The hissing sound that is often heard when an air conditioner is turned off after use is typically caused by the equalization of pressures within the system. Air conditioners work by circulating refrigerant, which undergoes a phase change from a gas to a liquid and back again. When the air conditioner is operating, the refrigerant is in a high-pressure state, and it flows through the system to absorb heat from the indoor environment.
When the air conditioner is turned off, the compressor stops running, and the flow of refrigerant ceases. However, there is still a significant amount of high-pressure refrigerant trapped in the system, particularly in the condenser and the refrigerant lines. As the system reaches equilibrium, the pressure within the system equalizes, causing the refrigerant to undergo a phase change from a high-pressure gas to a lower-pressure liquid.
This phase change process produces the hissing sound. As the refrigerant transitions from a gas to a liquid, it rapidly condenses, and the pressure difference causes turbulence and the release of excess energy in the form of sound vibrations. This results in the hissing noise that can be heard when the air conditioner is turned off.
It's worth noting that while some hissing sound is normal during this pressure equalization process, excessive or continuous hissing noises may indicate a potential issue with the air conditioning system, such as a refrigerant leak or a faulty valve. In such cases, it is advisable to have the system inspected and serviced by a qualified technician.