Hmmm. I looked at the ISY manual but it's not clear. I am using integer variables. in a program, I assign an integer variable (X) the result of a sensor temperature outside the house. In the same program, I assign a second variable (Y) the result of a temperature inside the house from a separate sensor (thermostat). What I want to do is get the difference between the 2 variables (Y -X) and assign that to a third integer variable (Z). Then, if the difference is greater than 15 degrees F, the program turns the furnace on.