What's the effect on copper tube (HVAC) after long storage?

Copper doesn't age. It will develop a coating of green patina, which is just copper oxide. But this is thin, and doesn't effect any of the properties of the copper. If you want to get rid of the patina, just rub it with some emery cloth. It's only a thin surface layer. .

HVAC copper is oxygen free and has a higher flexible strenth than ordinary water tubing it is the same size exactly. The fittings, however, are exactly the same copper wise, both are made by impact processes and sold interchangeably. So you can buy a copper 3/4 plumbing fitting or elbow at Lowes and it will work exactly in an AC line. Here is the catch, you cannot use ordinary solder to assemble HVAC, you must use a product called SilFos, that is a self fluxing silver/phosporous rod that requires temperatue to melt and flux at around 1200 degrees. Jointing in AC is called a "sweat fitting" No one today uses any "flare" fitting in the AC business or any other type, sweat fitting are used exclusively that includes internal parts in the condenser, and evaporator. So a MapGas+Oxygen unit or an accetylene Turbo Torch is used. Most HVAC use a turbo torch. You will also need to flood the lines with Nitrogen so as to prevent internal oxidization which causes trouble with internal AC parts when brazing copper.

Bill, if you don't know, you shouldn't feel obligated to answer. Copper most definitely age hardens, The speed depends on the heat as well as time. Compression fittings are more likely to leak on age hardened tubing because they can't force it into round and can't compress it or bite into it as easily. I am fighting a group of fittings in my house that are leaking right now. They are all in the hot water immediately out of a water heater that have been around 160-180 deg. for 16 years. The rest of the house is still fine. I cut a new piece of tubing (the same shelf age) and couldn't get it to seat either. I will most like have to replace several fittings and buy new fresh tubing that has not age hardened. I had similar experiences in my first house. All of this is anecdotal based on general engineering knowledge & 45 years experience. I have found oblique references but no direct authoritative references.