Why Is My Home’s AC Hissing? An AZ Tech Answers

If your central air conditioner is hissing, you could have one of the following situations:

• A refrigerant leak
• Too much pressure inside the compressor
• A noisy thermostatic expansion valve
• A duct leak
• A bad reversal valve (in heat pumps only)

Because a hissing air conditioner typically points toward a refrigerant leak, we’ll focus on this issue. We’ll explain:

Refrigerant, a heat transfer fluid, is stored in thin, copper tubing that runs throughout the entire AC system.

Since the refrigerant in the tubing travels under high pressure, a leak in the tubing can create a distinct hissing sound as refrigerant escapes. Think of it like squeezing a leaky balloon and hearing a hissing noise as air escapes.

Not sure if you have a refrigerant leak or another problem?

The first thing you should do to determine you have a leak is to look for other signs of a refrigerant leak:

• Bubbling noises
• Warmer air from air vents
• Ice or frost on refrigerant lines
• Your AC struggles to cool your home
• Higher-than-normal energy bills

Want an in-depth explanation of these signs? Check out our blog, “4 Signs Your AC May Need a Refrigerant/Freon Charge”.

When to repair the leak:

• When the leak is relatively small (have a professional inspect the leak)
• When your system uses R-410A, R-407C, R-134a refrigerant
• When you can’t afford the higher upfront cost of a new system
• When your AC is less than 10 years old

When to replace the AC:

• When the leak is extensive (again, have a professional inspect the leak)
• When your system uses R-22 refrigerant, which is being phased out
• When your AC is over 10 years old

Note: Beware of contractors who recharge your system without first locating and sealing the leak. Get a written contract before any work begins that specifies they will find and repair the leak before recharging your system.

Your compressor’s job is to pressurize refrigerant and pump it throughout the entire system. And sometimes, when things go wrong, pressure can build up inside the compressor. If this is your issue, you’ll only hear the hissing at the outdoor unit and the hissing will be very loud and accompanied by a shrieking sound.

Most modern AC compressors have a pressure sensor that will shut the system down if this happens but if your compressor continues to screech, turn off the AC and have a professional inspect the system.

Your thermostatic expansion valve is designed to remove pressure in the refrigerant before it flows into the indoor unit. The good news is that it’s completely normal for this valve to let off short, quiet hissing noises as it does this. If this is the problem, you’ll likely hear the noise at the indoor unit. Sometimes that noise can be described as a “whooshing” noise.

Be on the lookout for a whistling or screeching thermostatic expansion valve, though, as these are signs of low refrigerant or leaks in the valve.

Sometimes, misconnections or small holes in your ductwork can produce hissing noises as air leaks out of them forcefully. Even a very small leak can produce a loud hissing noise. If this is your problem, you’ll hear the hissing noise inside your home and only when the air handler unit (blower fan) is operating.

If you have a heat pump instead of a straight AC, that hissing noise could be the reversing valve. The reversing valve is responsible for reversing the flow of refrigerant, which allows the heat pump to both cool and heat your home.

But if the reversing valve has gone bad and is stuck between the heating and cooling mode it produces a constant hissing noise at the outdoor unit.

Not sure if you have a heat pump? We’ll show you 3 quick ways you can tell in our blog, “How Do I Know If I Have an AC or a Heat Pump?”