Did your 'reputable heating contractor' do a manual j calculation? You'll need that to start discussions with the GC, which is where you probably need to go unless you had a separate contract with the HVAV contractor.

If you had a contract with the general contractor then you need to go after him. You may have some recourse if he breached the contract. You may want to consult a lawyer for advice. Small claims court may be an option.

A Manual J calculation is good to have, but what it is not going to help if the duct is undersized. It is not required to start the discussion with the general contractor in my opinion.

Forget about court, the GC will just file bankruptcy if he hasn't already.

Chalk it up to learning experience and move on. This is why it's important to do your homework up front and hopefully make a better decision. (There are no guarantees, it merely comes down to a gut decision you have to make.)

There are all kinds of hints: does the contractor show up late? Is the contractor always busy? To some this sounds like wow this guy must really be good because he's always so busy... but is the reason he's busy is that all the work he's done recently is confounded with problems of various aspects of the jobs he's done?

I know it shouldn't be this way, but there are plenty of contractors that give the good ones a bad name.

Remember, there are two sides to every coin. Unless you're a magician.

#Presto! Nothing up my sleeve.

Get a couple more diagnoses and estimates. "Reputable" means different things to different people and is very hard to define and find! If they all say the same thing then go with the one you feel most comfortable with.