Mechanical Engineering for Design, Construction, Alterations, Remodeling, Upgrade and Maintenance of Buildings and Facilities
See below for links.
Mechanical Engineers in this field may do heating, ventilation, air conditioning, refrigeration, heating and cooling processes, piping, pumping, gases, water plumbing, waste plumbing, fire sprinkler systems, and overlap into electrical engineering, structural engineering, civil engineering and architecture.
This is the work I've been doing for many years. Every employer, project, customer and facility can be so different, on one hand it seems like this is the art of responding to continuously new and random problems and challenges. On the other hand, there are some general principles and truths that remain more or less constant.
The best mechanical systems in buildings and facilities are the ones that nobody notices, and nobody cares about. People spend their whole day there and they didn't give one thought to the mechanical systems.
If you're in this industry, you work for your own satisfaction of doing a good job.
Or for some reason, doing a good job is not your top priority. People notice your work. They're too cold. They're too hot. It's too stuffy. There's water on the floor. The (you name it) broke.
Links - Mechanical Engineering for Design, Construction, Alterations, Remodeling, Upgrade and Maintenance of Buildings and Facilities
(oops, too few links but I'll get to it.)
Design Phases, Mechanical, for Building Projects