How does car A/C work? Here's everything you need to know

The hot weather is back, and one of the worst things that you can run into is hitting the button for the air conditioning, and only getting hot air. Or maybe even no air. So that means something’s gone wrong with your car’s A/C. There are a variety of things that can go wrong, from fairly simple, to more complex. And before you run off to the auto parts store to try tossing a can of refrigerant into the system, there are some things you should know. Since that might not fix the problem, even temporarily, and even if it does, it won’t be a permanent fix. So let’s go over some basics about how air conditioning works, what might go wrong with it, and what to do to fix it.

How does car air conditioning work?

The heater in your car is nothing more than a small radiator. Hot water passes through what is called the heater core. Heat emanates off the core and this hot air blow into the car to heat the cabin. The air conditioner system works thanks to pressure changes in the refrigerant. These are generated by the compressor, which pumps the refrigerant. It creates a high-pressure side and a low-pressure side. On the high-pressure side, it forces gaseous refrigerant through the condenser. This is where the refrigerant condenses (get it?) and in the process, it releases heat, which is dumped into the outside air that passes over it. The cold liquid refrigerant passes into the low-pressure side of the system called the evaporator core. This is where the refrigerant evaporates (see, there’s a theme here), and the process absorbs heat. The heat it absorbs comes from the outside air that passes over the evaporator core, thus cooling that air before it reaches you in the car’s cabin.

In most cars, specifically internal combustion cars, the air conditioning system has nothing to do with heating. Instead, hot engine coolant is routed into basically a smaller radiator near the cabin called a heater core. It works the same way as the radiator, with outside air passing over the radiator, absorbing the heat from the coolant before entering the cabin to warm you up. This is also why if your car is starting to overheat, it’s good to switch on your heater to get some of the heat out of the engine’s cooling system.

But in electric cars, which don’t have large amounts of hot coolant to tap into, they need to generate heat in different ways. They can generate it by running electricity through high resistance wires, which become hot and transfer heat into the air, but that’s extremely inefficient. That’s particularly bad when you need that electricity to move the car. So a highly desirable feature is a heat pump, which is effectively air conditioning that can work both ways. Remember that we said there’s a high-pressure side, that expels heat, and a low-pressure side that absorbs heat. Switch those around, and you get the hot side warming up the air for the cabin instead of the cold side. It’s far more efficient than the resistive heating, too, thus being ideal for EVs.

How can my air conditioning fail?

These are some of the most common ways air conditioning can stop working, as well as what parts will have likely broken and need repairing or replacing.

Refrigerant Leaks: Leaks from ruptured hoses or poor connections result in system shutdown. Loss of refrigerant oil due to a leak can cause compressor failure.

Dirt: Anything other than refrigerant chemical and oil in an air conditioning system is considered dirt. Dirt can cause seized compressor bearings, expansion valve failure, decomposition of refrigerant and oil, and corrosion of metal parts.

Restrictions: These can happen for a number of reasons. The most frequent? Dirt, metal shavings, powered desiccant (moisture absorbing material), and foreign matter from the atmosphere. These substances cause high pressure, heat buildup, and loss of lubrication within the A/C system. The deadly duo (pressure, heat) cause “O” ring decomposition, brittle A/C hoses, and seized compressor bearings. Loss of lubrication can result in compressor failure.

Moisture: Combined with metal and refrigerant, moisture can cause the formation of oxides and acids, a sure death sentence to any A/C system. Additionally, moisture can freeze up expansion valves and powered desiccants.

All these items cause A/C system failure. How long can you go with an inoperative system before A/C failure? As long as it takes for these conditions to set in and cause failure, and it isn’t long.

Also worth noting is whether your problem is the temperature of the air, or the air flow. If you’re just not getting air flow, it could be an issue with your fan, or with the vent doors that let air through. It could even be that there’s something in your air vents blocking air flow. These aren’t directly related to the air conditioning system.

Can I just recharge the A/C?

If your air conditioner needs recharging it is because the refrigerant chemical is leaking out of the system. And it is possible to buy a can of refrigerant to recharge a system that’s gone low. But it has gone low because the system is leaking, which it’s not supposed to do. So just recharging it doesn’t fix the underlying problem. And that means that refrigerant will eventually leak out again. Not only will you have then wasted your money, but you’ll have also let materials harmful to the environment out into the atmosphere. Many cars currently use R134a, which is a powerful greenhouse gas, and much older cars use R12, which was discovered to damage the ozone layer. So for both your long-term bank account and for the environment, you should skip a quick fix recharge for finding the root of the issue and fixing that.

The other problem, is that, it might not simply be a lack of refrigerant in the system that’s causing it to not function. There are other components and malfunctions that can stop it from working, which we already discussed.

Should I take it to the dealer or can I go to an independent repair shop?

Considering the fact that a home recharge kit likely won’t fix the root problem, and the fact that other components could have gone bad, it’s best to go to a professional for a repair. And we know, that’s probably not what you wanted to hear, but we want to be straight with you. Professional mechanics will be able to more quickly diagnose the problems, and not only that, but they’ll be able to get the components and the refrigerant necessary for repairs. They can also capture and dispose of leftover refrigerant properly. If the vehicle is still under the factory warranty, then go to the dealer so the repair is covered under warranty. If out of warranty, go to whatever qualified shop you desire.

How do I know what refrigerant my car uses?

If you really do want to recharge your air conditioning yourself, you will need to get the correct refrigerant. There are three refrigerant chemicals typically used for automotive air conditioning: R12 (or Freon), R134A, and within the last few years, R1234yf has been introduced. Your vehicle will have labelling that indicates what refrigerant is used, but each air conditioning system has refrigerant taps of different sizes to help ensure that only the correct refrigerant goes into the vehicle. And you must not mix or use the wrong refrigerant in the system, as the components are designed specifically for each respective type.