Why Is My AC Hissing?

HVAC Logo IconBy Tom MoorAugust 4, 2023

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HVAC technician checking an AC's refrigerant level

If you hear a hissing sound outside (and it's not a snake), check around your AC condenser unit. The most common cause of an AC hissing sound is a refrigerant leak which should be repaired promptly by a certified HVAC technician to prevent system damage and health problems. However, other issues can also cause an air conditioner hissing noise. Keep reading to learn more.

Common Causes of an AC Hissing Noise

If you're asking yourself “Why is my AC hissing?”, one of the following issues is likely at play.

Refrigerant Leak

Refrigerants are a crucial component of every air conditioning and heat pump system. During the warmer months, the refrigerant absorbs heat from inside your home and dispels it outside through the AC compressor to provide cooling. Air-source heat pumps also use refrigerant during the cold-weather season but in reverse. During the winter, the refrigerant extracts heat from the outdoors and uses it to warm your home.

Refrigerant leaks occur for several reasons including:

  • Corrosion. Over time, the metal components of your air conditioner or heat pump, primarily the copper tubes found in the indoor evaporator coil, can corrode and develop small cracks or holes, leading to a refrigerant leak. This often occurs with aging systems and is a tell-tale sign the cooling system should be replaced, especially if it still uses R-22 refrigerant which has been phased out.
  • Improper Installation. Incorrect installation during the air conditioner installation process can cause refrigerant leaks. Shoddy workmanship can include improperly connected refrigerant lines or fittings that may compromise the system. It's crucial to work with a qualified HVAC company for installation.
  • Physical Damage. Damaged refrigerant lines during repair of maintenance often leads to refrigerant leaks. Lawnmowers and weed whackers can also damage the refrigerant line.
  • Compromised Joints and Connections. With age, the system's joints and connections can break down and develop a refrigerant leak that causes an AC hissing noise.
  • Manufacturing Defects. A factory defect, especially with the condenser and evaporator coil, may cause refrigerant leaks, although this is uncommon.
  • Freeze-Thaw Cycles. In colder climates, heat pump units go through a frequent freeze-thaw cycle during the winter. The expansion and contraction of the refrigerant lines may cause small cracks and leaks.

“You should call an HVAC specialist as soon as possible to come and fix the leak,” said Adam Graham, an industry analyst at Fixr. “The longer you leave the issue, the bigger it will become. Refrigerant leaks can also lead to compressor problems. Refrigerants can be hazardous, so fixing a leak is a job best left to professionals.”

Although newer refrigerants are far more environmentally friendly than R-22, all refrigerants pose potential dangers and should only be handled by certified HVAC technicians.

Air conditioners manufactured before 2010 use R-22 refrigerant which was phased out because of its ozone-depleting qualities. It was replaced by R-410A which is more environmentally friendly. Currently, R-410A is being slowly phased out in favor of alternatives with even lower global warming potential.

Refrigerant Leak Repair Cost

Graham said the cost to fix a refrigerant leak is between $636 and $1,184 on average. Recharging the refrigerant levels can cost about $207 to $385.

Air Duct Issues

You may also notice an air conditioning hissing sound coming from the air ducts. When ducts develop small leaks or gaps, conditioned air escapes and creates a hissing sound. Leaky ducts can lose as much as 20% or more of conditioned air, raising your energy bills and increasing wear and tear on the system. Have this issue fixed as soon as possible.

Clogged Air Filters

Clogged and dirty air filters restrict airflow to your HVAC system, causing it to work harder which can lead to hissing noises. Have your air filter replaced about every 2-3 months or according to the manufacturer's guidelines.

Pressure Issues

If the pressure is too high within your AC condenser unit, hissing or high-pitched noises often develop. If you hear unusual sounds coming from AC condenser unit, contact a local HVAC technician.

Faulty Expansion Valve

The expansion valve regulates the flow of refrigerant in the system. If it malfunctions, it can lead to unusual noises.

The Importance of Contacting an HVAC Professional to Repair Refrigerant Leaks

If the AC hissing noise is indeed a refrigerant leak, contact an HVAC professional as soon as possible. Do not try and repair the leak on your own. According to the U.S. Environmental Protection Agency regulations, any technician who maintains, services, repairs, or disposes of HVAC equipment that could release refrigerants into the atmosphere is required to be certified.

Heating and cooling professionals are trained and certified to handle refrigerants safely, ensuring leaks are repaired, refrigerant is properly charged, and the system operates efficiently. Attempting to handle refrigerants without the necessary expertise and equipment can not only pose health risks, but it can also damage the AC unit and result in costly repairs.

Although newer refrigerants are significantly more environmentally friendly than their predecessors, exposure to refrigerant vapors can cause respiratory issues, skin and eye irritation, and other even more serious health problems if inhaled in high concentrations.

FAQs on AC Hissing Sounds

How do you fix an AC hissing sound?

If your AC makes hissing noise when turned on, contact a certified heating and cooling professional. AC hissing sounds are commonly caused by a refrigerant leak, although leaky air ducts or a clogged air filter are other culprits.

What happens if AC refrigerant leaks?

If your AC's refrigerant leaks, the level of refrigerant will fall below what is required for efficient system performance. Your air conditioner will struggle to reach the set temperature, causing it to run longer cycles which will increase energy costs and system wear and tear.

Can you run your AC with a refrigerant leak?

Your air conditioner can still run with a refrigerant leak, but it will either provide inefficient cooling or no cooling at all. You should not run your AC unit with a refrigerant leak. Turn the system off, and contact an HVAC professional to recharge the refrigerant or recommend system replacement depending on certain factors.


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