The frosting directions of the refrigeration system are different, and the causes of frosting are completely different!

2025-04-27

I. Frosting Direction
If the frosting starts from the compressor's return air port and gradually progresses towards the evaporator, the possible reasons are:

The throttle valve opening is too large. The manifestation is that the condensing liquid return temperature and pressure are normal or slightly higher, and the frosting starts from the compressor's return air port and gradually progresses towards the evaporator.
The evaporator cannot absorb heat normally, or the cooling capacity generated by the evaporator cannot be carried away. The manifestation is that the liquid return temperature and pressure are normal or slightly lower, and the frosting starts from the compressor's return air port and gradually progresses towards the evaporator. After a long time, the sections from the compressor's return air port to the evaporator and from the throttle valve to the evaporator will all freeze, and finally, a low pressure will occur. The inspection methods are:
· Check whether the air filter is clean;
· Check whether the fan speed is normal;
· Check whether the evaporator is clean;
· Check whether the air supply system is blocked;
· Check whether the belt is too loose.

II. If the frosting direction is from the expansion valve to the evaporator, the possible reasons are:

The refrigerant is insufficient (leakage). The manifestation is that the condensing liquid return temperature is high but the pressure is not high, and the frosting starts from the throttle valve and gradually progresses towards the evaporator.
The expansion valve opening is too small. The manifestation is that the condensing liquid return temperature and pressure are normal or slightly lower, and the frosting starts from the throttle valve and gradually progresses towards the evaporator.
Check the liquid sight glass to see the refrigerant condition. If there is severe bubbling, it may be a system leakage; if there are no bubbles or very few bubbles of the refrigerant in the liquid sight glass, it may be that the expansion valve opening is too small or blocked. Generally, there will be a large temperature difference before and after the blocked part.
The surface of the evaporator fins is frosted, and the temperature of the evaporator at the frosted part is lower than 0°C. The frosting speed is related to the ambient temperature and humidity. There are two phenomena of evaporator frosting:
1). One phenomenon is that the frosting starts from the outlet of the evaporator, and over time, the entire surface of the evaporator is covered with frost. For the evaporator, this phenomenon indicates excessive liquid supply (the capillary system reacts obviously). For the entire refrigeration system of a household air conditioner, it may be that the return air system and the control system have problems:
a. Reasons such as dust blockage of the return air filter, dust blockage on the surface of the evaporator fins, a damaged fan capacitor or a decrease in capacitance, the fan motor not rotating, or a slowdown in speed will cause a reduction in the heat exchange load of the evaporator.
b. Cooling operation in a situation of low ambient temperature or long-term cooling operation when the resistance value of the return air temperature sensor changes. In a household air conditioner, if the control system has an anti-freezing protection function, after the refrigeration system has been running for a period of time and the control system detects that the temperature of the evaporator coil is lower than 0°C, the controller automatically turns off the compressor and the outdoor fan for protection, and the indoor fan runs at high speed. During the protection period, when the temperature of the indoor evaporator coil is greater than 10°C, the protection is cancelled and the refrigeration operation is resumed. If it is found that the frost on the surface of the evaporator does not disappear for a long time, it may be that the evaporator coil sensor and the return air temperature sensor of the control system are faulty or the control system does not have an anti-freezing protection function.
c. Some air conditioners have a continuous operation function. When the continuous operation function is turned on, the operation of the refrigeration system is not controlled by the temperature of the temperature sensor, and the surface of the evaporator will also frost after running for a long time. Just turn off the continuous operation function.
When repairing, the evaporator fins, the fan system, the return air temperature and the coil temperature sensor of the controller, and the ambient temperature at the time of operation should be checked first. Only when it is confirmed that there are no problems with the inspected components can it be determined whether the refrigerant charge in the system is excessive. An excessive refrigerant charge generally occurs after the system has been refilled with refrigerant. It should be noted here that when the refrigerant charge in the system is excessive, the surface of the evaporator may not frost when operating under high ambient temperature conditions, but the surface of the evaporator may frost when operating under low ambient temperature conditions.
2). The other phenomenon is that the frosting starts after the capillary tube throttles. Over time, part of the surface at the inlet of the evaporator is severely frosted, and most of the surface of the evaporator has no condensed water. For the evaporator of the refrigeration system, this phenomenon indicates insufficient liquid supply. For the entire refrigeration system of a household air conditioner, it may be that there is a leak in the system or a semi-blockage in the throttling system.
To distinguish between a leak in the system and a semi-blockage in the throttling system:
a. If the outlet air temperature of the condenser is high, the current is large, and there is partial frosting at the inlet of the evaporator without refrigeration, it is mostly a semi-blockage in the throttling system. The faulty parts are a dirty blockage of the filter before the inlet of the capillary tube or the indoor and outdoor connecting liquid pipes (thin pipes) are flattened, forming a secondary throttling.
b. If the outlet air temperature of the condenser is not high, the current is small, and there is partial frosting at the inlet of the evaporator without refrigeration, it is mostly that there is a leak point in the pipeline of the refrigeration system. The faulty parts are the system pipeline, the welding points, the interface nuts, the valve cores, the valve caps, especially the parts with oil stains on the system pipeline.
The control system of a household air conditioner should have a system capacity protection function. In the cooling operation mode, after the compressor has been running continuously for 30 minutes, if the temperature of the indoor evaporator coil is ≥ 25°C (some manufacturers use the condition that the indoor ambient temperature - the indoor coil temperature ≤ 4°C), the compressor and the outdoor fan are turned off, and the indoor fan runs at the set wind speed. To exit the protection state, it generally requires power off. Some control systems do not have a capacity protection function, and during the continuous operation of the compressor, compressor overheat protection may occur.

