I started in the HVACR Industry in about 1969, working with my Dad (an electrician) on some installs and trouble shooting for a local HVAC Contractor.
My career really moved forward when I went to work for Michigan Bell Telephone and got my 1st Class Refrigeration/Low Pressure Boiler Licences.
I have worked on Chillers Up to 850 Ton, and some pretty good size boilers (don't remember the HP). I later moved onto management and finally retired after more than 30 years with Ma Bell/Ameritech/SBC/AT&T.

The second time around...
After having been retired for 3 months I was going stir crazy at home (my wife said I was spending too much money on my hobbies) I went back to work. I responded to an ad for a HVAC Tech job for a maintenance company at a Manufacturing Facility, got the job, but then was recruited to another company at the same facility as an operator in their Energy Center.
No Boilers these days just Chillers (for process cooling), Compressors, Refrigerated Air Dryers, and METASYS.
Here I am in 1970 punching tubes on a 500 ton Trane Absorption Chiller
Klaus Koenig cleaning Trane Chiller