So I just bought a digital thermostat for my house- (the previous one is an old mercury switch) and while taking off the front plate I noticed that there were only 2 wires connected to contact plates, (I am thinking a 120 source) and a ground. The directions in the new unit says I won't be able to use the new one if it's a 120 source but I am not totally sure it's def a 120 source. Does anyone have any experience with these and do I just need to get a voltage meter to see if it's truly 120?