My old Honeywell rotary thermostat (mercury switch) is a 2-wire system and I want to replace it with a programmable thermostat (Honeywell RTH6300B). Problem is when I pulled off the faceplate, both wires were the same color and there is no letter coding on the wallplate where the wires are connected. My system is gas-fired hot water baseboard, heat only, no air conditioning. Any suggestions? Answer to similar questions on this website seemed to indicate that I could connect one wire to the W terminal and the other wire to the R terminal.

Thanks,

Ed

Just land the two wires for a heat only setup. In this case it will not make a difference which is which because you only have two wires to start with. Follow the manufactures directions that came in the box. Also be sure to check if the model you have will work on a two wire system.

Use W and R for your tie in with the old wires.

Here is some info for you.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1717.pdf

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1723.pdf