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

How to Wire a Honeywell Thermostat with 4 Wires




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 (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 are discussed below.



How to Wire a Honeywell Thermostat with 4 Wires

In the installations we’ve worked with, the most common wire color code guide for a four wire t-stat are as follows.  Of course though, you should check that the wires in your HVAC system match up with their expected functions.  If unsure, don’t just attach them and hope for the best.  Consult an HVAC tech if you’re confused 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’ve switched 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 spot on the wiring block.  When you only have 4 wires from the furnace to work with, and you cannot run a new cable with more wires, then you’ll likely have to give up some functions of today’s heat pumps, central heating, and central cooling systems.  In this case, we chose to give up control of the fan, to power our Honeywell Wi-Fi t-stat.

How to wire a Honeywell thermostat with 4 wires. Picture of a Honeywell WiFi thermostat wall plate, showing a typical 4-wire hookup.
How to wire a Honeywell thermostat with 4 wires. Honeywell WiFi Thermostat wall plate, showing a typical 4-wire hookup.

 

How to wire a Honeywell thermostat with 4 wires. Picture of the Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with 4 wires connected.
How to wire a Honeywell thermostat with 4 wires. Example of a Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with 4 wires connected.



Example of No Common Wire Thermostat

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.

How to wire a Honeywell thermostat with 4 wires. Picture of the mounting plate with four wires, of the Honeywell large dial thermostat, model T87N1026.
How to wire a Honeywell thermostat with 4 wires. Mounting plate with four wires, of the Honeywell large dial thermostat, model T87N1026.

 

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 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.

How to wire a Honeywell thermostat with 4 wires. Picture of the Honeywell T8775C1005(2) thermostat wall plate, front view, showing screw terminal connection points.
How to wire a Honeywell thermostat with 4 wires. Honeywell T8775C1005(2) thermostat wall plate, front view, showing screw terminal connection points.



Related Posts

References

Revision History

  • 2017-03-11: Originally published.