There are many questions about air conditioner issues with the most common being that it isn't cooling properly. Among the typical answers are low Freon and the standard diagnosis for low Freon is ice on the large copper pipe at the outside compressor.

From my understanding the small copper tube is the high pressure side with Freon that has been compressed to a liquid. When this liquid is allowed to boil in the inside evaporator things cool down.

Conversely the large copper pipe is the return line from the evaporator sometimes referred to as the suction line or low pressure side.

So my question is: under low Freon conditions how can the suction line freeze up?

Intuitively I would thing with low Freon the entire cooling ability would be below normal causing the evaporator to not produce as much cold air. A side effect of all this, I would expect the suction line to be warmer that normal.

Why does the suction line freeze up under low Freon conditions?

See the image in this question for a great diagram of an HVAC system: What's the most common cause of A/C refrigerant lines freezing?

  • This answer might be helpful.
    – Tester101
    Commented Jul 2, 2014 at 22:11
  • Read up on flooded evaporator cooling systems. There are several ways of regulating the transition from liquid to gas, some mechanically complicated with variable orifice expansion valves, others with fixed orifices that are cheaper to manufacture and maintain. Commented Jul 3, 2014 at 1:32
  • This might depend on the metering device used. When I had this problem, it was on a system with capillary tubes. If you put your ear to the high side line you could hear it spray, then wait for pressure to build, then spray again. There wasn't a constant injection of refrigerant into the evaporator.
    – Tester101
    Commented Jul 3, 2014 at 11:32

3 Answers 3


Chlorodifluoromethane, also known as R22 refrigerant has a boiling point of -40°F at 0 psi. As you increase the pressure of the refrigerant, the boiling point also increases. At 68.5 psi, the boiling point of R22 is 40°F.

In a normally functioning system, the refrigerant is sent into the evaporator at about 55-65 psi. Which means that the boiling point will be above the freezing temperature of water. The refrigerant absorbs heat from the air passing through the evaporator, boils, and flows on down the line at a temperature above the freezing point of water. The refrigerant will likely be below the dew point, however, so there will be condensation on the line.

If the pressure in the system drops, the R22 might be entering the evaporator a bit lower than the typical pressure. In this case, the boiling point of the R22 will be below the freezing temperature of water. As the warm moist air moves over the evaporator, the moisture condenses and freezes on the coils. Ice will start to form at the beginning of the evaporator coils, and slowly creep along its length.

The ice will act as an insulator, so the refrigerant in the line will not be able to absorb the heat it needs to boil. This causes the refrigerant to boil off further along in the evaporator, which forms more ice further down the line. This process continues all the way along the line, until the entire evaporator and suction line are covered in ice.

Once the refrigerant levels get too low, there's not enough refrigerant in the system to freeze the line. So eventually if the system has a leak, this behavior will stop and you'll simply get no cooling at all from the system.

NOTE: I could be completely wrong here. This is based on my limited knowledge of air conditioning systems. I am not an expert in thermodynamics, fluid dynamics, chemistry, or HVAC.


Superheat of the refrigerant should take place toward the end of the evaporator coil. If the the system is low on "freon" and the metering device is not a txv then the freezing up could very well occur because the pressure drop is not very well controlled. Non txv systems rely on a near perfect balance or charge of refrigerant. An overchared system can have a similar effect.


this is my opinion, imagine if this freon has very high pressure until it can't change to gas form, then this liquid can't absorb heat from evaporator coil, because the only way to absorb heat is change this liquid to gas form. if the pressure low, more amount of freon that can change to gas form so it can absorb more heat from evaporator coil, making the coil more cold

sorry, my english so bad

  • Welcome to Home Improvement. Just some feedback on your post. Your answer is essentially right, but it duplicates the accepted answer, which already explains it better. So in that sense, it isn't really contributing to the site's knowledge base. Also, the objective is factual answers rather than opinions, so starting your answer with "this is my opinion" is kind of a flag. If you aren't sure, research it first, then post an answer. Including a citation to an authoritative source is even better. Take the quick tour to familiarize your self with the site.
    – fixer1234
    Commented Dec 7, 2017 at 22:38

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