First I would find out what the statutes (laws) in your state cover about what an employer can say about you should a hiring employer check on your past employment. Chances are an employer stating an opinion about you gives you a legal right to bring action against them. If an employer doesn't want to get sued they'll keep their mouth shut and only verify your employment dates, pay, etc. based on what the hiring employer provides. Meaning if you give hiring employer B your dates of employment and they call employer A and give them the dates A verifies the dates. In some states it would be illegal for employer B to call A and ask for your employment dates. Employer A should not be giving that information out on request. The only case that I know of where employer A has a legal requirement in some states to tell employer B about you is if you violated some law while employed with A, for example sexual harassment, theft... If your previous employer freely shares/tells your hiring employer that you were fired/terminated where you didn't violate some law then I'd say you have legal recourse against this employer. If your interested in really finding out what they would say then what you do is pay a reference checking provider to contact them (do a google search) . If you find out they are giving false information or information freely that they shouldn't be then a cease and desist letter from an attorney should take care of the problem. If your past employer has any brains all they would do is verify the factual information and not give a reference good or bad. Believe it or not an employer giving a good reference might be a set up for the hiring employer.
Second don't give a hiring employer any information that you don't want to share. All you have to provide is your name, address, contact info. Really go ask the EEOC! In your case I'd tell the hiring employer what you did and just tell them you left because you weren't doing HVAC/R work that you went to school for and saw no opportunity there to land a starting position. That sounds like a good reason to me for leaving. Play it up and put the ball in your court so to speak. You had excellent grades, drive, ambition, and weren't being recognized for it. What ever you do don't bad mouth your previous employer or give any questionable doubt that would cause the hiring employer to start digging around for information about you.
As for the negative comment(s) posted about your grades just ignore them. Someone who is book smart is more apt to go look something up when they don't understand something. Ask yourself what kind of dumba$$ would you rather be working with .... someone who relies upon their intuition and wisdom and gets you both killed/injured or someone who has enough sense to educate them self about what they are doing? I would say yes there are good employers out there looking for people like you NATE certified and taking RSES training.
Oh really. I'd be finding me someone who you can rely upon to verify this and get me an lawyer to send that boss a not so nice letter. I'd bet he'd shut his pie hole real quick like. What do you suppose he'd do to you if he found out you were bad mouthing him? I'm sure he wouldn't like it anymore than you would about him bad mouthing you.that sucks i got fired due to a personal problem with my old boss but cant get a new job because of him b.s.ing in supply houses.
I may sound like I'm beating up on employers. Unfortunately too often we hear about the not so nice ones. There are good employers too.