I am trying to install a Honeywell programmable thermostat with an Equipment Interface Module. The thermostat requires a common wire in order to receive power. The diagram shows only 3 wires going from the thermostat to the module - red, yellow and green. The diagram then shows the module being wired to the common (C) on the furnace. How is this wired? There is currently a red wire on the furnace (C) leading to the A/C. Do I simply add another wire to the (C) on the furnace and run to the (C) on the module? The (R) on both the furnace and the module are already connected. Also there is a small wire connecting the (R) and (C) on the module.

List all brands and EXACT model numbers of all equipment being used. Also list the old thermostats brand amd EXACT model number.

Furnace is Bryant Model#376CAV024040
New thermostat is Honeywell VisionPRO IAQ with Equipment Interface Module model#TH9421
Old thermostat is Honeywell manual thermostat, possible model#LR1620, just a basic thermostat

Your going to have to open up the furnace and attach a wire to C (24 volt side)on the control board inside the furnace that goes to the thermostat if you have a extra wire going to the stat that is.

I'm not sure that everyone understands exactly what's going on here.

The mnauls are here:

Buy Honeywell VisionPro IAQ Programmable Universal Thermostat with Total Home Comfort Control | Honeywell YTH9421C1002

I haven't looked at how sensors get hooked up. I think the stat uses different resistance values so it can deterine if it's an indoor or outdoor sensor and I think it connected to S1 and S2 at the stat.

The only wires that connect from the stat to the interface module is #1, #2 and #3.

You must connect (C) and the jumped combination of (R)(Rc) and (Rh) to the furnace transformer.

(R) would be the original R and C might be easy or hard to find. Look on the door for where C is.

If you can't locate (C) that way and you only have single stage cooling; Just Y being used then (C) would be the other wire heading to the outside unit. This assumes that two wires go to the outside unit.

If you have any other troubles, holler.

Your furnace board has

W Y R C & G, so hookup should be cake.

In reality C & R have 24 VAC across them all the time hence it powers the interface module.

Rc and Rh jumped is what used to be R, but it's at the thermostat side.

R is the non-common side of the transformer at the furnace side. If it was a thermostat with Rc and Rh connected to your system, R at the furnace would be connected to Rh/Rc at the thermostat. That's why the jumper between R/Rh and Rc.

Does this now make sense?

C is common for a reason. It's common to the outside unit. e.g. 24 VAC between C & Y turn on the AC relay outside.

24 VAC between G and C turns on the fan.

So, it's a contact closure between R and G that turns on the fan and a contact closure from R and Y turns on the outside unit.

A contact closure between R and W turns on the heat.

Heating is not described as power between two pins because the furnace board has other things to do. In gas heat the furnace controls the fan, so you cannot turn the furnace on as easily as the AC unit.

For cooling, the thermostat controls the fan, so the thermostat determines when the fan comes on and handles short cycling.