How to Wire a Honeywell Thermostat with 4 Wires

Honeywell T8775C1005(2) thermostat wall plate, front view, showing screw terminal connection points.

If you installed more than a few thermostats, you’re likely to have come across many that have four wires. Typical 4 wire thermostat (t-stat) wiring examples follow. Some have the common (C) wire, while others do not. Both examples of how to wire a Honeywell thermostat with 4 wires we describe next.

How to Wire a Honeywell Thermostat with 4 Wires: Wire Colors

In our experience, the most common wire color code guide for a four wire t-stat is as follows. Of course though, check that the wires in your HVAC system match up with their functions. If unsure, don’t just attach them and hope for the best. Instead, consult an HVAC tech if you’re not sure about how to “ring out” each wire.

The Red Wire

The red wire (R, RC, RH) connects the high side of the output of the control transformer to the t-stat.

The Blue, Brown, or Black Wires

Also referred to as the C wire, this wire connects the low side of the output of the low voltage power to the t-stat. 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 t-stat which included no blue, brown, or black wire. Also, these particular t-stat require a C wire. So we decided that we could do without the fan switching function that the G wire normally provides, in favor of the remotely programmable capabilities of a Wi-Fi smart t-stat that must have a C wire in order to function. Later on though, we hope to run new t-stat wire, that has more conductors, so we can get the C as well as the G wires.

The Yellow Wire

This wire controls the cooling (first stage). The t-stat connects this lead to the R wire when it calls for the cooling system to come on.

The White Wire

The white wire controls the heating (first stage).

G Wire Change to C Wire

In the next two pictures, we switch the G fan wire into the C (common) wire. This involves getting into the furnace. Then you must move the green wire from the G to the C terminal on the wiring block. Now you might only have 4 wires from the furnace to work with. Why? Because you cannot run a new cable with more wires due to wall and floor barriers and such. In that case then, you’ll likely have to give up some functions of today’s HVAC systems. Here, we choose to give up control of the fan, to power our Honeywell thermostat.

A common Honeywell thermostat wall plate, with a 4 wire hookup.

Another 4 wire wall plate example.

How to Wire a Honeywell Thermostat with 4 Wires: A No Common Wire Example

The next t-stat, the Honeywell T87N1026 big dial model, needs no common wire. So the G wire here does control the HVAC fan. Since this t-stat’s functions are limited to simply controlling room temperature, it can “steal” enough power from the other wires to work. Again, no blue wire here.

A round t-stat wiring example.

The next t-stat, the Honeywell T8775C1005(2) non programmable t-stat can control both heating and cooling, as well as manual fan operation, with just four wires.

This t-stat also needs no C wire. For this one, the red wire would attach to the R screw. The green goes to the G screw. The yellow to the Y, and the white wire to the W terminal. This is for single stage operation of both heating and cooling systems.

The Honeywell T8775C1005(2) thermostat wall plate, front view, showing screw terminals.