Why is the Low-Pressure Pipe of a Car Air Conditioner Hot to the Touch?
2 Answers
Causes for the low-pressure pipe of a car air conditioner being hot to the touch include compressor or expansion valve failure, evaporator blockage, insufficient refrigerant flow, or excessive pipeline pressure, as well as excessive fouling in the condenser copper tubes. Under normal circumstances, the low-pressure pipe of a car air conditioner should be cool, while the high-pressure pipe should be hot. If the low-pressure pipe becomes hot, the air conditioning cooling effect will be poor. Below is additional information: 1. Compressor or expansion valve failure: This can cause the low-pressure pipe of a car air conditioner to become hot. The air conditioner compressor plays a role in compressing and driving the refrigerant in the air conditioning refrigerant circuit. The air conditioner compressor extracts the refrigerant from the low-pressure area, compresses it, and sends it to the high-pressure area for cooling and condensation. The heat is dissipated into the air through the radiator, and the refrigerant changes from a gaseous state to a liquid state, with an increase in pressure. 2. Checking the refrigerant: If the expansion valve is blocked and the air conditioner is not working, check whether the refrigerant is sufficient. If it is insufficient, the system needs to be vacuumed before adding refrigerant, and then use a pressure gauge to check whether the pressure in the high and low-pressure pipelines is normal. 3. Identifying the high and low-pressure pipes of a car air conditioner: The thin one is the high-pressure intake pipe, and the thick one is the low-pressure pipe. You can also distinguish them by the letters next to the compressor connection. Some compressors are mostly marked with S or D to distinguish them, where S is the low-pressure connection and D is the high-pressure connection.
I've been repairing air conditioning systems for over a decade, and from what you're describing, it's almost certainly due to insufficient refrigerant. You mentioned the low-pressure pipe feels scalding hot, right? Normally, it should be ice-cold with condensation. When the compressor runs without refrigerant to compress, those pipes become red-hot like soldering irons, rendering the entire AC system useless. Other common scenarios include: a completely clogged expansion valve blocking refrigerant flow, or excessive moisture in the dryer bottle causing ice blockage in the pipes. If the condenser gets caked with mud and leaves, it can lead to poor heat dissipation, causing high-pressure heat to backflow into the low-pressure side. More severe cases involve compressor clutch slippage or internal wear causing similar issues. You need to immediately test both high and low pressures with gauges - a minor refrigerant leak is one thing, but a seized compressor will cost you a fortune to repair. Remember, when you notice cooling efficiency dropping, don't push it - unless you want blistered hands from touching those scorching pipes.
