How to wire a new Honeywell RTF6580WF thermostat to replace a Chronotherm IV

The C-wire substitution requires you to use the G-wire. When using the G-wire as a common you have to change it to C at the thermostat, and then to a common in the bottom panel of the unit. In your situation, your specific unit uses an older style control board made by White Rogers. This early revision of the board did not have a terminal bus on the board the thermostat, air conditioning, and fan speed 24v connections. Wires were used instead if a terminal bus, and only has 4 connections (your W, G, R, Y wires). To have your new thermostat work, you will need to remove the thermostat wire at the unit. Then you must either ground it to an exterior panel of the unit, or using spade, attach it to one of the open neutral buses on the control board. After doing this and adding the G-wire on C at the thermostat, it will work fine.

Now that the thermostat is working, you have lost the ability to control the fan. A little trick if you want to retain a way to control the fan without having to run a new 5-wire is to get a piece of 18/2 wire and run it into the control panel of the unit. Attach one wire to R and one wire to G. Attach a switch box to the side of the unit and run the wire into it. Connect the 2 wire to any light switch and mount it into the switch box. Throw your switch cover plate over it, and you have yourself a home made, manual fan switch that you can activate by going to the unit.

For Reference:

• W - Heating
• R - Continuous 24v Power
• G - Fan
• Y - Cooling

• C - Common

  R is used to energize, C is common (For Optional Accessories like A/C, Etc..)

• Closing R and W will initialize the heating cycle
• Closing R and G will initialize the blower to run on low speed
• Closing R and Y will initialize the cooling cycle and the blower on high speed
• Closing R between Y on certain units will only energize the A/C relay and not activate the blower. These units require the thermostat to energize the Y and G terminals together to initialize the cooling cycle and the blower on high speed. Make sure to check that the Air Conditioning is working properly, and that the blower is running on high when the Thermostat is calling for cooling.

There should be no problem connecting the C wire from your thermostat to the C terminal in the furnace. Take a look at this diagram, which is a rough approximation of your system.

Notice the cable going to the condensing unit has a red wire connected to the Y terminal in the furnace, and a white wire attached to the C terminal. When the thermostat calls for COOL, it does so by energizing the Y terminal (connecting R to Y). This provides power to the coil of the contactor, which allows it to close the contacts and turn the condensing unit on. The other side of the contactor coil is connected to the C terminal, as to provide a complete circuit.

You'll notice in the first image that two relays in the furnace, the thermostat, and the contactor coil all connect to the same leg of the transformer. This is because the thermostat basically works like a switch, connecting the R wire to the C wire through control circuitry. Anything that needs 24VAC power, will be connected to both legs of the transformer.