I can't speak to the rest of the country, but my brother is an HVAC guy, and he's done quite well for himself, attaining a master's certification. Around here, the Pipefitter's Union handles both plumbers and HVAC fields. Here's a good estimates of the hourly wages he gets (he's on the high end), and this does not include benefits, which are another 50-60% of his wages.
LINK.
From the stories I've heard, they're paid well because when they're installing roof top heaters and A/C units, either the engineers or the manufacturing plant have fouled them up so much that they require a massive overhaul just to work.
And he didn't get an engineering degree because he couldn't hack it in a classroom. He does quite well when you put a tool in his hand, though. It's almost savant-like, what he can do with something mechanical.