MIS called me, as I'm a Bulidings and grounds TECH. Limited A/C knowledge. He said the server rooom was getting hot. Servers put out some heat. There are 2 units in the room. A Reem and a Rudd unit. The room has no fresh air only room air. Each unit takes room air and for the bottom which blows over the evaporation coils. The condensers are on the roof 2 of them. The filters at bottom of the room units were very clean. I went to the roof and found on the Reem unit a frozen Suction line. As well as a frozen Evapoator coil. I live in syracuse NY. Where the nights get cold. I remember a Contracor many years ago when it was 29 degrees out went to the roof units and overrided something electronically. Because the same thing happened.Low refrigerant will do this i think. 410A
Frozen suction line and evaporator coils can be caused by a few different items. But basically the A/C unit is making more cooling than the air flow can dissipate. Dirty filters is the first place to look. They will restrict air flow. Not enough air flow means inability to dissipate cooling to the room, means freeze-up. If the filters are good, look at the coil itself. Is it dirty? Shut the system down, defrost the coil then you can inspect it. If it is dirty, you have to clean it. dirt on the coil face will restrict air flow. Lack of air flow can mean freeze-up. Now check to make sure the blower wheel is clean. A dirty blower wheel cannot mover air like it should. If it is dirty, you have to pull that blower wheel and clean it. Not very much dirt on those fins can reduce the efficiency of the wheel a lot. Not enough air movement across the coil means freezing. Now think about it this way, you want to direct as much warm air across those coils as you possibly can. Is there any way to augment the flow of the warm air in the room across those cooling coils? If the room thermostat is calling for cooling those compressors are going to run. When they run they are going to produce a certain amount of cooling regardless of other conditions in the room. If you can't dissipate that cooling off the coils they will over cool and freeze. With a frozen coil, not enough air flow across the coil is 95% of the time the problem. Either move more air or shut the units for a short period of time to let the warm air catch up. This is a good start. Good Luck.
Low charge and low air flow will cause freezing of the suction line. Usually only part of the coil will be frozen if it is low on refrigerant. If it is very cold outside and it is a normal air conditioner it will freeze up. Air conditioners are designed to work when it is hot outside. When the temperature drops so does the head pressure. The head pressure drives the suction pressure. If there is not enough pressure the coil goes below freezing and starts to ice up. Usually units designed to be used while cold outside would have some sort of head pressure control. Once it has frozen all you can do is defrost it and see if it still freezes. If it froze because it was too cold out all will be well once it warms up. Check for anything restricting the air flow such as closed dampers or a dirty coil.
After you check out all the items that Paul Logan and User76730 suggested, if the coils still freeze you can do the following. 1) Have the HVAC service contractor that services your A/C units install a head pressure control device as user 76730 suggested. I used them on any A/C unit that ran when the outside air temperature was below 60 degrees F. 2) If you need cooling when the weather is very cold, Have a contractor install a fresh air cooling system that brings in cold outside air and exhausts the hot ceiling air. This type system is rather inexpensive to operate does a good job of reducing the rooms temperature.