i agree that the pressure is low and the delta-t is high. sounds like you could have some duct issues. i have found that the majority of systems do not have enough return air and many times inadequate ducts in general for modern refrigerant systems. that said, there are some fundamental service questions that still need to be addressed. first of all, what type of metering device does it utilize ? if it is a txv, your sub-cooling does not sound too bad. if it is a fixed type metering device, the focus should be superheat. i will address each scenario separately -
fixed - verify piston is correct size. use manufacturer's chart for the match of your equipment. verify superheat by manufacturers recommendations. corrrect superheat will determine the freon level. if the superheat is correct and the air flow is correct, performance will be attained.
txv - verify sub-cooling is within acceptable range (usually 7-10). if properly sub-cooled liquid is delivered to the txv, the remote sensing bulb on the txv, strapped on the leaving end of the coil, will regulate flow and superheat. typically, a txv does not need to be adjusted out of the box if it is properly matched. however, in some cases, adjustment can be made to raise or lower superheat. the important thing is to charge until sub-cooling is correct.
of course, standard service procedures must be performed prior to diagnosing these problems. correct vacuum, matched equipment, correct duct design and of course correct system sizing are all prerequisites. assuming there are no problems there, freon level based upon the above guidline should finish off the process and make the system perform.
all manufacturers include in their data, a blower performance chart indicating the correct fan speed based upon static pressure in the duct system. using a magnihelic, the static can be measured across the blower to determine the total static, then compared against the chart. if the system has a multi-speed motor, the fan speed may need to be changed. the industry is moving toward variable speed motors that sense current draw and adjust fan speed to match the static, but unfortunately, it is possible to compensate for a poor duct design with such a design. in that case, the problem can be masked and efficiency compromised.
remember, each duct is capable of delivering a specific cfm of air. added together, those ducts need to delliver 2000 cfm of air for a 5 ton system (typically 400 cfm per ton). if the supply ducts and the return air are too small, the ratio of heat load to refrigerant is out of balance. this could contribute to the high delta-t.
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