You cannot just follow what the wires are on the other thermostat. The other unit could use different wires for different terminals. For Central AC, you need to go to the furnace and look to see what colors go to what terminals. Radiant heat is usually going to be electric, forced hot water or steam. If it is electric, it is going to be 240 vAC.
If all you want is A/C, go to the Furnace for the downstairs A/C, and write down on a piece of paper, what color or wire is connected to what terminal. Then when you wire the thermostat, you wire it exactly the same way as it is on the furnace. What is the manufacturer & model of the thermostat you are trying to use?
You may be better to post this over at diychatroom.com. There are hvac techs in that forum, that can better assist you in this.
If all you want is A/C, go to the Furnace for the downstairs A/C, and write down on a piece of paper, what color or wire is connected to what terminal. Then when you wire the thermostat, you wire it exactly the same way as it is on the furnace. What is the manufacturer & model of the thermostat you are trying to use?
You may be better to post this over at diychatroom.com. There are hvac techs in that forum, that can better assist you in this.