There is a 3-wire conductor running from the heater to the mercury thermostat, but only two wires are connected (the Red and White wires). The third wire is not connected on either end.
My first question has to do with what would happen if, when disconnecting and reconnecting the wires at the thermostat end, I got them mixed up and put the Red wire back on the screw where the White wire originally was and the White wire back on the screw where the Red wire originally was. In other words, will it matter if I got these two wires backwards? I have been assuming that the old mercury thermostat is really just an on/off switch, and the circuit is either completed or not completed. So, my thinking is that it wouldn't matter if these two wires were mixed up on the thermostat end.
Is that correct? -- that it won't matter if the wires were put back on the thermostat "backwards"?
My second question is, if I decide to replace the old wire from the heater to the thermostat, what kind of wire should I use? Since only two wires are now being used for the existing thermostat, I assume that replacing it with two-wire thermostat wire will be okay. But, if I later change to a programmable thermostat for example, will I need more than a two-wire conduit to connect the new thermostat up?
NOTE: This is for a gas-fired baseboard hot water, heater-only, system. There is no AC involved.