my old stat has a X2 wire, black, which is not listed in compatibility chart.

[butch74]

Same as the subject. No X2 wire listed and no C wire either.

[Jenelyn_O]

Hi butch74,

Thanks for reaching out. I'd like to take a closer look at it. A few questions: in what country are you located? What is the make and model of your system? Please take a photo of the wires connected to your system's control board. Ensure that the labels of the wires are visible. 

Best,

Jenelyn