I just purchased a Honeywell ct87758A,C to replace an old round, manual Honeywell unit. The old one was taken off without labeling the wires. There is a white, red and gray one. Any idea which one goes with which letter? It only controls the cooling system.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit.

Specifically to your problem. Does the new thermostat have terminals labeled R, Y, and G? If so, go the furnace or air handler, and if it also follows the convention I gave above, simply list which color of wire connects to which terminal, and wire the thermostat the same way. If one of the wires isn't used, don't worry about it.

This thermostat only controls the AC. The new thermostat does have those letters and the old one had only R,O,G,Y. Excuse my ignorance, but where do I look a A/C unit to see which colors match up with the letters?

Most central A/C units use the same blower as the forced air furnace. Often it is wired to give a higher speed for air. The furnace should have a contact labeled Y that the thermostat switches the power from the control transformer to when it calls for heating. The Y terminal should have a second wire running out to the coil of the compressor relay along with the common from the transformer. Identify the wires at the furnace or air handler connected to the R and Y, and connect the same ones at the thermostat, and it should work. Some systems may also require the G terminals to be connected.