If you live in an area prone to extreme weather, such as snowstorms and hail, covering your outdoor air conditioner will help prolong its life and save you from costly repairs. For temperate climates, an air conditioning net cover is not necessary, but can still be used to keep stray leaves, mud, rocks, grass, and other debris out.
Cold winter weather does the most damage to air conditioning units. Snow accumulation on top of the unit is not a problem. But melting snow is a big problem.
If the temperature rises above freezing, the snow will melt inside the air conditioner. Depending on how old your air conditioner is and what metal it's made of, water can cause rust and corrosion. On top of that, water may refreeze around the condenser coils, slowly weakening them.
Freezing rain and snow can also cause ice to form on the air conditioner fins, pushing fans out of position. When you turn the air conditioner back on in the spring, the air won't flow the way it should, and you may notice that your home isn't as cool as it used to be.
Air conditioners are not designed to block leaves, seeds, rocks and grass due to the need for airflow to operate. A small amount of debris won't do much damage. But problems arise when leaves and debris start to build up.
Wet leaves and grass that build up on the bottom of your air conditioner can start to rot, producing formic acid. The accumulation of this acid on copper pipes can eventually lead to corrosion. This, in turn, causes small holes in the pipes for refrigerant to escape.