It may not seem like a big deal if your AC unit's coils are frozen over; it's just an occupational hazard when you're an air conditioner, right? But unfortunately, frozen-over coils reduce your unit's effectiveness and can even cause complications that can kill the unit altogether. For example, if the coils remain frozen, the freezing may spread up the line to the compressor. Once frozen coolant gets there, the compressor may fail; it can't compress the frozen substance, yet something has to give. This is especially bad news because the compressor is one of the most vital and expensive components of the unit. Here are four reasons a freeze-over may occur.

1. Check for low refrigerant

Yes, low refrigerant levels can paradoxically result in a frozen-over AC coil. But that doesn't mean it's cooling your house well; quite the contrary, in fact. So if your unit is frozen over and you've checked the other possibilities offered here, it may be time to call in a technician to check and top off the refrigerant levels. The reason you can't top it off yourself is because the refrigerant is a somewhat-restricted substance, so you're unlikely to be able to obtain it legally yourself, unlike your AC repair professional. 

2. Check for overuse

Although AC units can work seeming miracles by keeping your house drastically lower than the outside temperatures, they do have their limits. Your AC unit is especially likely to fall victim to overuse if you're in the middle of a record heat wave or if the unit is slightly too small for your house (for example, if you added a room to your house recently). 

3. Check for low outdoor temperature

AC units are designed to keep your house comfortable on days that are too warm to be comfortable. If the outdoor temperature is as low or lower than a comfortable living temperature, your AC unit may not be at the top of its form, and in truly cold temperatures the AC unit's condenser may become damaged if you run the unit at all.

4. Check for blockage

Blocked airflow can also cause the coils to freeze over. This is because when the unit can't get the air it needs fast enough, it works harder than it normally would to pull air in, meaning it becomes overworked (see point 2) much more easily.

These four problems can all be the cause of your AC unit's unfortunate freezing episode. If you're not sure which one of them is to blame, be sure to call in an air conditioning repair expert rather than simply guessing.