Our 3 ton envision series wasn't heating so I called our dealer and he determined that we were low on refrigerant. He filled it up and put an electronic leak tester on it but couldn't find anything. He said it probably wouldn't leak in the future.....but how can that be. It had to leak from somewhere and I certainly don't want to pay for another diagnosis fee in 10-12 months if it does. Any thoughts. How far should I push them to find the leak?? The unit is only 2 years old and I have the 10 year warranty.