What causes the Evaporator to freeze on an Air conditioning system.?
Common causes are:
* Low Ambient, Running when the outdoor temperature is too low. Below 65?F
* Low refrigerant charge
* Under sized liquid line
* Blocked capillary tube
* Indoor Temperature too low Below 72?F
* Over sized Compressor
* Blocked Liquid Line Filter
Low AmbientThe mechanical refrigeration system used in air conditioning equipment is design to work when the outdoor temperature is above 65?F. If the AC runs when the temperature is too low will cause the evaporator coil to drop in temperature and freeze over. Two ways to prevent this is to use a low ambient control to shutoff the AC when it's too cool outside or use a speed control to slow the condenser fan. A roof top unit or larger air handlers will use an economizer to draw in outdoor air.
Normally the evaporator is hidden inside the ductwork or the air handler and cannot be seen. If the AC is a packaged system such as a roof top unit, you may have to remove the air filter or the fan access panel to see if it is frozen. If the freezing has gone on long enough the compressor will freeze over, which can be seen on a roof top unit through the condenser air grill or a side panel.
On a split system the evaporator is mounted in a remote location. The freezing can be seen on the suction piping. The suction piping should be insulated, but anywhere there is a gap in the insulation or if you pull back the insulation you can see the freezing.
Freezing causes water to drip outside of the drain pan as onto the floor or ceiling tiles.
When your AC freeze's over, it causes some serious problems. On a roof top unit, the water drips down the return air ductwork and collects, providing a place for mold and bacteria to grow or it will drip directly on to the ceiling causing damage to tile or electrical system. Freezing can also cause liquid refrigerant to return to the compressor, damaging the piston, valves and crankshaft, which can be very expensive to fix.
The most frequent cause of freezing is a lack of airflow. This can be from dirty air filters, which choke the airflow to the evaporator, a blocked return air or too many supply air dampers being closed. Other less common causes are under sized or poorly designed ductwork, dirty evaporator coil, excessive dirt built-up on the fan blades, broken fan belt or a failed fan motor.
Evaporator coils can freeze from a lack of refrigerant as well, but this will only happen if the refrigerant is slowly leaking, eventually enough refrigerants will leak out and the cooling will stop completely. In this case the leak has to be located and repaired.
A frozen evaporator can also be caused by improper refrigerant feed to the evaporator. Such as an undersized liquid line that is too long (common on residential system), blocked capillaries tube on the evaporator or a plugged liquid line filter.
Determining the cause is key to a proper repair, far too many mechanics try to resolve coil freezing by jamming more refrigerants into the system. If your mechanic tells you the system was freezing because it was "a little low on refrigerant" than they should also tell you why it was a little low. Where was the leak and is it repaired? AC systems do not require the occasional " top up of refrigerant " and in fact the additional refrigerant will reduce efficiency and can damage the compressor.
Some times the cause of the freezing can be very expensive to repair, such as under sized ductwork, where extensive interior renovation can be required to replace the ductwork, in this case, a freeze stat can be add to the system to shut down the compressor when the freezing occurs. A freeze stat is a small snapdisc switch that will clip to the end tubing on the evaporator and will open when the evaporator falls to 34?F or 1?C. This will prevent the coil from forming ice and damaging your building or equipment. Even if your systems is not prone to freezing, a freeze stat is a low cost safety device to have in place. Sometimes all it takes is the coil to freeze over once and it can cause thousands of dollars in damage and you may not even be aware of it for some time. Such as, when the compressor bearing are damaged by liquid slugging, but continue to another year.
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