AC is blowing hot.

Is it a closed system...most definitely. The problem with cars is they rattle, vibrate, and develop micro leaks and operate under pressure so the refrigerant can quite easily leak out. Home A/C systems are closed as well, but no one drives them all over the country, they're stationary and aren't susceptible to the leaks that cars are. So when someone says the A/C works fine except it needs a charge, they're really telling you they have a leak when they shouldn't have and it really doesn't work fine at all. Typically the leak is at the compressor shaft seal. then the condenser and then any of the connections that use O-rings.

A shop will need to diagnose the a/c system, and they may find that it needs a recharge. If so, then they will inspect the system for leaks, and make the the repairs before recharging the system. Go here to learn more: http://repairpal.com/heating-ac Before you take the car in, please read this: http://repairpal.com/Heating-And-Cooling-Issues