So . . . talking about the same post office I was talking about a few days ago. I thought I'd log back on the next day, but I had cool stuff like the Floresville Peanut Festival come up. I just HAD to go!
Anyways . . . we did all this work to find out what was wrong with that York unit, and learned that the other contractor was right about the compressors being bad. While I was there, I was able to talk the management company into letting me clean the other unit on the premises, and perform basic maintenance.
So the next day I spent about half the day pulling that other unit (it's a Trane by the way) apart and cleaning everything so it would function properly and not leak all over the place . . . pain in the butt, but it was a $600 (for me) pain in the butt. When I got done, I noticed that the liquid line was running cool . . . about the same temperature as the suction line. The liquid line actually carries liquified refrigerant, which is compressed from its gas state, so it's supposed to be warmer than the suction line which carries refrigerant in its gas state. You see, refrigerant cools down as it is sprayed from its liquid state through a metering device into the evaporator coil. Spraying it allows it to form back into its natural gas state, cooling it considerably. Just think of your bottle of Windex that you spray on the windows to clean them . . . or those misters used at football games to keep the players cool . . . or a fuel injector. All of those things vaporize a liquid by spraying it.
Needless to say, there needs to be a perfect balance of refrigerant so it will be liquid going into the home and vapor coming out of your home. If this balance is thrown off, it will damage your compressor or you will notice ice accumulating in your unit. That's why HVAC contractors are hired. We know what we're doing! We have to understand pneumatics, hydraulics, elctricity and plumbing to do what we do. It's not easy, and there are people out there who know more about each of the things we work on than we do. It's just amazing how scentific all this stuff is.
Back to the subject now. 15 ton Trane unit, post office, cold liquid line, suction line freezing up. There was a reason for the last two paragraphs. See if you can follow now. We checked the unit out by actually following the liquid line from where it came out of the wall outside backward to the unit, inside the unit to a filter/dryer that was attached.
Now, do you want to hear something cool? The the liquid line was about 20 degrees warmer going into the filter/dryer than it was coming out of the filter dryer! Based on everything I said in the previous paragraphs, that means something was happening to the refrigerant as it passed through the filter/dryer. It was entering as a liquid, but exiting as a vapor. How the hell could that happen? Well, this is a filter we're talking about here, so it gathers up particulates in the refrigerant as it passes through the filter/dryer. It just so happens that the filter was clogged so much that the refrigerant had to move through tiny holes in the filter to get to the other side. So as it passed through the filter, it was being sprayed through those tiny holes and turned into a vapor as it moved to the other side. That's why the liquid line was so cold when it is supposed to be the warmer line. Remember? When refrigerant becomes a vapor, its temperature drops considerably.
That's exactly what was happening in this situation. We're not sure how long this went on, but that puts back pressure on the compressor, shortening its life, so we needed to get the filter/dryers changed out as soon as possible.
I'm getting tired of typing now, so I'll continue later.