Are you sure there are only two wires between the thermostat and furnace? Normally a furnace would have R or Rh ( 24 V), W (heat), and G (fan). The C wire connection is the ground on the 24 V side of the transformer. The control board might be labeled Gnd or C. If you can find the brand and model number, a search may locate the installation manual. The manual may have a drawing of the control board. This will help guide you to make the correct wiring connections.

thanks mike_home - I will double check this weekend. I'll report back here - thanks!

• Video that explains which Nest thermostats work with two-wire furnaces
• Nest-E installation video for two-wire system
• Nest Generation 3 installation video for two-wire system

Thanks - the videos help for sure. but I've read many times that without a c-wire, there can be reliability/power/wifi connectivity issues. So for the Nest-E (which btw do I understand is not available anymore?) and the 3rd Gen - they are both options without a c-wire, but are they reliable without c-wire?

Most likely the Nest 3rd generation will "work" the day you install it. The problem is the internal battery may not charge. Users have reported after a few days the internal battery is discharged and the thermostat no longer works. If you have an extra wire, or can install additional wiring, then that would be the best solution.

Another option is to install the Nest Power Connector. at the control board. This module is designed to charge the Nest battery when no C wire is available. Here is the video if you are interested:

How to Install a Nest Power Connector for Your Nest Thermostat

Okay I'm back with some photos and what I believe to be good news on my quest to easily add a C-wire. Here is a pic of the panel at the furnace - empty C terminal! There are definitely only 2 wires - W and R - in the existing setup. Also showing the existing Honeywell thermostat, W + R.

So am I correct in assuming I can just run 1 more wire at the c terminal and that's my c wire? Then with a newer Nest, it's the W, Rh, and C connections at the Nest, correct? Small cottage so I will easily be able to run this wire in the attic over to the thermostat.

Thanks!

closeup of terminals - only W + R connected:

broader view of panel:

and existing thermostat, which will be removed for a new Nest with C-wire:

"So am I correct in assuming I can just run 1 more wire at the c terminal and that's my c wire? Then with a newer Nest, it's the W, Rh, and C connections at the Nest, correct?"

Yup. I think the terminal on the Nest thermostat is labeled "W1" instead of "W" because the thermostat can support multi-stage systems. If you wanted to be ultra-careful and have a voltmeter, you could check that the voltage between R and C on the furnace board is about 24 volts AC before connecting the thermostat.

In my excitement I totally forgot to look for model number, but I still will.

Mike - are you saying that if I connect G furnace to G nest, with the Nest I'll have the option to run the fan independently of the heat kicking on? (sorry but I'm not sure what you mean by "it is not necessary with your furnace". What's not necessary?

"if I connect G furnace to G nest, with the Nest I'll have the option to run the fan independently of the heat kicking on?"

Yes, in many systems the thermostat turns on the fan by sending power over the G wire. With your furnace, we know the G wire is not required because your system was controlled using only two wires; when the thermostat turned on the furnace, it also turned on the fan.

If you want to be able to turn on the fan independently, connect a G wire. My Honeywell thermostat allows the fan to remain on for several seconds or minutes after my heat pump turns off. This allows the fan to push residual heat (or cool air) through the duct work after the compressor turns off.

I concur with what wdccruise has stated. Your furnace turns on the fan whenever the W terminal is energized. If you had AC the same thing would happen whenever the thermostat is calling for cooling and Y is energized.

Having the fan run for 5 - 8 seconds after the gas flow is shut off and the flame is out and independent fan control is a nice feature. Add the additional wire if it is easy. If not then you don't need to worry about it.

Okay I'm back at the house and have installed the new nest! Although I do have a few follow-up thoughts/question (mostly regarding the fan discussion from above).

btw - furnace is RUUD, although I still can't figure out model??

Remember that in my existing setup it was heat only, 2 wires W + R. And I think I already had the fan blowing off the residual heat, as the operation was as follows:

- call for heat

- furnace would activate with power vent and igniter heating up

- after about 20 seconds or so, furnace would actually fire

- after about another 10-20 seconds, fan would kick on

- when temp reached, furnace would fire down, and fan runs for another 20 or so seconds.

So I think the fan has already automatically been doing what you described (aka fan not coming on right away, and also running a bit longer after furnace fires down) without any G wire. Since I already had this with only 2 W + R wires and no G wire, I assume this a function of the control board in the furnace??

But now in the new set-up with Nest (latest model) - I've run a 5-wire 18awg to the thermostat, and have W + R + C + G connected. Call for heat operates as I've just described. So I'm in good shape there. However - it does NOT operate the fan independently/manually. I've double-checked all the thermostat wire connections at the thermostat and the furnace - they are correct and intact. Also just FYI I did tell Google setup which wires I have, the type of fuel and forced hot air, etc. So it believes it has a fan control, but when I attempt to run the fan manually nothing happens, despite Nest thermostat telling me the fan is running for X amount of time...

