You air conditioner can seem like your best friend on a sweltering day in the middle of the summer. If it is not working correctly, it can make life miserable in a very short time as your home heats up. One symptom of failure that you might notice from your air conditioner is a hissing sound. This doesn't always mean there is something wrong, but it could. It is a good idea to know the possible sources of this hissing sound when it happens.
The most serious problem that may cause a hissing sound to emit from the air conditioner is too much internal pressure in the compressor. This can range from a mild hissing noise to a loud hissing or a noise some describe as a "scream." This type of noise from the compressor can indicate a dangerous situation. If your compressor is making this type of noise, shut down the air conditioner and call a qualified technician to repair the problem.
Internal Valve Leak
A similar but usually quieter hissing sound can come from the compressor area in an air conditioning unit if the internal valves are leaking. This hissing may sometimes take on the sound of a trumpet or trombone if the condition worsens. The noise caused by leaky valves is especially noticeable when the system powers down. A service technician will be required to replace or repair the leaking parts.
Sometimes a hissing sound coming from the air conditioner is not a major problem. During normal operation the refrigerant, such a Freon, circulates through a system of pipes to cool the system. It compresses the gas and then expands the gas rapidly through a port called an expansion valve. In some systems the expansion of gas inside the unit will make an audible hissing noise routinely as part of the cooling process. These hisses are just a few seconds long and happen in regular intervals. They are not a cause for concern.
If the hissing sound from your air conditioner is coming from the registers or vents in your home, then there is probably no major problem with the mechanics of the system. The simple rush of air forced through by the fan could result in a hissing sound because the ducts are excellent for carrying sound. Also, if some of the registers are partially closed then air could be entering through tight spaces, causing increased sound. Try opening the registers wider to turn it from a hiss sound to more of a low-key whoosh.