little machine (picture from Wikipedia) containing the evaporator. Oftentimes, you control this evaporator unit with a remote control. The cool air from the evaporator is dumped straight into the room containing the evaporator unit instead of being pushed through a duct to all the rooms in your house.
The window unit is a small box that contains the entire AC system. It is designed to sit in your window and have one side stick outside. This type of system in more common in a small space like an apartment because it is far easier to set up and cheaper to buy than a split system. To work correctly, the compressor and condenser are on the outside of the window and separated from the evaporator inside by some kind of styrofoam insulation. You can see one of these units on the desk at the start of this video.
A portable unit lives completely inside your room and often has wheels. It vents the hot air from the condenser to the outside via a flexible tube that you attach to a window or other hole in your wall. Portable units usually work best for smaller spaces when a more efficient window unit is impractical (the window is too small, etc.). Buying tip: if you have a choice, look for a portable unit with two hoses instead of just one. Double hose units are usually more energy efficient than their single hose counterparts, as explained by this mini-rant on YouTube.
Here’s a few common terms that you might come across in your repair adventures.
- Blower fan: The fan used by the evaporator. In split systems it usually looks like a big hamster wheel.
- Freon: Used interchangeably with the term refrigerant.
- A-coil: The evaporator in a split system. The name comes from the shape of the fins in the evaporator which resemble an “A” or upside-down “V.”
Car AC Troubleshooting
While this page is not primarily focused on car AC repair, I can provide a few useful videos on the subject. The first series of videos comes from Ratchets and Wrenches on YouTube:
- How Car AC Works for Beginners. A car AC system uses the exact same concepts described in the “How AC Works” section above, but each component is in a different spot in a car. Additionally, the compressor in a car doesn’t need to use its own motor inside, it can steal rotational power from the engine. It’s useful to know about the belt and clutch that connect the compressor to the engine so you can troubleshoot these parts.
- How to Diagnose Problems With Your Car’s AC System. This can help you narrow down your AC issue to a single component.
- How to Add Refrigerant to Your Car’s AC System. This video shows you how to top off the refrigerant in a car AC system.
The next set of videos comes from the venerable ChrisFix. They show the whole process of fixing a car AC system by locating the leak with UV dye refrigerant (turns out it was on the compressor), replacing said compressor, and refilling the whole refrigerant tube. Awesome!
- How to Find AC Leaks in Your Car (UV Dye)
- How to Replace an AC Compressor in Your Car
- How to Properly Recharge Your AC System
Car AC repair
- How to diagnose problems with car AC on YouTube
- How to find leaks in your car AC with UV dye on YouTube
- How to add refrigerant to car AC on YouTube (adding more refrigerant to existing amount)
- How to recharge car AC on YouTube (refilling refrigerant from empty after changing a compressor)
How AC works