Today's A/C systems are designed to operate as silently as possible, which makes strange noises originating from the unit stand out. If your A/C system suddenly starts producing hissing noises, it may be due to one or more serious issues that should be addressed as soon as possible. The following goes in-depth about these potential issues, as well as the repairs you and your HVAC technician can make to eliminate hissing sounds from your A/C system.
Potential Causes
A continuous hissing sound coming from your A/C system can signal a number of issues:
- There could be a refrigerant leak somewhere within your A/C system. Refrigerant leaks often occur when a pinhole leak develops within the copper or aluminum refrigerant line or when an O-ring fails. As the refrigerant forces its way through the leak, it can produce a low-pitched hissing sound until the unit completely empties of refrigerant.
- Damaged or worn internal refrigerant valves can produce a loud hissing noise during A/C operation. This noise usually goes away when high- and low-side pressures stabilize upon system shutdown.
- Excessive internal pressures inside of the A/C compressor can initially create a continuous hissing noise. If left unchecked, the hissing noise can give way to a screaming sound, indicating the potential for severe A/C system damage.
- In some cases, the hissing sound may have nothing to do with your A/C system's refrigerant. The cause of the problem may be a loose or poorly fitted supply air register that's letting air push past the register's edges.
If the hissing sound occurs at regular intervals and only last for several seconds each time, then it's likely due to the operation of the A/C system's expansion valve. Believe it or not, this is actually normal -- in this case, the brief hissing sound is simply the expansion valve regulating the amount of refrigerant that passes through the valve. If you still want peace of mind concerning this sound, you may want to have your HVAC technician verify the expansion valve's normal operation.
Making Repairs
To get to the bottom of your A/C system's noisiness, start by checking the supply air registers in each room. If your A/C system is equipped with variable speed blower fan, you can diagnose hissing sounds coming from the registers simply by changing the fan speed. If the hissing sound changes pitch as you change fan speeds, then it's the registers that need your attention. Make sure each register fits flush against the wall or floor. Loose registers should be refitted and re-tightened to eliminate gaps. If any gaps remain around the register, seal each one with silicone caulk or mastic.
For hissing noises emanating from the A/C system itself, carefully listen to the A/C unit as it operates and while the unit is turned off. If you hear hissing sounds coming from the A/C while it's turned off, then it's likely due to ongoing refrigerant loss. If you only hear continuous hissing sounds while the A/C system is in operation, it may be due to an internal valve or compressor problem.
For these issues, you'll definitely want to get your HVAC technician involved. To stop refrigerant leaks, your HVAC technician may replace the offending section of refrigerant line and recharge the refrigerant to proper operating levels. For noises stemming from the compressor or internal refrigerant valves, your HVAC technician may replace the offending components and inspect refrigerant levels afterwards.
How to Prevent Hissing Sounds
Being proactive about A/C maintenance can help prevent hissing sounds and other problematic noises from occurring in your A/C system. Not only should you have your HVAC technician perform an annual inspection of your A/C system, but you should also keep up with ordinary maintenance tasks, including regular air filter changes and duct cleanings. For information about how often your A/C needs to be maintained or cleaned out, speak with an air conditioning installation professional.
