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Frozen AC coil?

As Don mentioned, the two common causes for a frozen evaporator coil are an undercharge of refrigerant and reduced airflow.

If only a portion of the evaporator coil is frozen, chances are that there is an undercharge of refrigerant. However, if the entire coil is frozen, chances are you have restricted airflow through the coil. What follows are some reasons/explanations for the partial/full coil freezing scenarios:

If the system is undercharged, the operating pressures and temperatures in the evaporator will be lower than desired. This means that the liquid refrigerant in the evaporator coil will actually boil at a temperature that is lower than 32 degrees F, causing ice to form on the coil. Now, because there is a deficiency of refrigerant in the evaporator, the amount of liquid enterng the coil is reduced, so the liquid boils into a vapor faster. Once all of the liquid refrigerant boils, the temperature of the vapor in the evaporator rises (superheat). In this scenario, the warm cooling coil cannot adequately cool the space.

Now, if the airflow through the coil is restricted, there is no shortage of refrigerant in the coil. What happens now is this... In order for the liquid refrigerant to boil in the cooling coil, we have to add heat to the refrigerant. The source of this heat is the air that passes through the evaporator coil. With a restricted airflow, the amount of air is reduced and, therefore, so is the amount of heat that is available to boil the refrigerant. With a reduction in heat, the operating pressures and temperatures of the evaporator will drop, bringing the coil temperature to a point below 32 degrees, once again causing frost/ice to form. Since there is no deficiency of refrigerant, the entire coil will freeze.

If you understand the concept of evaporator superheat, the following will prove invaluable. A low evaporator temperature and pressure can indicate either an airflow problem or a refrigerant deficiency on the low side of the system. The evaporator superheat will help us determne which scenario is the correct one. High superheat is an indication of an underfed evaproator coil, which is indicative of a refrigerant undercharge. Low superheat, which is an indication of an overfed evaporator, will be present when an airflow problem exists.

Here are some concepts that you might find useful:

Low evaporator pressure is an indication that the evaporator might have a deficiency of refrigerant.

High evaporator superheat is an indication that the evaporator might have a deficiency of refrigerant.

High evaporator pressure is an indication that the evaporator might have an excess of refrigerant.

Low evaporator superheat is an indication that the evaporator might have an excess of refrigerant.

Therefore, if the evaporator pressure is low and the superheat is high, the evaporator has a deficiency of refrigerant.

Also, if the evaporator pressure is high and the superheat is low, the evaporator has an excess of refrigerant.

Now the cool part... if the evaporator pressure is low (an indication of an uncharge) and the superheat is low (an indication of an overcharge), there is an airflow problem thorugh the evaporator.

Of course other factors need to be considered, such as the type of metering device in the system.

Hope this helps!

Comments

  • TimcoTimco Member Posts: 2,969


    I am earning my EPA cert this week, and ordered a few courses on AC. In the mean time, I have a coil on a FA system that after running a few months turns into a giant block of ice that reaches past the pan and starts flooding the blower compartment and into the garage. Can anyone suggest the first thing to check, or suggest places to start? I am really looking forward to getting into refrigerants...

    Tim
    Just a guy running some pipes.
  • don_207don_207 Member Posts: 12
    Hi Timco

    I think you will find this side of the trade alot of fun.

    I'm not going to get into all the thing you should be checking and testing at this point because it would be a overload of info.

    I will say the biggest two problem with a unit freezing up is low on refrigerant and the other being airflow.

    Check for dirty coils,dirty blowers,the three month so called airfilter that is a big problem on system where, the distribution was not design for the filters pressure drop.

    Every two month leads me to believe the customer has a very restricitive filter along with not cleaning or changing when needed.

    Lets us know how we can help.

  • TechmanTechman Member Posts: 2,144
    Coils freezing

    Good Luck Timco!,Start the system with an unfrozen coil.What are the following, outdoor temp;temp of air leaving the cond unit; indoor air temp & humidity;temp of insulated suction line & liquid line at the cond unit.Also, what are the hi and lo pressurers?The reading should be taken at startup and ,say ,30-45 minutes later.
  • mtfallsmikeymtfallsmikey Member Posts: 765
    Thanks!

    Dr. Silberstein!...Also a good refresher for those of us who have been turning wrenches and knocking tin for a lot of years. Sometimes we forget or neglect the basics...
  • Eugene SilbersteinEugene Silberstein Member Posts: 1,380
    My Pleasure!

    :)
  • don_207don_207 Member Posts: 12
    I like the part

    About deficiency of refrigerant in the evaporator.

    You know how much money I've won over the years betting with guys that believe you would have 0 degree super heat with a frozen coil and low on refrigerant.

    I also like the word might,it like that other word that often used around here..it depends.
  • Steve_168Steve_168 Member Posts: 39
    Frozen coils

    Hidy Ho professor!Just as smart as ever you are!Now a question, what happens on a normally operating system when the daytime temp is 75-85* and the nite time outdoor temps drop down into the hi 50's .And if a system is a little low on freon,would a small iceberg start that would creep across the entire evap coil after a few weeks or so?
  • Eugene SilbersteinEugene Silberstein Member Posts: 1,380
    Big Time!

    Yes indeed..... Just keep the Titanic away from it!
  • TimcoTimco Member Posts: 2,969


    Thank you very much for the great advise and explanation. I will have gauges on this next week. Can anyone elaborate on the comment regarding 50*-60* overnight temps? We are having 65* temps overnight now but it will be back down into the 50
    s soon. This sounds like a good cause but why the issues with cooler overnight temps? Fix for that?

    Tim
    Just a guy running some pipes.
  • Eugene SilbersteinEugene Silberstein Member Posts: 1,380
    Testing

    Testing
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