[jsfrisbie]

I have been wanting to replace my 23 year old home thermostat with one that is wifi enabled. So I purchased a Honeywell RTH6580WF. I have a central heating unit converted to propane, with A/C. When checking my current wiring for compatibility, I found that while a C terminal is required on the new thermostat, my current thermostat does not utilize it.


According to Honeywell folks, a work-around is to move the G terminal at the furnace side to C terminal. When I checked on that, I noticed that the C terminal at the furnace is already being utilized along with Y terminal, on another wire run. According to my furnace literature, that wire run “goes to CC” (compressor contactor) which I presume is for the A/C.


While investigating all this, I noticed that the wire bundle from the furnace to thermostat has an un-used wire (for reference, color blue). I discussed the possibility with Honeywell of leaving the furnace G wire alone, and hooking up the blue wire to C terminals at furnace and thermostat, thus obviating the need to move the furnace G wire. Since the C terminal would be shared the Honeywell folks would not commit to a definite answer (at the time I did not know the destination of the “CC” wire run).



So, my question: Will it be OK to hook up the un-used blue wire at the furnace to the shared C terminal and to the C terminal at thermostat? I can provide photos if that will help.

 

[BayouRunner]

It wouldn't hurt to use the blue wire if it's not hooked up to anything else. The g is normally for the blower motor

 

[yuri]

Yes you can use the blue wire on C at the furnace. C is Common same as the white/neutral wire in your house wiring and has no voltage on it.

They did not know as they could not see the wire going to the AC.

 

[jsfrisbie]

Thermostat C and G wire question

Thanks guys. I will give it a go.

 

[jsfrisbie]

Re: trying to install a new thermostat: used the furnace shared C terminal

Well,
Yesterday I was FINALLY able to work on my new thermostat install, based on Yuri and others suggestions of hooking up the un-used blue wire to the furnace C terminal, and then to C at the thermostat.

I am pretty darn certain that I wired everything correctly. However, when I restored power to the system the new thermostat did not power up.:vs_sob:

As I mentioned in an earlier post, at the furnace is a second wire run (two wires: C and Y terminals) which goes to the compressor contact (CC) (for AC?). I figured (I am not all that electrically astute) that between those two terminals there must be voltage since it is supplying something to the CC, so I checked voltage at C and Y; it was zero.

I noticed a note in the new thermostat instruction guide: "Important! C wire is required and is the primary power source. Without a C wire, your thermostat will not power up." However, Yuri said the C at furnace is neutral and that seems to be born out by thermostat not powering up.

So, I am really puzzled, and I did not think of this before. If C is not hot and my old thermostat does not have a C anyway, which terminal on my old thermostat is 24VAC? Set-up on old is G, Y, W and RH (which is jumpered to RC). Those correspond to G, Y, W and R at furnace. I found the owner's guide for my old thermostat and it contains a diagram that shows output from a 24VAC transformer going directly to RH, but also outputs from the transformer going to G, Y and W with labelled boxes "Fan", "Cool Pump" and "Gas Valve" in the lines between transformer and terminals.

I am completely stumped.:sad:

 

[ben's plumbing]

Re: trying to install a new thermostat: used the furnace shared C terminal

Well,
Yesterday I was FINALLY able to work on my new thermostat install, based on Yuri and others suggestions of hooking up the un-used blue wire to the furnace C terminal, and then to C at the thermostat.

I am pretty darn certain that I wired everything correctly. However, when I restored power to the system the new thermostat did not power up.:vs_sob:

As I mentioned in an earlier post, at the furnace is a second wire run (two wires: C and Y terminals) which goes to the compressor contact (CC) (for AC?). I figured (I am not all that electrically astute) that between those two terminals there must be voltage since it is supplying something to the CC, so I checked voltage at C and Y; it was zero.

I noticed a note in the new thermostat instruction guide: "Important! C wire is required and is the primary power source. Without a C wire, your thermostat will not power up." However, Yuri said the C at furnace is neutral and that seems to be born out by thermostat not powering up.

So, I am really puzzled, and I did not think of this before. If C is not hot and my old thermostat does not have a C anyway, which terminal on my old thermostat is 24VAC? Set-up on old is G, Y, W and RH (which is jumpered to RC). Those correspond to G, Y, W and R at furnace. I found the owner's guide for my old thermostat and it contains a diagram that shows output from a 24VAC transformer going directly to RH, but also outputs from the transformer going to G, Y and W with labelled boxes "Fan", "Cool Pump" and "Gas Valve" in the lines between transformer and terminals.

I am completely stumped.:sad:

..old t stat..r=24 volt, g= blower control.,y=compressor contactor,w= gas valve..c= common white from compressor contractor.. new t stat..r= 24 volt, g= blower, y compressor contactor, w =gas valve.c= blue common ..if you did not blow a fuse..should work

 

[supers05]

Ben is right.

For a more detailed answer:

As for all electricity, you need a complete path or loop to make it all work. The source, The load, and the connections in-between. C, is very similar (but not the same) as N on the high voltage side. They both are current carrying and are required for everything to work. It is live, but just happens to be very close or at the same voltage as ground. When you're measuring a single wire, you're measuring the difference between it and C. (Voltage difference=Potential=volts, etc)

Power is flowing through both R and C equally, and the only difference is that we reference one to ground.


Another component of a circuit is a switch. (control) That's what your old stat is, and half of your new stat.

