I'd like to replace the wiring from my Carrier Weathermaker 8000 to my thermostat to add a C wire. I assume I need to turn off the Carrier first, but there is no power switch that I can see on the Carrier - can I just unplug it? Also, do I need to turn off the gas before I turn off the furnace? My A/C is also connected to the Carrier. Any other pointers that I should consider?
Check your circuit breaker panel. There should be breakers for the furnace and ac unit. Turn them all off. Also, turn off the gas just as a precaution.
It is advisable to turn off the furnace power before you start opening it up and working on the wiring. If the furnace is off and not operating the gas valve in the furnace is already off so that should give you the idea that it would also be a good idea to also turn off the gas and that such action would not damage the furnace unit. Turning everything off is really common sense anyway in case some unforseen thing should go wrong while working in and around the furnace.
It is common that there will be an electrical box mounted near the furnace in line with the power wiring going to the furnace. This electrical box will often have a shutoff switch for the furnace mounted in its cover plate.
You should take a careful look at the existing thermostat cable to see if there are any unused conductors in the cable. It is quite common to see installations where there are one or two spare conductors in the cable. If yours is that way you can use a spare conductor for your C wire and not have to string a new cable.