Why does the compressor start when the defrost function is turned on when the car is turned on?

We all know that the AC light on the car's air conditioning panel is on to prove that the air conditioning compressor is working. However, some careful car owners found that the AC light turned on when the heater was turned on to defogging in the winter, and it also came on automatically after turning on the defogging function. What is going on? Could it be that the air conditioner circuit is broken? In fact, this is a normal phenomenon, and this is how the car air conditioning system is set. The reason why the windshield fogs up

When the windshield fogs up, the fog is actually a lot of tiny water droplets

It is very cold outside in winter, and the temperature of the windshield is naturally very high when it is exposed to the outside. Low. When we sit in a car, a large amount of water vapor is produced when we breathe. When the water vapor encounters the cold windshield, it will liquefy and form tiny water droplets that adhere to the inner surface of the windshield, affecting our vision. The principle of warm air defogging

The essence of windshield defogging is to let water droplets re-evaporate

The essence of turning on warm air defogging is to let the water droplets adhere to the windshield Evaporate quickly, and we have learned in junior high school physics that if you want to speed up the evaporation, you can increase the air flow and increase the temperature. Obviously both of these points can be achieved by turning on the heater.

The drier the airflow, the better the windshield defogging effect

But what junior high school physics did not tell us is that the drier the airflow, the better the defogging effect. To give an extreme example, You use a garment steamer to use steam to remove fog. Do you think it is effective? This requires temperature and airflow. Therefore, the humidity of the air flow for defogging must be low, and the drier the better. Because the same airflow can hold more water vapor. The AC light comes on during defogging to dehumidify the airflow and improve the defogging effect

Car air conditioning system structure diagram

Cold air conditioners naturally have a dehumidifying effect because the airflow is cooled by the cold evaporator At the same time, the water vapor in the air flow will liquefy in large quantities, thereby reducing the humidity of the air flow. Therefore, cars with automatic air conditioning will turn on the AC switch when turning on the heater and defogging. The purpose is to let the air flow first pass through the air conditioner for dehumidification, and then be heated by the heater water tank before being sent to the cockpit.

In fact, this setting is basically a joke in the north, because the winter in the north is too cold, dry and cold, and the air humidity is already very low, so there is no need for additional dehumidification. Besides, the weather is so cold that even if the AC light comes on, it is impossible for the air conditioner compressor to start, because the evaporator will freeze into ice lumps as soon as it is started, and the air duct will be completely blocked. You must know that there is a temperature sensor on the evaporation box of the car air conditioner. If the temperature is lower than 3 degrees Celsius, the compressor will not work.

Comparison of air humidity in the south and north in winter

But winter in the south is different. The temperature is not very low, but the air humidity is really high. In this weather, it is basically basic to use warm air to remove fog. The effect is almost the same as using a garment steamer. All the air blowing on the windshield is warm and humid air. It cannot hold more water vapor on its own, so it naturally cannot achieve the best defogging effect. So at this time, you need to turn on the air conditioner compressor, first dry out part of the water vapor in the air, and then use dry airflow to remove the fog. The effect will be much better. Should I turn off the AC light?

You can turn off the AC directly

I think this is the issue that everyone is most concerned about. I just want to defog and don’t want to turn on the air conditioner, so can I turn it off? The answer is of course yes, just press the AC button to turn it off. It will not have any impact on the air conditioning system. If it does, the problem may lie with the manufacturer who designs the air conditioning system, not with the car owner.

But before turning off the AC manually, you should consider these two issues:

1. Is the air humidity in your area high? If the air humidity is high, it is very likely that the AC will be turned off. To reduce the defogging effect, don’t hesitate at this time, just turn on the AC again.

2. If you are in the north and it is several degrees below zero outside, then it does not have much impact whether you turn it off or not. At most, you can save electricity money for the indicator light. Because at this temperature, the compressor cannot work even if the indicator light is on.

But when the outdoor temperature reaches ten degrees, the compressor may really start. At this time, just turn it off, because the air humidity in the north is not high, so there is no need to rely on the air conditioner to remove the humidity. How to operate manual air conditioner?

When the automatic air conditioner turns on the defogging function, it can automatically turn on the compressor to dehumidify. However, my car has a manual air conditioner. How can I achieve this operation? In fact, the method is very simple:

Car air conditioner mechanical temperature adjustment knob

The first is this purely mechanical temperature adjustment knob. This knob essentially controls the damper in the heat exchanger. The direction of airflow. Turn toward blue and all airflow will pass through the air conditioner evaporator and not through the heater water tank. Turn toward red and the airflow will only pass through the heater water tank, not the air conditioner evaporator. If you twist it towards the middle, the air flow passing through the two will be controlled proportionally. Therefore, if you want to dehumidify this type of manual air conditioner while turning on the warm air and defogging, you must press the AC button to turn on the compressor, and then adjust the temperature adjustment knob to the appropriate position according to the defogging effect. Under normal circumstances, it can be placed at the position of 75% warm air.

Potentiometer-type temperature control knob

There is also a manual air-conditioning temperature control knob that is potentiometer-type, but the adjustment method is the same as a pure mechanical knob. It is generally adjusted to half of the red area. The location is about right.

When defogging the manual air conditioner, remember to select the external circulation mode, which will achieve better results. Because there are people inside the car breathing the extra water vapor, the humidity of the air outside the car is in most cases lower than the humidity of the air inside the car.

By now I think everyone should know whether it is necessary to turn on the AC when turning on the warm air to defogging. Simply put, when the air humidity is high and the defogging effect is poor, you can turn on the AC dehumidification and increase the dehumidification. Fog effect. If the outside air humidity is not high, as long as it does not affect the defogging effect, it is okay to turn off the AC.