Sir:

My house is old and I only have 2 black wires coming from it. Where do they go?

Y -o o- Rh
G -o o- Rc
o- W

I want to put one black to Rh and the other black wire to W on the RTH6300B. Please advise. Fred sends.

Here are your directions

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

Make sure you have fresh batterys installed in the thermostat.

One of you two wires you have goes to RH-RC. There is a jumper you leave in place so you have to attach one of your wires there.

Next and last wire you have goes to W on the new thermostat.

Read all of the directions and program and set up your new thermostat with you type of system you have.

Word of caution. Many times the new style thermostats will not work on older systems but there is no reason not to try to hook it up.