Older model robertshaw thermastat needs to be rewired. It is for gas heat and a.c. There is no green wire. There is one yellow wire, one black wire, two red wires and a red jumper and three white wires. I know where the red wires and jumper go but nothing else. I cannot get the fan to go on automatically. Could the black one be used for green and why so many white and only one yellow.

All wiring has to be to the standard color code of the industry. When you have colors like you do it is impossible for us to be able to tell you how to wire it.

Here is something you can do for yourself.

Go to the unit and see what color wires are attached to what terminals. Then make up a color code list IE (The Y terminal in the furnace is wired to the yellow wire in the thermostat wire set) then do the rest of them. That way when you are back at the thermostat all you have to do is transpose what color was used on lets say Y at the unit and place that color wire on the Y terminal on the thermostat. This is kind of like sounding the wires out but it is the only way to be sure when you have a non-standard set of wires to work with.

If you need to use a wire that has many wires of the same color in the bundle then you will have to sound out the wire you eant to use with a OHM meter or some other device that will allow you to test for continuity.

Standard colors for both standard units and heat pumps.

Red= transformer
Yellow= contactor A/C
Green= fan
white=heat
blue= reversing valve energized in heating.
Orange=reversing valve energized in cooling