A properly operating airconditioner should have a consistent temperature
split across the return and evaporator coil. A desired split would
range between 16 to 21 degrees F.
To obtain a TD split:
Place a thermometer in a grill or plenum closest to the air handler
and another thermometer close to the return air filter. Allow system
to run for 15 or 20 minutes and stabilize and for thermometers to
get an accurate reading. The difference in the two thermometers will
be the split.
Thermometer at return air is 75 degrees F.
Thermometer at plenum or grill is 55 degrees F.
75 - 55 = 20
You have a 20 degree split.
It is very important to use a quality digital thermometer to get
an accurate reading.
Do not use a laser infrared thermometer. These are not nearly accurate
enough to get a good reading and will usually give a different split