The furnace board/gas valve, compressor contactor, and the display and smarts of the new stat are all side by side and called loads. (parallel) All loads require a path to C and R. (or Hot and N, or + and - of a battery, depending on the application)
(ground can be a wire and anything, even you can be a load. So that's how you can be shocked by only touching only one wire, your feet are touching the other wire -ground)

This is why your new stat requires C and the old one didn't. That blue unused wire needs to be connected to c at the furnace and the stat. (sharing it with the compressor contactor, and everything else, like a humidifier, or HRV, etc.)



Measuring with a voltmeter:

Y-C will only=24v if the thermostat turns on the compressor. (Y-R will be opposite. =0v when the compressor is on, 24v when it's off)

W-C will only=24v if the thermostat turns on the heat. (W-R, same as above)

G-C will only=24v if the thermostat turns on the fan. (On mode or with the compressor in auto mode. Never with the heat in auto mode) (G-R, same as above)

R-C should always=24v when the furnace has power.

Cheers!

 

[roughneck]

The C should be connected at both ends. You should have 24 V at the thermostat, between R and C. You might have to ring the blue wire out to make sure it's not broken.

 

[DOUG1111]

The C should be connected at both ends. You should have 24 V at the thermostat, between R and C. You might have to ring the blue wire out to make sure it's not broken.

YES - roughneck is correct!!!! Your blue @ the T/S may not be connected @ or to the transformer "c" side.

 

[jsfrisbie]

Thanks all you guys.
What I get from your replies is that I should be able to get it to work somehow.
No, I have not yet blown a fuse. :wink2:
I checked continuity of the blue wire between furnace and t-stat; it is good.

I will check voltages at blower, CC, gas valve no problem.

What I do not get yet is 24V input. Since my old system seems to be supplying it via R (at furnace) instead of C (as expected by new t-stat), should I, at the new t-stat swap R and C wires, hooking up the furnace R wire to new t-stat C terminal and the C furnace wire to t-stat R terminal? Keep in mind that new t-stat R and RC terminals are jumpered.

 

[beenthere]

Re: trying to install a new thermostat: used the furnace shared C terminal

Well,
Yesterday I was FINALLY able to work on my new thermostat install, based on Yuri and others suggestions of hooking up the un-used blue wire to the furnace C terminal, and then to C at the thermostat.

I am pretty darn certain that I wired everything correctly. However, when I restored power to the system the new thermostat did not power up.:vs_sob:

As I mentioned in an earlier post, at the furnace is a second wire run (two wires: C and Y terminals) which goes to the compressor contact (CC) (for AC?). I figured (I am not all that electrically astute) that between those two terminals there must be voltage since it is supplying something to the CC, so I checked voltage at C and Y; it was zero.

I noticed a note in the new thermostat instruction guide: "Important! C wire is required and is the primary power source. Without a C wire, your thermostat will not power up." However, Yuri said the C at furnace is neutral and that seems to be born out by thermostat not powering up.

So, I am really puzzled, and I did not think of this before. If C is not hot and my old thermostat does not have a C anyway, which terminal on my old thermostat is 24VAC? Set-up on old is G, Y, W and RH (which is jumpered to RC). Those correspond to G, Y, W and R at furnace. I found the owner's guide for my old thermostat and it contains a diagram that shows output from a 24VAC transformer going directly to RH, but also outputs from the transformer going to G, Y and W with labelled boxes "Fan", "Cool Pump" and "Gas Valve" in the lines between transformer and terminals.

I am completely stumped.:sad:

C is not a power wire/terminal despite what it says in the manual of your thermostat. C is Common. Which as said before is similar to neutral. C is common to all of the loads the transformer supplies power to.

Use a meter at the thermostat location to see if you read 24 volts between the Blue wire that goes to C, and the red wire that goes to R/RC. If not, you don't have a completed path back to C at the furnace.

 

[DOUG1111]

The "C" is something, somewhat, simular, to the negative side of a 12 volt car battery. The negative (-) on a 12 v battery goes to one side of electrical device (ground- negative)) On your car battery the positive side is the hot wire (+)- On your transformer the "C" is common (- negative) goes to one side of your switching devices - contactor - relays etc. On your transformer the other wire (red) is the power wire (hot) -going to your thermostat RC-RH - So when you turn the T/S to heat - the T/S connects the red RC RH to the W (white) & G (green fan) The T/S turns the W & G to a hot wire. Hope this helps??

 

[HVACDave]

Thanks all you guys.
What I get from your replies is that I should be able to get it to work somehow.
No, I have not yet blown a fuse. :wink2:
I checked continuity of the blue wire between furnace and t-stat; it is good.

I will check voltages at blower, CC, gas valve no problem.

What I do not get yet is 24V input. Since my old system seems to be supplying it via R (at furnace) instead of C (as expected by new t-stat), should I, at the new t-stat swap R and C wires, hooking up the furnace R wire to new t-stat C terminal and the C furnace wire to t-stat R terminal? Keep in mind that new t-stat R and RC terminals are jumpered.

You shouldn't be swapping any wires between r and c at the stat. R on furnace should go to R on stat, likewise for C, G W, and Y. the jumper for R and RC is still needed. You need to check to be sure you have 24 volts between R and C at the thermostat to know that it is getting power. Obviously the door needs to be in place on the furnace for this to happen (providing it has a door switch). If you see 24 volts from R to C, and attach the thermostat and nothing happens, then you have a stat problem.

 

[jsfrisbie]

Thanks to:
beenthere
DOUG1111
HVACDAVE

I will follow up and check for 24V between R and blue (C).