What Does Car A/C Button Do When Heater Is On? (Explained)

The A/C button in the car keeps the car’s compressor running so that it removes moisture from the air in the cabin. 

Is it normal for this button to be on when the heater is running? Is it bad for the car’s units to run simultaneously?

In today’s article, learn all you need to know about your car’s heater and air conditioner!

Here’s What The Car A/C Button Does When The Heater Is On

The A/C button keeps the car condenser on while the heater warms the car. This helps to increase visibility and decrease fogging on the windshield. Last, it also helps maintain the health of the climate control system. 

Hyundai Tucson climate Control
Hyundai Tucson climate Control switched on

The air conditioner is always turned on when the car’s climate control system is initiated in modern cars.

At least, so long as the car’s automatic climate control system is tuned on or the defrosters are activated. 

Car defrosters are an essential part of the car’s climate control system. Without this, it would be virtually impossible for most people to see the roadway clearly, especially at night. 

Thanks to the car’s A/C unit, the defrosters can remove moisture from the windshield. 

However, just because the car’s A/C button is on doesn’t mean the system will blow cold air.

If the heater is on, all the A/C unit does is compress the air moisture and dispel it from the vehicle. This way, hot, dry air enters the car rather than hot, humid air. 


Is It Okay To Have Both A/C And Heating On Simultaneously?

BMW Climate Control with controls switched on
BMW Climate Control with controls switched on

It is okay to have both the A/C and heating systems on in your car simultaneously. This important feature in your car allows the vehicle to pump dry air into the cabin. If the car didn’t dry the air before pushing it into the vehicle, the windshield would be too foggy to see through correctly. 

It might sound counterproductive, but running the car heater and air conditioner together is essential. This is the intended purpose of the car’s climate control features. 

The cold fins on the A/C unit pull the water from the air. This water is separated from the air and dumped below the car while the dry air is pushed inside. 

Similar to house A/C units, car units contain freon. Freon is a chemical that artificially lowers the temperature of the atmosphere.

This process also causes evaporation of the water particles in the air, allowing the system to dry the air. 

In some cars, you must turn on the heater and the A/C to defrost your windshield. This was especially true in older cars. However, you can automatically turn on this feature in most modern cars by selecting the defrosters. 

What Is Dual Climate Control In A Car?

Audi Auto Dual Climate Control
Audi Auto Dual Climate Control

Dual climate control in cars is a feature that allows two temperatures to flow at the same time. This feature is good if both front-seat passengers want a different temperature blowing on them. In short, the right-hand passenger can use the heater while the left passenger uses the A/C. 

Dual climate control is a luxurious feature in some modern cars.

Although not standard for all new vehicles, it’s becoming more popular in SUVs, vans, and other larger cars. 

Back in 1986, German engineers produced the first BMW with dual climate control. This feature was somewhat simpler back then. It included separate vents that could be tempered to the liking of the left and right. 

Now, this system is more advanced. 

The system works by having several sensors positioned throughout the vehicle. While most cars have this feature only in the front seats, some family vehicles have these sensors in the back seats too.

The sensor communicates with the computer after you input your preferred temperature into the car computer. The sensor tells the computer whether that preferred temperature has been achieved and acts accordingly. 

In these scenarios, the A/C and heater can be initiated simultaneously to adjust the preferred temperatures. 

Why Does The A/C Button Automatically Come On When Turning On The Heater?

Car dual zone climate control
Car dual-zone climate control

The illuminated A/C button simply indicates that the car compressor is activated. The A/C will not turn on or push cool air into the cabin unless you initiate it. All the A/C does in this scenario is compress dry air in case the defrosters are triggered. 

The indication that the air conditioner is on only means the air is being compressed. When the air is compressed, it is dehumidified and turned to dry air

Warm air typically has a higher water vapor content than cold air due to the evaporation of water. When cold air hits warm air, the water particles are dissipated and brought to the earth as rain

This is a bit of a simplification of the weather system, but it helps explain why cold air is drier than hot air. Hot air holds water in a gas form, while cold air turns it into a liquid state. Although rain is wet, removing the water from the air leaves behind cooler, drier air. 

This is similar to how your climate control system works in your car. The car A/C cools the air, compresses the moisture, and expels it from the car.

This allows the vehicle to dehumidify the cabin and increase visibility whenever the defrosters are engaged. 

Does The A/C Control The Heating (In Auto Mode)?

The A/C and heater always work together if the car has an automatic system. Unless you select to handle the climate control manually. In older car models, you need to turn on the air conditioner to dry the air before turning on the heat.

The air conditioner and heater work together. The A/C doesn’t control the heater, but the heater doesn’t control the A/C either. 

The automatic system offers several benefits. These include the following: 

  • Automatically moderates the temperature 
  • Provides Comfort
  • Fuel efficiency
  • Automatic and less distracting

While this automatic system is ideal, it’s not available in all vehicles. It’s preferable to the old system, which requires the driver or passenger to modulate the controls manually. 

Does Automatic Climate Control Save Fuel Compare to Manual?

Audi A6 climate control manual sync
Audi A6 climate control manual sync switched on

Automatic climate control does help save fuel and energy if used properly. Proper use of automatic climate control includes proper temperature inputs and reasonable adjustments. If you select a moderate temperature, the car will work to maintain it and minimize energy consumption. 

Automatic climate controls are more energy efficient than their manual counterparts. This is achieved by regulating the moisture and keeping the cabin at a moderate temperature. 

Like home A/C units, it’s more efficient to keep the system on than to turn it off. This is because the system moderates the temperature and maintains a comfortable level. It also regulates the humidity of the cabin and removes moisture consistently.

By consistently removing the humidity, the A/C achieves two necessary functions. 

  • It helps maintain the health of the exhaust and climate control systems in the car
  • It saves energy by preventing the climate control system from overworking to remove humidity

The A/C and heater need to work hard to get the temperature and humidity regulated again. 

Check out this great article to check on how to save fuel while using climate control.


The car’s air conditioner and the heater must work together to keep the car’s environment comfortable. The heater provides warmth in the winter months, but even in cold weather, the A/C is needed. 

The air conditioner works to remove the moisture so the heater can bring optimal heat to the cabin. This isn’t too different from how all heating and air conditioning units operate. 

Home, airline, and industrial heaters and airconditioners always work together! They need one another to make the environment comfortable. 


Your Mechanic: How the Defroster Works


FJC: Air Conditioning Products, Tools, Equipment and Refrigerant: Is it OK to Run the A/C While the Heater is On?

Cars: What Is Dual-Zone Climate Control?

Curb Classic: Cold Comfort: The History of Dual-Zone Automatic Climate Control

American Meteorological Society: Dry Air

National Geographic: Rain

OTO: Automatic Climate Control – Pros and cons

WOOD Air Conditioning INC: Should You Turn Off Your AC When You’re Away?