hissing, perfect AC

JHZR2

Staff member
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44,835
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New Jersey
Hello, Recently Ive noticed something in most of our car AC systems that I have not remembered hearing/noticing before... If one turns on the AC with the vehicle on, fan blowing, etc., there is a notable hissing noise from the vents. It is the AC system, not a vacuum actuated air diverter or similar device - I know that for sure. It is a hissing sound, almost like a very muffled sound of evaporation, which obviously I'd assume to have occurring in a closed rankine cycle... but I never used to hear it, and I don't hear it in every vehicle or every time. The catch is, the static, resting pressures and the operating refrigerant pressures all seem to be fine... Any thoughts? Have I just not been observant enough, or does this indicate anything in our aging vehicles??? Thanks!
 

Kestas

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13,820
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The Motor City
I think you're just observant. Either at, or just before the evaporator is a small orifice. This is where the refrigerant gas goes from high pressure to low pressure. This is called the "Joule-Thompson effect", and renders the fluid cold. Any moving, turbulent fluid generally makes a lot of noise.
 

JHZR2

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New Jersey
OK, I am acceptimng that it is just the turbulent fluid going through a phase change... that can be noisy - think of water dropping on a hot plate... that is kind of like refrigerant going into the evaporator, right? I am aware of J-T effect/valves, and the cycle in general... Just odd that on multiple vehicles, within the last 6 months, I note it unlike ever before. Weird. Just wanted to verify that it didnt mean low lube oil, refrigerant, growth in the system or some other situation. Thanks!
 

JTK

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13,156
Location
Buffalo, NY
Good question. You didn't notice the hiss on older vehicles. Maybe it's more compact automotive HVAC systems or quieter blowers now?
 

JHZR2

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New Jersey
Maybe... but for example, I just noticed it this weekend on our 94 toyota previa, which has an original, never opened, AC system and 222k miles. Ive noticed it for the last 6 months or so in my 04 saab. I never notice it in my 91 BMW, which has been retrofitted to 134a and works great/cold. Sounds like it is nothing to worry about...
 
Messages
1,714
Location
CA.
Some of the Chevy cars and trucks I've owned over the years made the hissing noise. A/C systems were great in those vehicles no problems at all. As long as the compressor is quiet and not short cycling, I wouldn't worry about it. JMO AD
 
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