"Go online and do a search on 'HVAC' to find reference photos; also, look out your window, taking sketches and pictures. Then obtain a bag of balsa blocks. Shape the evaporators, large ducts, etc., from these blocks; cover with styrene or cardstock; add louvers and grills cut from old diesel shells or built up. Paint silver with appropriately placed rust. Cheaper and more fun than buying $20 bags of details that don't go very far anyway, and you get something nobody else has, which is becoming a rarity nowadays."
You're advocating modeling?! HOW DARE YOU!!!!!!!!!!!!! What's wrong with you? Have you no principles? Everyone knows that anything you use on a model railroad MUST come in a box preferabably with walthers on it.
A courier with a package has been despatched to your residence. It contains a bottle of hemlock. I urge you to drink it for the good of the hobby. ;-)
Eric