Air conditioners use a cycle that involves changing a refrigerant between a liquid and a gas to decrease the temperature of a room. The design of virtually all air conditioners assumes that the system isn't going to run when the exterior temperature is below 60 degrees Fahrenheit, since it's below the comfort area of most people. In most cases, it's not safe to run an air conditioner below this temperature.
Running A/C at Low Temperatures
Air conditioners are designed to operate within a relatively narrow range of temperatures. The manufacturer assumes that the air conditioner will never run below a certain temperature. The minimum temperature can vary between models, but it's usually around 60 degrees F.
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The air conditioner is designed to manipulate the properties of the refrigerant between a gas and liquid. When the exterior temperature drops below the minimum temperature, the refrigerant starts acting differently. The air conditioner usually still runs at lower temperatures, but the A/C experiences excess strain to compensate for the changes.
Ultimately, running the system at these temperatures would damage the compressor. The amount of time it takes to ruin the A/C compressor varies substantially based on the design of the compressor, operating parameters of the system, and external temperature. It's difficult to predict how long it would take to blow out the compressor as a result.
Using the Fan Only
Air conditioners usually are two-part systems. The main part of the air conditioner is the heat exchange system. However, most air conditioners also include a fan for air circulation. While it is possible for an air conditioner to function without air circulation, it takes far more time to change the temperature in a room without it.
It is safe to run just the fan of an air conditioner in cooler temperatures if the heat exchange system is disabled. Not all air conditioners allow you to run the fan without the heat exchange system.
Special Cases to Consider
While most home air conditioners cannot be run below 60 degrees F, this doesn't mean it's impossible for an air conditioner to safely run below this temperature. The reason most air conditioners cannot be run below this temperature is a design choice, not an actual physical limitation.
Some air conditioners are designed to be used over a larger temperature range than home models. These systems are much more expensive, which is why they're almost never used outside industrial plants. Equipment, such as manufacturing equipment, generates heat all year round. The interior of these plants would be too high for comfort without an air conditioning system that works year round.
Air conditioners designed to be used all year round usually have a much lower minimum temperature. Usually the minimum temperature is below the lowest seasonal temperature for that area, so there is no risk of running these systems during cold weather.
Air Conditioner Safety
In most cases, especially in homes, you shouldn't run an air conditioner at 58 degrees F or lower. The air conditioner does function, but you could burn out the compressor fairly quickly. It's safe to run the fan if the heat exchanger is disabled.
It's also safe to run an air conditioner that is designed to be run all year round. The general rule of thumb is that you should never run an air conditioner at 58 degrees F or lower unless you understand the system well enough to know it is designed to be run at lower exterior temperatures than most models or can be run with the heat exchanger disabled.