Differential Temperature is the drop in supply temperature after cooling is engaged, or the rise in supply temperature after heating is engaged.
Use this information to solve performance issues. If an HVAC is only producing a 2-degree differential temperature when calling for cooling, this may be an indication that the compressor is low on Freon.
An HVAC running 2 compressors should cause a drop in supply temperature of at least 10 degrees lower than the room temperature.
An HVAC that uses gas heating should causes a rise in supply temperature of at least 20 degrees higher than the room temperature.