III. Evaporator Freezing
The entire evaporator freezes because during the vaporization and heat absorption process of the refrigerant in the evaporator, it does not obtain more heat! It lowers the temperature in the limited space outside the copper tube wall below 0 degrees! That is, when the temperature of the cooling medium (air) is lower than the dew point temperature, dewdrops are formed on the outer wall of the evaporator! When the temperature drops below 0 degrees, freezing occurs!
Why does the temperature drop below 0 degrees? It is because the amount of air treated on the outer surface of the evaporator is small! So the heat transfer is more sufficient! Therefore, the first two analyses of the moderator are all centered around why the air volume is insufficient! As for the third point: if the outlet of the evaporator freezes and the superheat degree of the return air decreases! It is possible that there is too much refrigerant! Because one of the necessary conditions for the evaporator to freeze is that the evaporation temperature must be lower than 0 degrees! And an excessive amount of refrigerant will precisely cause the evaporation temperature to rise!
Add a factor that is impossible: the enthalpy value of the air state point at the air inlet of the evaporator is small! That is, the inlet air temperature is low or the humidity is low! In addition, the evaporator manufacturing process does not use a hydrophilic treatment! As a result, the condensed water cannot leave the evaporator smoothly! The heat exchange time is too long!

Insufficient Refrigerant. In the case of insufficient refrigerant, the pressure in the system will correspondingly decrease, and the decrease in pressure leads to a decrease in the evaporation temperature of the refrigerant, usually dropping below zero degrees Celsius, which causes the moisture in the air to directly condense into frost instead of being in the normal water state. At the same time, in this case, the temperature-sensing probe of the indoor unit coil of the split air conditioner (which detects the temperature of the indoor evaporator coil) can still detect a certain temperature drop of the coil (if not, the air conditioner will shut down for protection). In this case, the indoor unit of the split air conditioner will freeze, and the process should be: start the unit - the high-pressure pipe of the outdoor unit is frosted - the inlet of the indoor unit evaporator is frosted - the indoor unit freezes - ice cubes are blown out from the air outlet (in severe cases) - the indoor fan is stuck (in severe cases).
Dirty Evaporator. When the indoor unit evaporator is very dirty, the air volume of the indoor unit will be severely reduced, so that the heat required for the refrigerant to evaporate in the indoor unit cannot be supplemented from the indoor hot air (I don't quite understand the change in the evaporation temperature of the refrigerant in this case and look forward to someone supplementing it). At this time, the indoor unit evaporator will gradually start to frost from condensation to freezing. And the process should be: start the unit - the air volume is small (the air outlet effect is good, but the indoor temperature cannot drop, and sometimes foggy water vapor will be blown out from the air outlet. This phenomenon usually occurs in places with a very poor operating environment, such as bars, karaoke rooms, barber shops (blocked by hair), restaurants, etc.
Slow Fan Speed. The situation is very similar to that of a dirty evaporator, but through actual observation, the operating environment is good, the evaporator is clean, and the human body feels that the wind speed is low.
Excessive Continuous Operation Time of the Air Conditioner.
This state is especially common in some enterprises and institutions, but it mostly occurs in larger cabinet-type indoor units. Small split wall-mounted units generally do not have this situation (it is estimated to be related to the low cooling capacity).
In larger units, especially 5P air conditioners, they will also freeze after running for a long time, but the freezing part is different from the above situations. The freezing part is generally at the end close to the evaporator, especially on the return air pipe (low pressure, suction) of the outdoor unit. Sometimes, the ice cubes will be very thick. However, this state does not affect its refrigeration effect.
Excessive Indoor Humidity and Long Operation Time. It is similar to the previous situation, but it mostly occurs in split wall-mounted units, especially in some hotel kitchens, cold dish rooms, and other environments that require long-term operation and have a very high environmental humidity.
The Connecting Copper Pipe between the Indoor and Outdoor Units is Flattened. This situation also occurs frequently. The manifested state is mostly that frosting or freezing starts from the flattened part (it will freeze if the operation time is longer). Exploring the reason, it should be that the refrigerant undergoes secondary throttling when flowing through the flattened part, and there is an improper evaporation at the throttling part. Moreover, the flattened part mostly cannot exchange heat with the outside air, resulting in a gradually lower temperature at the flattened part and finally freezing.