I have a heat-only natural gas furnace with a fan. (Bryant 315aav036070adja) The thermostat I'm trying to install is a Honeywell TH4110U2005/U.

My old thermostat only has two wires (V+ and Vg), which go to a controller panel by the furnace which sends Rc, W/W1, G, C, and Y1 to the furnace. There's also a green wire through the wall that doesn't connect to anything on either end.

The installation instructions for the Honeywell say:

R Power

Rc [R+Rc joined by Slider Tab]

C 24VAC common

W Heat

G Fan

Can I simply disconnect the V+ and Vg wires from the control panel, and connect them to whichever wires correspond to power and heat (then do the same thing with the unused green wire to whichever wire corresponds to the fan)? How do I figure out which of the output wires in the control box are for heat/power/fan?

Old Thermostat:

old thermostat

Control Panel:

enter image description here

Control Panel Wiring Guide:

enter image description here

Honeywell Thermostat:

enter image description here

  • How many wires are in the cable between the thermostat location and this control board/interface-module of yours? Sep 25, 2021 at 21:29
  • Only three wires. Red (V+), white (Vg), and green (doesn't connect to anything on either end).
    – Ash
    Sep 25, 2021 at 23:24
  • Is replacing that cable an option? Sep 26, 2021 at 0:03

1 Answer 1


You can make the 3 wire cable work but you won't have the ability to control the fan manually.

Step 1: remove the control module and connect the two cables together like this:

3 wire cable > 5 wire cable

Red > red

White > white

Green > Brown

You won't use any of the other wires in the 5 wire cable.

Now for the thermostat, connect:

Red > R

White > W1

Green > W2

Normally, green is for the thermostat to be able to manually turn the fan on but you need that wire for your second stage heat. You'll also need to consult your thermostat manual for configuring for 2-stage heat.

This thermostat also has the ability to be powered by the furnace control transformer (the C terminal) but you don't have enough wires, so you'll need to use batteries only.

The only other option is to run a new 5 conductor cable from the furnace to the thermostat.

  • They have an interface module between their furnace and their thermostat Sep 26, 2021 at 1:46
  • Yes, I modified my answer
    – DrSparks
    Sep 26, 2021 at 1:58
  • You said to hook up the green wire from W2 (Aux) in the thermostat to the brown wire (Y1) at the control panel. But wiring guide says that Y1 is used for "multi-speed cooling"?
    – Ash
    Sep 26, 2021 at 16:48
  • Oh never mind, it looks like the brown wire actually goes to 0/W2/B. It's a hard to see angle.
    – Ash
    Sep 26, 2021 at 16:51
  • I tried your suggestion. It doesn't appear to do anything. The heat won't come on.
    – Ash
    Sep 26, 2021 at 19:59

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