We have a two wire old analog thermostat it has a black wire that I think was red before (had some wiring redone recently) and a white one. There's also a purple one but I am not sure what its connected to.
Old thermostat is a White-Rodgers it says Type 1C20-101 and then E1 0539T.
The new one is a digital wifi touchscreen from Honeywell and it asks for a C wire also. RTH858WF is the model number.
http://wifithermostat.com/wifi-programmable-thermostats

Can we use it? Any help will be appreciated :)

If the new thermostat can be battery powered, you probably don't need the common wire.

If the new thermostat can be battery powered, you probably don't need the common wire.

Yes. It has a battery! Ok awesome I will try that. Also the electrician changed the wires from small to thicker. Is that OK? Jw.

How thick is "thicker".
Thermostat wire should be 18 gauge. Telephone wire is usually 22 gauge & house wire is 12 or 14 gauge. The lower the gauge number the thicker the wire.