Supply and Temperature difference. In South Carolina, you would like a supply vs. return differential of about 20 degrees F. There are a large number of variables that we as home inspectors will not be able to determine, so in general, a difference (supply vs. return) should be about 14 degrees F to 22 degrees F. If your temperature drop temperature is less than 14 degrees F or greater than 22 degrees F your HVAC system needs servicing by an certified licensed HVAC contractor. If the air on the outlet side is too warm, you could have one or more of the following:
•Too little refrigerant in the system.
•Too much air passing over the evaporator.
•Too high fan speeds. If the outlet air is too cold, you could have one or more of the following:
•Dirty air filter.
•Clogged evaporator coils.
•Ice on the coils.
•Too low fan speed.