Such a seemingly simple task that I can't figure out how to do. I need to activate a program when the difference between two temperature values exceeds a threshold...say greater than 10F difference.

I can easily check a temperature value in an if statement that it is relative to some absolute value or even another variable.. If I could just assign a variable the value of the temperature sensor + an offset, then I'd be able to do it.

So what's the trick? What am I missing to be able to do something so basic?

what is the source of the temperature readings?

Using either CAI 8 or Insteon thermostats. Same issue. There is no way to assign the temperature value from either of these to a variable. 

Insteon thermostats can definitely be assigned to variables, at least in version 5.0.14.  Here is a program I use to measure the difference between the set point and the actual.

temp dif from set - [ID 0086][Parent 0082]

If
        'HVAC Mt. Tabor / Mt. Tabor-Hall-Back Wall' Temperature is not -50.0°
     Or 'HVAC Mt. Tabor / Mt. Tabor-Hall-Back Wall' Heat Setpoint is not 0°
 
Then
        $i.temp.dif.from.set  = 'HVAC Mt. Tabor / Mt. Tabor-Hall-Back Wall' Heat Setpoint °
        $i.temp.dif.from.set -= 'HVAC Mt. Tabor / Mt. Tabor-Hall-Back Wall' Temperature °
 
Else
   - No Actions - (To add one, press 'Action')
 

CAI webcontrol has two options.

1) Write PLC code on the CAI that posts directly to a variable using ISY REST interface

2) Install ioguys webcontrol software on a PC or RPi to interface your CAI and ISY.  I have no temp probes on my CAI, but here is a program that I created just now that I'm pretty sure would set a variable based on temp reading from CAI.

DST1 - [ID 014A][Parent 0093]

If
   - No Conditions - (To add one, press 'Schedule' or 'Condition')
 
Then
        $DST  = 'WebControl8 / Temperature1' Status 
 
Else
   - No Actions - (To add one, press 'Action')
 

 

So my problem is that when I was selecting the little right arrow for the source to assign to the variable, it was cycling between Number, Variable, and System. After restarting the UI, I now see a fourth option which is the device pulldown along with status. So with that I can do what I wanted.