AC Making a Hissing Sound? Possible Causes and Solutions

AC unit in the backyard of a residential home

One of the best ways to beat the summer heat is to lounge around in your comfortably air-conditioned house, reading a few books or binge-watching your favourite Netflix series. Like all appliances, however, air conditioners are prone to breakdowns and maintenance issues. One common issue is when you hear your AC make a hissing sound. The most common reason your AC will make a hissing sound is due to a refrigerant leak – but it’s not the only reason. In the article below, we’ll review some of the most common reasons your AC might be making noise and what you should do about it.

Refrigerant Leak

The most common culprit behind a hissing noise coming from your air conditioning unit is a refrigerant leak. Even the tiniest of leaks could cause a hissing sound due to the extremely high pressures on the lines the refrigerant must travel through. During installation your air conditioner technician should run a test known as an ‘air test’ to ensure the pressurized lines can withstand the constant pressure required to operate your system efficiently. The AC and connected copper tubing are designed to withstand pressure as high as 500 psi (pounds per square inch). Refrigerant travels at high speeds, but not at 500 psi. Therefore, if the pressurized line tested by the technician does not leak at 500 psi, then the refrigerant should not leak when under less pressure. However, the older your AC unit is, the more likely a leak will eventually develop. It is also possible that damage could occur from fallen debris or corrosion. In the sections below, we will review how to address such issues.

Related Topic: Why Does it Seem Like My AC is Always Running?

Refrigerant Leaks in Your Copper Tubing

The copper pipes that connect your air conditioner to the indoor coils on top of your HVAC system are known as a line set. Upon installation, your heating and cooling technician makes an airtight connection between the line set and air conditioner using a method known as brazing.

When brazing is done properly, the connection should last as long as your HVAC system. However, if the brazing isn’t done correctly, a leak could develop and create a hissing noise. A leak may not occur immediately, but it could form over a period of time and worsen quickly.

If you notice a hissing noise coming from your AC, you should turn off the system immediately and contact a service technician. If the sound is the result of a refrigerant leak, that is not something you can repair yourself. A heating and cooling technician will need to repair or replace your line set or possibly even the entire air conditioner unit, depending on where the leak is located. Once the line and leak are repaired, your AC can be recharged and your system will be up and running in no time.

High Pressure Within Air Compressor

A less common cause of a hissing sound coming from your air conditioner is due to high pressure inside your compressor. High pressure is caused when the refrigerant or freon running through the air conditioner isn’t cooling enough. Although not common, this problem can occur on very hot or humid days for systems that were improperly installed or that are older.

If the air compressor is the cause of your AC’s hissing noise, you should turn off the system immediately and contact a service technician. The technician will determine the exact cause of the hissing and recommend the necessary repair. For more information about AC noises and problems to listen for, visit:

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AC Damaged by Debris

Another common reason for a leaky air conditioner or line set is because something damaged it. Falling branches, debris, or a physical impact from a lawn mower, weed trimmer, or other object can all cause damage to your AC or line set. The best way to prevent such damage is to put some protection or barrier around your AC unit. There are several ways to achieve this:

  • Build a fence around your AC
  • Set up pipes or cones around your AC
  • Put your AC on a platform above the ground

If you’re serious about keeping your air conditioning equipment out of harm's way, then any of the above methods will do the trick. The most sure-fire form of protection is to build a fence around your AC unit. A fence is both effective for protection and service-friendly in case of maintenance, provided there is a gate to access the unit.

For more information about how to build a great fence around your AC unit, visit this link:

Get Professional AC Repair

Staying cool on hot summer days inside the comfort of our home is something we often take for granted, until something goes wrong. A do-it-yourself repair on a hissing AC is not an option. Proper diagnosis and repair will require the expertise of an experienced HVAC technician. The professionals at Aire Serv have the tools knowledge and experience to diagnose, repair or replace your AC problem quickly. Whether it’s a refrigerant leak or a problem with your compressor, if you hear a strange noise coming from your AC unit, don’t wait. Call the pros at Aire Serv to identify the source of the problem so you can get back to your book, movie, or favourite series.