So...a lot of speculation on my chiller's problem. I purchased this chiller (1/2 hp) used from a member here on the forum about 5 years ago. When I first tried it (several months after purchase [my first mistake]) it did not work. It turned on and made noise (sounded like the compressor was working), but did not really chill. I left it in place as I had a secondary, smaller chiller (1/5 hp) that was getting the job done anyhow. 4 years later the smaller chiller stopped functioning. It sounds like the problem with the smaller chiller is the compressor as it no longer 'kicks in', but I have not had that chiller looked at by anyone yet.
I purchased a new relay and overload switch? from Aqualogic for both chillers and had the parts sent to me 2-day air. Total cost including shipping was $100.
So...I took both chillers out and replaced the parts I got from Aqualogic. Not really difficult to do, but not trivial either, particularly for the large chiller. Anyhow, after changing out the parts neither chiller acted any differently. I don't think the parts were the problem(s). However, I did run the big chiller for a bit with the cover off and I could see that at least part of the chiller was indeed 'chilling down'. So, I figured there was hope, maybe just needed to be recharged.
So...I had the chiller repair guy look at the big chiller. His assessment was that it did not have any significant leaks since there was still a fair bit of pressure in the system even though it had been sitting around for several years. He fiddled with the system for a while and tried to recharge it, but it didn't seem to help too much. Finally he completely drained and 'purged?' the system and then got the proper info from Aqualogic (sometimes the time change, they are on the west coast, helps you out) as to how much refrigerant is supposed to be in the system. He added that amount and it started working pretty well. Still, as I mentioned earlier, the compressor makes a fair bit of noise.
Anyhow, I reinstalled the chiller and it's been working like a champ since then. BTW, the definition of an optimist is an aquarist that glues the chiller PVC connections in place!
I've yet to have the smaller chiller worked on. I'm fearfull that the compressor is dead.