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Your car AC unit and heater are one thing.

Twenty degrees below zero and yes, your car AC unit and heater are critically entwined.

A car AC unit and heater are not independent systems like you would think. In theory their separation stands to reason. Since when is a fridge integral to your wood stove cranking out heat? It makes sense that systems so different in purpose would have little overlap in design. 

A wood stove burns bright in an alaskan home. This is a heater unit completely separate from a car ac unit.

Climate Control, not Heat + Air Conditioning

Since the 00’s, climate control is considered standard equipment rather than a car ac unit and heater. Similar to HVAC systems (heating, ventilation, air conditioning) in homes and buildings, vehicle climate control systems have integrated functionality. In fact, car AC units are not merely joined with car heaters to form the ‘climate control’ in vehicles, they are dependent on each other. 

What is a car AC unit?

It used to be that cars came with or without an AC unit. Now you cannot commonly find a passenger car sold without AC because climate control requires that the air be “conditioned” or modified before it enters the cabin or interior of the vehicle. Air conditioning literally refers to a process of modifying the characteristics of the atmosphere surrounding us in a given space. 

How does your car AC unit modify an atmosphere?

The answer is simple. Evaporation. The main principle used in treating air before it enters a room or vehicle cabin is ‘evaporation’. When you sweat, tiny water droplets form on your skin which feels cool. Most of us think perspiring cools us because we are wet, like taking a cold shower. This is a common misconception. Evaporation and getting wet are two completely different ways of cooling down. 

Thermal transmission– cooling via temperature trade-off

Thermal transmission is what happens when one thermal mass exports heat or cold to another thermal mass. When your 98*F hand touches an ice cube, the ice melts and your hand feels cold because there has been a temperature trade-off. When you hop in a cold shower, the cool water is absorbing heat from your body and the water is warming up while your body is cooling down. This process of trading temperatures is entirely different from the process of evaporation, which also has a cooling effect. 

car ac unit and heater not working as shown by a thermometer registering only 110 degrees at the car fan.

Evaporation– cooling via chemistry more than physics

When we perspire, the water droplets that form on the skin are very near 100 degrees Fahrenheit because they just emerged from pores in a 98 degree human body. This warm water is poised to turn from a liquid into its vapor form, hence the name ‘evaporation’ or “process of turning into a vapor”.  When water is vaporized and changes into floating rather than sitting molecules, heat is extracted from the skin in a chemical process. Evaporation steals heat.  If you are dying to know more here is a full and brilliant explanation of this process on Khan Academy.https://www.khanacademy.org/science/biology/water-acids-and-bases/water-as-a-solid-liquid-and-gas/v/evaporative-cooling

Back to cars — How vehicle climate control uses evaporation, summer and winter. 

Air conditioning cools an atmosphere because it uses the process of evaporation to remove heat. A by-product of vaporizing water is condensation. Ever notice a pool of water under your vehicle if it idles in a hot parking lot? You are witnessing the evaporation process without a 55 mile-an-hour breeze removing the vapor. The vapor turns back into a liquid (condenses) and falls to the ground. 

Condensation and defrosting

This is where the automotive industry got creative and formed the “climate control” concept which fully integrates heating and cooling. Here is what they figured out: Frost on a windshield depends on moisture-laden air either on the inside or outside of the glass. Ever set your coffee mug on the dashboard while you buckle your kids up? If it’s -25 out you can be sure your windows are good and solid fogged up! How about getting in the car after a 10 K ski? Does your driver’s side window ice up immediately? This is water vapor being released into the tight bubble of your cars’ interior. Remove the vapor and you remove frost. Hence the name defrost!

Frost collected on trees and hanging in the air. In winter, Alaska car ac unit and heater combinations are truly critical.

Your car AC unit and heater is your defroster

Simply put, engineers have cleverly used the evaporation process in air conditioning to remove vapor from air before it enters the cabin. This is the main principle in keeping your windows clear–remove as much vapor as possible from the air. Don’t breathe, don’t let your dog pant, and don’t bring steaming hot coffee on the commute to work. However, it turns out we will breathe, drink coffee and bring our dogs everywhere. The solution? Condition the air inside and remove that vapor!

Car AC unit removes moisture from the air.

Climate control systems run the air conditioner automatically when the defrosters are on. If your AC is not working, your defroster is not working. 

“The A/C button means the compressor that runs the A/C’s cooling function is running and the coils are cold. It does not mean you’ll get cold air though, that depends on the temperature of the air going through the heater. It is being used in a literal sense as an “air conditioner” as in it conditions the air to make it comfortable regardless of temperature. Generally speaking, it does this by reducing the humidity.”

When a car AC unit is enabled and the heater is disabled (the temperature-dial set all the way to cold), you’ll get cold air, because the AC cools the air and no heat is added.

However, if the car AC unit and heater are both on at the same time, your engine’s heat is more than enough to mix with cooled air from the AC unit to produce warm temperatures. This is where the AC unit is crucial. The coils that produce cold in the AC unit collect moisture in the form of condensation. The AC unit pulls moisture out of the air before it enters the vehicle cabin. Air that has been dehumidified or “conditioned” feels nicer whether hot or cold.

This is what we call Climate Control, and it’s why your defroster depends on your car AC unit and car heater working.

 Common symptoms of car AC unit and heater issues:

  • On the coldest setting the vents feel like a fan rather than fridge.
  • Your car interior has a sweet, humid smell. 
  • Moisture inside your vehicle on the passenger side floor
  • Foggy windows, summer or winter

If you’ve experienced these symptoms and think your car AC unit and car heater are not working the way they should, schedule a time to come by the Garage. It is stinking hot this summer. Control your climate right now. Be comfortable summer and winter!

Too Long; Did Not Read

Not to worry! We have you covered. From Reddit a fantastic synopsis:

Your car’s climate control works as such:

Air is drawn from either inside the car or from outside the car, based on whether the recirculation button is pressed.

That air goes to either the “hot” path, the “cold” path, or some mixture of the two.

That’s what the red/blue knob selects between. Fancier climate control systems may make this decision automatically based on a temperature setting.

On the hot path it goes through a heat exchanger from the engine.

This is “free” heat because the engine would be hot anyway. This is why the heater doesn’t work until the engine warms up, and why you can crank the heat if your radiator is struggling to keep your engine cool.

On the cold path it goes to the A/C unit which may or may not be running.

The A/C button turns this unit on or off. The A/C unit is much like the A/C in your home, but smaller. It has a big compressor which takes a significant amount of power to run. This affects fuel economy and can drain your battery if your car will let the compressor run with the engine off.

The air from those two paths is rejoined and directed to whichever vents are selected.