There are slide rules for HVAC that list the required super-heats and sub-cooling for fixed orfice and txv applications, Both York and Carrier have nice ones and offer them for both R22 and 410A. But if you check Jim Wheelers artical in the subjects of interest section It will help.
Good luck and enjoy this is a good forum for the trade. ENJOY
Subcooling is also usefull information for any type of metered system. While charging by superheat is the preferred method for fixed metering devices and subcooling for TXV's, subcooling is still important data for fixed metering systems.
Let's say you have a fixed metered system with a compressor that keeps going out on thermal overload. You check the superheat and it is right on the money, but the cooling is not sufficient and the compressor goes out on thermal during long cycle times. What do you do?
Too many times a tech will replace the compressor. If the tech replaces the compressor and the liquid line filter/drier the issue is usually resolved and the tech is reinforced that his/her diagnosis was correct.
Lets say the tech does not change out the filter/drier and the same issue occurs with the replacement compressor. I have had techs call me to complain about bad runs of compressors many times.
OK, now lets back up to the original diagnosis. The tech checks the superheat and it is right on the money. Then...the tech checks the subcooling, yes, check the subcooling on a fixed metered system.
Hey! What's going on? The superheat is correct but there is 30Ί subcooling.
Well, there is most likely a blockage between the condensor coil and the evaporator coil. What is there? There should be a filter/drier (my first suspect), a service valve and the evap coil metering. More than likely one of them is partially blocked.
While the problem is most times the filter/drier, it could be any one of these components. I have seen where a tech had replaced an internal filter/drier that was just before the service valve only to have the same problem. This guy was so beligerant with me over having told him to replace the filter/drier that I did a field check on the system.
What first attracted my attention was that the piece of copper tubing bypassing where the internal filter/drier was, had a bit of sloppy brazing. As diplomatically as a RoBo can, I insisted that the system charge be claimed and the liquid line service valve be taken out.
When we pulled the service valve, a small bead of brazing rolled out of the condenser side of it. We put the system back together, evacuated and charged it up to where it was working fine. The tech then took me to lunch.
Point is; check all temperature and pressure relationships before coming to a conclusion of the condition of a system.
Manufacturers generally require the subcooling to be around 14°. Goodman, for example, requires 12° - 15°. Older Lennox required about 14° - 16°. Newer Lennox use the approach method. Rheem has a chart based on the outdoor temperature/liquid line pressure. It is best to go by what the manufacturer recommends, though.
As RoBoTeq pointed out, the main point is to be sure that the TXV never gets any vapor. A unit is also more efficient with lower subcooling. If the subcooling is too low, though, then the system stands a better chance of getting hit with vapor as the condenser gets dirty.