A/C problem - loud hissing at evaporator
 

The return line (uninsulated) line coming from the evaporator has a loud hissing sound. I turned on the other A/C unit and it does not have the same sound. Unless you put an ear up to the line, the air handler is louder. On the upstairs unit, the hissing sound in the evaporator is louder than the air handler/blower fan. (I have 2 units in my house, one is for the upstairs and the other for down stairs, and are independantly controlled).

Can someone diagnose the issue? Last fall I had to have the evaporator replaced, since they detected freon leaking in that area. I do have guages which i typically use on my car. I think they are the same ones used on home A/C units.

Might be low on freon and/or air in the system. Any frozen lines?

I haven't opened up the evap compartment yet, to look. I didn't let it run very long, though.

The smaller (uninsulated) line usually comes from the condensor and goes to the evaporator. The hissing could be coming from when the liquid refrigerant is going through the metering device at the evaporator inlet.
Hooking up gauges will tell you more. As long as the coil isn't frozen and the filter is clean,and the unit is cooling you might be o/k.

^^^^ Are you talking about the expasion valve? I was thinking about that, but read it as he could hear the refrigerant in the lines.

My house has 2 A/C systems, one for upstairs and one for downstairs. When I operate the downstairs unit, the fan system is louder than any sounds of the refrigerant. This is on the small line next to the evaporator. I need to put my ear on the line to hear anything.

When I operate the upstairs unit, the sound is quite lous. Like the hissing in a natural gas line that is flowing gas.

According to the technician yesterday, the compressor is not producing much pressure on the outlet. If that is the case, then the refrigerant isn't liquifying in the condensor, which would be the hissing I'm hearing.