I have a Miller furnace CMF80-PO and I would like to add a common 'C' wire for my new 3M-50 thermostat. Only 4 screws, T1(RH) T2(W) and the other two are F1 and F2. It don't have the regular code letters, it got 2 wires connected to those (yellow wires). I wondering if 1 of those wires could be use for a common 'c' wire connection?

Why /do you want a common wire?

I bought a Filtrete 3M-50 thermostat. I need it for the WiFi connections so I could be able to register and control the thermostat online.