Countermeasures for Common Problems of Defrosting during Winter Heating of Air-cooled Modules

2025-04-11

As the cold and heat source of a central air conditioning system, the air-cooled module has the advantage of serving two purposes with one unit. At least one four-way reversing valve is added inside the unit, enabling the switching between the refrigeration and heating working conditions and improving the utilization rate of the unit. During refrigeration, the condenser adopts air cooling, eliminating the need for the cooling water system required by water-cooled chillers.

When the air source heat pump unit operates for heating in winter, its finned coil heat exchanger functions as an evaporator. Due to the relatively low evaporation temperature, the temperature of the coil surface also drops, even below 0°C. When the outdoor air flows through the coil driven by the fan, the moisture contained in it will precipitate and adhere to the surface of the coil to form a frost layer.

So in principle, frosting is a very normal phenomenon for an air source heat pump. It doesn't mean there is a problem with the machine just because it frosts. Since any air source heat pump machine has an automatic defrost function, as long as the machine can defrost normally, the impact on the heating effect of the heat pump system is minimal. Therefore, what we need to pay attention to is whether the machine can defrost normally. Because if the defrosting is not normal, it will affect the heating effect of the machine. Compare the situation before and after defrosting, and notice that a stream of hot air will come out of the machine.

The impact of the frost layer on the heating effect of the air-cooled module is mainly reflected in the following two aspects:

Impact on thermal resistance of heat conduction The frost layer increases the thermal resistance of heat conduction and reduces the heat transfer coefficient of the evaporator. In the early stage of frosting, due to the frost layer increasing the roughness and surface area of the heat transfer surface, the overall heat transfer coefficient increases to some extent. However, as the frost layer thickens and the impact of the thermal resistance of heat conduction gradually becomes the main factor affecting the heat transfer coefficient, the overall heat transfer coefficient begins to decrease again.
Impact on air flow The increase of the frost layer increases the resistance of air flowing through the finned tube evaporator and reduces the air flow. This impact on the performance of the evaporator is the main aspect of the negative impact of frosting.

Generally speaking, the greater the relative humidity of the outdoor air, the more severe the frosting of the machine. Under the climatic conditions in the Yangtze River Basin, when the relative humidity of the outdoor air is less than 50%, the finned coil rarely frosts. When the outdoor air temperature is lower than -5°C, as the temperature drops further, the air tends to be dry and the frosting decreases.

Countermeasures for Common Problems of Defrosting during Winter Heating of Air-cooled Modules1744344727392

So how should we deal with the problems of air-cooled module defrosting failures?
Problem: What should we do if there is local icing on the evaporator?
This may be because there is no frost at the defrost sensor, and the defrost point is inaccurate. The solution is to manually force defrosting and move the defrost sensor to the frosted area.
Problem: What should we do if the evaporator doesn't defrost?

The defrost sensor or the ambient temperature sensor is faulty;
There is a lack of refrigerant or the system is blocked;
The four-way valve doesn't reverse.
The solutions are to replace the sensor in sequence; add refrigerant according to the standard and unclog the system; check whether the valve is powered and whether it operates normally.
Problem: What should we do if the defrosting is not clean?
The problem may be caused by setting the coil temperature for exiting defrosting too low, resulting in unclean defrosting. In this case, you need to adjust the defrost parameters and patiently observe until the defrosting is completed. It may also be that the position of the sensor probe is not in the area with the most severe frosting. In this case, just move the probe.
Problem: What should we do if all the fins are frosted?
It may be due to setting the coil temperature for entering defrosting too low or setting the defrosting time interval too long, preventing the defrosting action from occurring in a timely manner. You need to adjust the defrost parameters, increase the coil temperature for entering defrosting, and shorten the defrosting time interval.
Problem: What should we do if there is frequent frosting in a short period?
The finned heat exchanger is blocked, the fan motor is damaged, or the air volume configuration of the fan is too small. The solution is to clean the heat exchanger and repair or replace the motor.
Problem: How to solve the efficiency problem of the air-cooled module at low temperatures?
In low-temperature areas, an ultra-low-temperature air-cooled module unit should be selected. It can be used in environments above minus 25 degrees Celsius, and its heating efficiency is 30%-50% higher than that of conventional machines, ensuring its use under low-temperature conditions.
Select the appropriate type reasonably. When selecting the heating capacity of a single ultra-low-temperature air-cooled module unit, refer to the local winter meteorological data, find the appropriate heating capacity corresponding to the local winter air conditioning outdoor calculated dry bulb temperature, and calculate the energy efficiency ratio to avoid the situation where the efficiency at low temperatures in winter cannot reach the required heating effect.
The ultra-low-temperature air-cooled module unit will perform better when operating at low temperatures. However, sometimes we need to consider some external factors and our own conditions, as well as whether we have carried out timely maintenance, etc.
Problem: What are the causes and solutions for the freezing and cracking of the plate heat exchanger of the air-cooled module unit?
The plate heat exchanger is one of the key components of the air-cooled module unit. In air conditioning maintenance work, it is common to encounter the freezing and cracking of the plate heat exchanger, which can lead to water entering the system and even the scrapping of the compressor. What exactly causes the freezing and cracking of the plate heat exchanger of the air-cooled module unit?
This situation is actually related to the design defects of the control system of the air-cooled module unit. The anti-freezing protection temperature setting of the control system is too low. In addition, the quality of the temperature sensor is poor, and there is a resistance drift situation.
This is the fundamental cause of the freezing and cracking of the plate heat exchanger of the air-cooled module unit. In response to this situation, it is recommended that if the anti-freezing protection parameters of the controller of the air-cooled module unit can be set on-site, the protection temperature should be set at 3-5 degrees Celsius, which is more appropriate. It is very unprofessional for some manufacturers to set the anti-freezing protection temperature at 1 degree when the product leaves the factory. An excessively low protection temperature will prevent the control system from providing effective anti-freezing protection.
In addition, when selecting the temperature sensor, a copper-encapsulated temperature probe should be chosen, which has high temperature sensing sensitivity. For the controllers of the air-cooled module units of many manufacturers, the anti-freezing protection parameters cannot be set on-site. A simple temperature control switch can be installed on the plate heat exchanger and connected in series with the high-pressure protection or low-pressure protection in the control system. In this way, an additional protection barrier can be added to further protect the plate heat exchanger of the air-cooled module unit and prevent major losses that make the air conditioning unit irreparable due to the freezing and cracking of the plate heat exchanger.