hissing, perfect AC

[JHZR2]

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]

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.

 

[KilgoreBass]

Maybe the sound of the refrigerant as it goes through the orifice restriction? If its cooling OK, and pressures are OK, I wouldn't worry.

 

[CROWNVIC4LIFE]

My 05 Crown Vic and all the cars in our fleet do it everyday of the week here in the jungle as we never shut it off....No worry..Everything is OK.

 

[JHZR2]

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]

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

 

[JHZR2]

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...

 

[ADFD1]

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