I am upgrading my old cheap mercury thermostat with a Honeywell programmable model - RTH6300B (a 5-2). When I removed the old unit there were two wires - black & white (labelled B and W). The new unit has 6 connections - C,G,W,Y,R & Rc. There are a number of alternate letters given:

C - X or B
G - F
W - W1, H,O,B
Y - Y1, M
R - RH,4,V
Rc - R

I am unsure what would be the correct connections for the existing B & W wires. Any ideas?

Thanks in advance for any assistance.

Make sure you bought the right thermostat. Should be compatible with one stage heat only.

If so, connect the white wire to "w" and the black to "R"

I have a similar problem.

I am installing the Honeywell RTH6300B, replacing an old Honeywell mercury switch thermostat.

No model number on the old one, but it only has two terminal screws, unlabeled. It must be a simple on-off switch for heat. I have heat only, no cooling, no fans, etc.

Alas, the only two screws on the old thermostat are not labeled.

And, the new thermostat's manual doesn't tell you which two terminals simply turn heat on/off.

Any suggestions?