I only 2 wires, a white or yellow and a black, what letters on my new thermostat should they be connected to. I only have a heating unit.

R and w

r and w

Thanks Joshdta a but my new thermostat only has the letters... E, AUX, Y, G, O, L, R, B, and C(optional).

r and w

Thanks Joshdta but my new thermostat only has the letters... E, AUX, Y, G, O, L, R, B, and C(optional).

R and e and you will have to put the new stat to emergency heat. As this is a heatpump stat