The liquid line was replaced today (the original copper pipe was undersized) and a TXV valve fitted in the roof next to the coils.
The unit is filled with R22.
Forgive me if my explanations sound pathetic, I'm no a/c expert, and I only have basic knowledge of this stuff (although at times I belive the "techs" know about as much as I do )
I will correct my first post here, the tech installed an accurator to the system the first time around, it did not have a TXV installed until today.
Today, after the system was turned on after installing the pipe and valve, the techs filled the system with R22. They filled it until the sight glass was clear (yeah I now know this is wrong, I just read another thread here). The suction pipe inside the outdoor condensor was still starting to frost after a short time, so a tech went in the ceiling to adjust the TXV. Now I don't know how to read or understand a vacuum gauge reading, but one tech at the condensor unit was reading the temps to the guys upstairs adjusting the TXV. It started at about -12 degrees C (10 degrees F) and they adjusted it to between -5 and -6 degrees C (23 - 21 degrees F). The temperature coming out of the air vents was 12.5 degrees C (54 degrees F). Inside temp according to the thermostat was 24C (75F) and outside temp was about the same. After the techs adjusted the TXV, the suction pipe inside the condensor did not frost up.
Well, about 1 hr after the techs left, and the unit had been running, the suction line again freezed up almost as bad as it had been before ! As usual, the inside temp stays at around 23 -24C (73 - 75F).
I've thrown my hands up in the air, I'm totally confused. The techs said that the coils are clean and that "this was not the problem anyway".