How do I install an Honeywell RTH221B thermostat?? I have a red, white and green wires on the old tthermostat. Where do the new terminals Y, G, Rh, Rc, and W connect to the old wires?? :)

Is this for heat only? A/C only? Or both?

It is heat only on a water baseboard heat--no fan.

List the old wire colors.
List the exact model and brand of the old thermostat
List exactly where the old wire colors were attached to the pld thermostat, I know you followed the thermostat instructions when it told you to identify and mark the old wires before you removed them.

Your instructions for the new thermostst since you might have lost them,

http://customer.honeywell.com/techlit/pdf/69-0000s/69-2060EFS.pdf

If the terminals are the same the only difference should be connect Rc to Rh and use as R.