Okay so back to the furnace board - see pics below. on the right side of the board there are internal connections for, among other things, "heat" and "cool" (even though we don't have AC), and there is one for "Fan" - but it's empty. I assume this might be the reason fan is not active manually?

I realize I don't need per se, but now that we've talked about it - I'd love the option to run the fan in the warmer weather to circulate/distribute air throughout the house when we have a few window AC units running.

Thanks again for all the help!

the whole board:

New thermostat connections look good - W, R, C, G:

on right side, terminals for "heat", "cool". "Fan" is empty

better pic - "fan" is empty.

"However - it does NOT operate the fan independently/manually."

As you know the system is configured to operate the fan when heat is called and by a two-wire thermostat. Hence there is no need for the G wire to control the fan. To find out whether you could modify the configuration to allow fan control by the G wire, I think you'd have to find the installation manual for the furnace.

Bummer. But I’ll look into it the next time I have somebody come out for a cleaning. Thank you again!

You did a nice job on the thermostat wiring. You may have a future as a HVAC tech!

I am not an expert, but I think the no wire on the fan connector is the reason the fan is not turning on. The fan, heat, and cool connectors are connected to the motor wire taps. This furnace can be set up to have a different heating and cooling fan speeds. The speed for fan only mode can be a third speed. There is a H/C connector which can be used when the heating and cooling speed are to be the same. In your furnace it is unused and capped. You can't use the H/C connector if the heat and cool wire taps are used. Your control board has two unused taps connected to M1 and M2.

I found this Rudd furnace installation manual which also uses the same control board. I think this is a common White Rogers furnace control board model 1012-83-9203. I am posting this so you can get an idea how this is supposed to work. There is an explanation on page 46.

The blue fan wire is connected to the "Cool" terminal on the circuit board and the yellow fan wire is connected to the "Heat" terminal on the circuit board. These terminals are powered whenever the thermostat calls for cooling or heating, respectively. That explains why no wire is connected to the "Fan" terminal on the circuit board and the G wire is not used.

The instructions on page 46 explain that terminals "M1" and "M2" are parking terminals for fan wires that are not used for cooling or heating. If there are fan wires connected to "M1" or "M2", presumably you could move one of those wires to the "Fan" terminal. Power to the "Fan" terminal is presumably controlled by the G wire.

Hi!

My furnace has this board. The C (bottom left) and G(center) goes inside the furnace and there is no C and G wire that goes to my thermostat.

I have enough wire that goes to the thermostat, so I was wondering if I just have to connect one wire to the c and one to the G (with a female connector?) on my furnace and in the thermostat?

grasswhisperer,

If you look on page 51 you will see the four speed tap wires going to the fan motor. Each color is labeled. Red is low, yellow is medium-lo. blue is medium-high, and black is high. You could switch either the red or black wire from M1 or M2 to the fan connection. I would suggest trying the red first since that is the low speed. The black is the highest speed, and the red is the lowest speed on most blower motors.

If you attempt to do this yourself, then make sure the power is off. If you are not comfortable moving the wire then wait for the next time a HVAC tech is doing a service.

Pier,

Yes you can make the wiring connections to the G and C terminals between the control board and thermostat. Shut off the power before making any changes. If you short out the C and R terminals you are likely to damage the 24 V transformer.

If you have additional questions please start a new posting.

What a helpful forum!! Just catching up here. I will dive into all this and formulate a plan. I’ll report back with progress, or more likely some more questions or confirmation. I’ll be sure to continue cutting the power when I work. Thank you!

Okay so a few thoughts to make sure I'm absorbing all of this correctly:

On my system:

- I've got M1 (black) and M2 (red) (shown in upper right corner of the panel, pics above) going to the fan motor. Standard, as well as what's shown on page 51, indicates that M1/Black is connected to HI fan speed, M2/Red is LO. These are parking lots for unused fan speeds.

- HEAT terminal is connected to Yellow which should be Medium-LO. (this is different speed than shown on Page 51, but obviously I assume my system has been wired this way with purpose and I will not touch. I assume this is the proper fan speed for heat in my setup as it won't overheat.)

- COOL Terminal is connected to Blue which should be Medium-HI. (again also different than page 51, but also my system doesn't have cooling anyway)

Okay - so - if I understand correctly - if I move either the M1/Black/HI over to FAN - that might operate the fan independently and this will be at HI speed. Or, alternatively, move the M2/Red/LO over and the fan will operate independently at LO speed. And mike_home you've advised to try initially with Red/LO.

I will try when back down there. However I have a few questions already. With this M1 or M2 modification:

- is there any chance the fan, when in HEAT mode, will operate any differently? That is, any change in fan speed or change its current behavior of fan kicking in only after furnace has fired and fan running a bit longer to dissipate residual heat? I don't think so but just wondering. Again I don't want to change the fan speed or operation when heating - it's already working as desired.

