Kevin,

The facts are that most believe that only taking the temperature differential (between the return and supply air) is a means of determining if the unit is functioning or cooling as needed. Having a TD of 15-22 degrees is only an indicator of the system. Not how it is actually performing. Although, you will always hear the real estate agent state that the unit is good because of the TD being in that range.

Keep in mind, that a blocked coil is one thing that you (or most) do not determine during the normal visual inspecton and without removal of components of the unit. It is possible to have a blocked coil and still have a temperature diff between 15-22 degrees--- MEANS NOTHING!

The presence of a blocked coil will provide the TD with skewed numbers to go by. The range (of temperature differential) becomes greater as not enough air moves thru the coil, providing everyone relying soley on the TD with bad information.

The air flow restriction is obviously an issue that is very decieving. Conversely, the opposite is true with too much air flow-- the numbers become too low/ narrow, which may not allow enough humidity to be removed from the air.

If you take the TD in a home when you first get there, that would cause issues with your numbers as well. The unit needs to be running (in my mind) about an hour to determine if it's cooling or not (notice I did not say functioning).

Richard