Trane 13i TXV Replacement


I have a relatively new Trane 13i - only 3 years old. I have a couple of questions about it.
First, I'm having cooling problems. A tech came out, and said that he suspected a problem with the TXV valve, but would have to come back when it was warmer (tomorrow). He said several things:
1. That there was getting low suction pressure. 2. He adjusted the Freon, but could not get it above 32 degrees. 3. He said that they had been replacing a lot of a certain kind of TXV in this unit. If it was the TXV, he would also have to replace the coil (he said that the replacement would include both of these items) 4. Even though the part was under warranty, he would charge $600 to do the replacement.
Does this seem right? Do you normally replace a coil, or any other part, when you swap the TXV? I have heard that the Trane has this valve fairly accessable - you take down the Freon, pull the valve, charge with Nitrogen, braze the new part back in, and then charge the freon again. Sounds like a fairly straightforward effort.
Also, I've seen on other sites, that you normally replace the drier when you swap out the TXV. True?
Next problem - can anyone tell me what the proper size of the copper tubing should be, between the fan unit in the basement, and the compressor outside? I actually had a second unit installed in my upstairs (not a trane), and the service guy on the second unit asked me if the Trane installer had replaced the tubing when he installed the Trane (the trane was a replacement for an old contractor grade unit that I replaced). The reason he asked, was that he said that the copper piping was too small of a diameter for that High efficiency unit, and that with the restrictive, smaller tubing, I wouldn't get anywhere near the correct SEER. How do I find out the correct tubing diameter for this unit (its an RXC0255hpA0).
Thanks in advance.
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Is this a factory authorized Trane repair shop? If not and labor is suppose to be covered as well, that's probably why he wants to charge you. Because Trane will not pay him. If it is a Trane shop and you think it's all suppose to be covered, call Trane for a clarification.

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