- Does this mean there is no way to run the fan independently at the same speed as heating? Seems like not - can only run independently at a speed different than Heat (?) I can live with this but just wondering. Similar to the option to run H/C both at the same speed, but for Heat + Fan instead of Heat + Cool.

Also when I'm in there I will try to see if the Fan motor connections are labeled Lo, Med-lo, hi, etc just to make sure I'm operating with the correct assumptions.

Okay - sorry for all the questions - I might be a HVAC apprentice soon enough! :)

Thanks again

"is there any chance the fan, when in HEAT mode, will operate any differently?"

I don't see anything that would cause that to happen because the "Heat" and "Fan" terminals should control the fan independently.*

"Does this mean there is no way to run the fan independently at the same speed as heating?"

If you wanted to do that, you'd have to add a jumper wire between the "Heat" terminal and the "Fan" terminal.

---

*If you wanted to verify the independence of the "Heat" and "Fan" terminals, you could connect the Black fan wire to the "Fan" terminal and apply power to the G wire. The fan should run at HI speed (if the G wire really does control the "Fan" terminal). Then adjust the thermostat to call for heat; the fan should slow down to Medium-LO.

Yep makes sense. I will investigate.

Also - since I don’t have cooling in this system - Couldn’t I just move the blue (med-Hi) wire that’s on “cool” over to “fan” and leave cool empty? Sorry if one of you already suggested that above, I couldn’t find it.

Your furnace has a PSC (permanent split capacitor) motor. It is a simple AC motor with multiple windings. Each winding causes the motor to rotate at a different speed when energized. Your motor has four speeds which is fairly common. The control board will only energize (apply 120 V) to one winding at a time. The motor could be damaged if more than one winding is energized at any time.

The heating motor speed will not change as long as you don't change the current wire. For the fan selection you can choose the two unused speeds (M1 and M2) or move the wire from cool since you don't have AC. If you really wanted the fan to have the same speed as heat, you could install a wire jumper between the heat and fan terminals. I don't recommend you do this. The variation in air flow between two adjacent speed settings is that that noticeable. It is not worth the risk of adding a extra wire.

Did you ever find out what model you have? There should be a model plate somewhere on the furnace. You should find a wiring diagram attached on the inside of the cover panel.

"Couldn’t I just move the blue (med-Hi) wire that’s on “cool” over to “fan” and leave cool empty?"

Sure. For a system without AC, the "Cool" terminal is really just a parking terminal.

If you had also run a wire to the Y terminal, in the summer you could set the thermostat to cool. Even though you don't have an AC condenser and coil, I think the fan would run until the thermostat setting was satisfied.

Great insights. Thanks!! I’ll keep you posted

Hello folks - I'm happy to report success! I moved one of the M terminals over to "Fan" and voila! Can operate fan independently now. Awesome! Thanks so much for all the help, and enjoy the holidays!

btw - STILL can't find the model number, but I do have a pic of the wiring diagram. The wiring from the motor is slightly different from the diagram supplied earlier in this thread, but at least I understand how it works now. Again thanks so much.

Hello folks - I'm happy to report success! I moved one of the M terminals over to "Fan" and voila! Can operate fan independently now. Awesome! Thanks so much for all the help, and enjoy the holidays!

btw - STILL can't find the model number, but I do have a pic of the wiring diagram. The wiring from the motor is slightly different from the diagram supplied earlier in this thread, but at least I understand how it works now. Again thanks so much.

I am glad to hear you were able to install your new thermostat and independently turn on the fan. Good work!

Hey guys - resurrecting this thread as I am now attempting to install a Nest Thermostat at another location (different house.) For this one we've got both heat and AC controlled with a Honeywell thermostat. Pics below.

My question is regarding - yet again - the infamous c-wire. I thought for sure I would have a c-wire in this setup - but I don't even see a terminal for a c-wire on this thermostat!? What am I missing? btw we've got an oil-fired boiler for baseboard heat (along with the AC...)

Thermostat type:

Existing Thermostat wiring for heat and AC

Type of control board I assume at the newer-style boiler (can send more pics if needed).

"resurrecting this thread"

See the note at the bottom of Figure 4A of the Installation Instructions.

In your setup there appear to be two zones, each with its own thermostat.

Thanks! Yes - we have 2 zones. Upstairs has heat + AC (wires as shown). Downstairs is just heat (only 2 wires there). Both Honeywell thermostats as shown.

But for now - I'm also just wondering how could this somewhat newer thermostat (my existing honeywells) not even have a terminal for common wire? I was under the impression that if I had both heat and AC, there is typically also a c-wire...

Just when I thought I was learning a thing or two... :)

"I'm also just wondering how could this somewhat newer thermostat (my existing honeywells) not even have a terminal for common wire?"

It doesn't need a C wire to power the thermostat because there are batteries in the cover that power the display.

A basic programmable thermostat like the Honeywell you have pictured typically don't have a C wire connection.