I have been in the heating and cooling field for about 10 years now. I have been setting my AC units up by superheat. I have a chart which tells me the superheat for a specific day based on a few variables. To set the unit up I use the Outdoor Ambiant temp, Indoor Dry Bulb, and Indoor Wet Bulb Temps. This is a chart I got while at a Heil, or ICP Training. The question I have is, Will this same chart of formula work for different brands of split systems. I would think it would however I have been installing a lot of Luxaire Equipment and when I initially charge the system based on the extra length of linset and the matched coil it seems that the system is always overcharged based on my chart. Then to get my chart value I have to remove some of the freon that I put in after pumping the system down. I understand that this chart is based on having 400 CFM per ton of ac and I always
try to install my ductwork accordingly.

The second question I have is why do the coil temps vary so much with different equipment brands. I just but a 4 Ton 12 Seer unit in my home. The home already had the ductwork installed so I just retrofitted it. I used a 4 Ton 12 Seer Luxaire condenser with a 5 Ton Air handler and coil. I have been instructed that this will gain me a little in the effeciency. I did not however install the TXV valve I used the recommended pin instead. I understand that this hurts the Seer a bit. It seems the system has a hard time keeping up in my home. The freon is boiling at about 44F (74Lb) which seems high. (High Pressure on the Suction side) and the High pressure seems low at 175 lb of pressure. I matched the pin however it seems I may need a pin with smaller oriface. Is this a product of larger coil or of the 12 Seer of the unit. Or is my unit over charged. I added 29oz which was what the manual said for the lineset length as well as the coil installed.

Any help would be appreciated.. Thanks.