If you’ve installed more than a few thermostats, you’re likely to have come across many that utilize four wires, as shown below in the units we’ve hooked up. Typical 4 wire thermostat wiring examples follow. Some have the common (C) wire, while others do not. Both examples are discussed below.
In the installations we’ve encountered, the most common wire color code guide for a four wire thermostat installation are as follows. Of course however, you should verify that the wires in your particular HVAC environment match up with their expected functions. If unsure, don’t just attach them anywhere and hope for the best. Consult a professional HVAC technician if you’re confused about how to “ring out” each wire function.
- The red wire (R, RC, RH) connects the high side of the output of the control transformer to the thermostat.
- The blue, brown, or black wire (C) connects the low side of the output of the control transformer to the thermostat. Note that in our case, we had to use the green wire for C, as discussed below, because there were only four wires coming from the furnace to the thermostat which included no blue, brown, or black wire. Also, these particular thermostats require a C wire. So we decided that we could do without the independent fan switching function that the G wire normally provides, in favor of the remotely programmable capabilities of a Wi-Fi smart thermostat that must have a C wire in order to function. Eventually, we hope to run new thermostat wire, that has more conductors, so we can get the C as well as the G wires.
- The yellow wire controls the cooling (first stage). The thermostat connects this lead to the R wire whenever it calls for the air conditioning to come on.
- The white wire controls the heating (first stage).
In the next two pictures, we’ve converted the G fan wire into the C common wire. This involved getting into the furnace at the other end of the wires, and moving the green wire from the G to the C terminal on the wiring block. When you only have 4 wires from the furnace to work with, and you cannot run a new wire with more conductors, then you’ll likely have to sacrifice some functionality of today’s heat pumps, central heating, and central air conditioning systems. In this case, we chose to give up independent control of the fan, in order to power our Honeywell Wi-Fi thermostat.
The next thermostat, the Honeywell T87N1026 big dial model, does not require a common wire. So the G wire here actually does control the HVAC fan. Since this particular thermostat’s functionality is limited to simply regulating the room temperature in a non programmable sort of way, it can “steal” enough power from the remaining wires to function properly. Again, no blue wire appears.
The next thermostat, the Honeywell T8775C1005(2) non programmable electronic thermostat can control both heating and cooling in the home, as well as independent fan operation, with just four wires, without any common wire required. For this one, the red wire would attach to the R screw, the green to the G, the yellow to the Y, and the white wire to the W terminal, for single stage operation of both heating and cooling systems.
- 2017-03-11: Originally published.