Home AC Coils Freezes Up

[grengadgy]

I found this good definition of why an ac coil freezes up.

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.

 

[grengadgy]

If a thermostatic expansion valve is as the metering device the above case is very true and you see TXV in ACs 3 ton and larger. If your AC is 2 1/2 ton or or smaller yours will probably be capillary tubes or a fixed orifice which can ice up simply from an improper freon charge.

Improper air flow through the evaporator coil can cause a freeze up. Look for dirty air filter or dirty coil. No air flow, bad fan motor or start/run capacitor. All of these are about freeze ups.