My understanding from when I had a new furnace installed a couple years
ago and was discussing the option of also getting central air hooked up at
the same time was this - Household central air units are designed to
provide a max differential air temperature of 15 degrees.

If it's 85 degrees outside, the a/c can reduce that to 70.
If it's 100, then the best it can do is drop the temp to 85

This is wrong. I live in Arizona where it was about 112 degrees yesterday.
That would mean the temp in my house would be 97 degrees----We keep it at 78
and our A/C unit has no problem at all. I have customers that keep their
homes around 68 degrees and their units cycle just fine. A few years ago
when it was 122 outside my unit didn't cycle off much.

I think the installer may have been referring to the temp split a unit
operates on. The air coming out of the vents is about 15 to 20 degrees
cooler than the air going into your return vent. This does not mean the the
unit will only cool 15 to 20 degrees below the outside temp.

AZCAIG

www.arizonavintagetrailers.com