Where I live, it’s already pushing 90 in the heat of the day, so a/c is your first, last, and only line of defense against sweating your ass off. Unfortunately, this frosty image means that my a/c unit wasn’t cooling much of anything, despite the ice on the outside of the coil. In this case, ice is not cool.
There are tons of reasons an evaporator coil will freeze over like this — one of the most common is restricted airflow across the coil. This allows the temperature of the coil to drop below 32 degrees, causing the humidity/condensation to ice over. It’s also why the repairman will ask you how long it’s been since you changed the filter, since a clogged filter is often one of the root causes.
If this happens to you, shut off your system immediately. Running it while the return can’t get enough air to the system will damage the compressor if you leave it for too long.
Lucky for me, it turns out the filter in the return was too restrictive, and as soon as the unit thawed and a slightly less restrictive filter was popped in, all was right with the world.
Note: Keep in mind that if you’re not sure about what to do with your a/c unit — never guess. Just call a pro.
Furnace Filter Tips [Healthy House Institute]