Air conditioners can be turned on their side for shipping and storage, but they should never be run or installed that way. All air conditioners are also dehumidifiers, and the dehumidification system relies on gravity to take water away from the unit and drain it from the bottom of the air conditioner's case. An understanding of how an air conditioner works can help explain why a vertical rather than horizontal positioning will lead to unit damage.
Sideways Installation Could Damage the Compressor
The compressor of an air conditioner is a bit like a small electric motor in that it has gears and turning parts that require lubrication. The lubrication in the air conditioner compressor comes in the form of oil, typically located in the compressor case itself. When an air conditioner is turned on its side, this oil settles on the bottom half of the compressor and potentially deprives part of the compressor of oil, which can lead to damage and, eventually, compressor burnout.
Condensation Won't Drain Properly
Window air conditioners, just like all other air conditioners, dehumidify the air to improve indoor comfort. The air conditioner pulls indoor air in and blows it across cold copper coils filled with refrigerant. These coils "sweat" the humidity in the air in the form of condensation. This condensation drips, through gravity, down into a condensation pan. If an air conditioner is positioned on its side, this water will drain down into the case, potentially damaging the unit and leaking water back into the home.
Ventilation Grates Can Collect Rain Water
Air conditioners have ventilation grates, or fins, on the sides and bottom of the unit, and these bring sufficient airflow into the hot side of the unit, allowing for the dispersal of heat removed from the home. If an air conditioner is situated on its side, rainwater will drain into these grates and damage delicate internal mechanisms.
Storage, Shipment and Installation
If an air conditioner has been stored on its side, or if it has been shipped that way, take care to ensure it has been turned horizontally for a period of at least 24 hours prior to running the unit. This allows sufficient time for any oils and lubricants to resettle back to their proper levels, providing protection for internal gears and moving parts.
When you move the unit to the location where you plan to install it, you may have to set it on its side temporarily prior to setting it in place. It's fine to do this, as long as you don't install the unit sideways and you don't turn it on for 24 hours.