Why do some central air units get the white frost on the lines when its low on R22 and others don't?
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They will all form frost on the suction line at some point . It all depends on the size of the leak itself . At first , when a unit springs a leak , there is no frost buildup . As it loses freon , the pressure and evaporating temperature go down . Once the freon starts evaporating (boiling) at or below +32 f , the cooling coils form ice on them .
The coils become too cold for the freon to boil in them , so it boils in the suction line between the cooling coils and conpressor after it leaves the cooling coils . The moisture on the suction line freezes and forms ice .
Once the unit has lost enough freon , the ice on the line begins to disappear .
As I mentioned earlier , it all depends on how fast the freon is leaking out . A slow leak will have the frost visible for some time , whereas with a fast leak it can come and go without being noticed .
It is more a matter of equipment design than anything else.
When the refrigerant is low, the liquid transport line that takes the compressed refrigerant to the evaporator is not full of liquid - it has a vapor / liquid mixture.
Gasses (vapor) are compressible, liquids are not. This means that the vapor / liquid mixture enables more liquid evaporation which causes chilling before the liquid reaches the evaporator and the frost lines. The amount of evaporation that happens determines the amount of frost line regardless of design. However, the distance to the evaporator, the design of the equipment, and other factors influence evaporation rate. It is a complex design issue and pretty hard to predict in equipment design - further it is not a performance issue but a leak issue - so makers are not so concerned about this matter.
just because theres frost does not mean its low on freon or there is a leak the coils could be stopped up someone may not have pulled a vaccum when it was installed the air temp. could be causing it to frost up and its not insulated at that part consulated a professional in your area to be sure but dont take that there is a leak or low on freon go out side with the tech check the superheat it should be between 15 and 20 degree drop and if he does a leak check watch over him
usually frost lines show up at a certain pressure and temp. a little less or more loss of freon will by pass frost stage
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